Combining very large quadratic and cubic nonlinear optical responses in extended, Tris-chelate metallochromophores with Six π-conjugated pyridinium substituents

Published on Web 02/18/2010

Combining Very Large Quadratic and Cubic Nonlinear Optical

Responses in Extended, Tris-Chelate Metallochromophores

with Six π-Conjugated Pyridinium Substituents

Benjamin J. Coe,*^,†John Fielden,^†Simon P. Foxon,^†Bruce S. Brunschwig,^‡

Inge Asselberghs,^§Koen Clays,^§Anna Samoc,^|and Marek Samoc^|,⊥

School of Chemistry, UniVersity of Manchester, Oxford Road, Manchester M13 9PL, U.K.,

Molecular Materials Research Center, Beckman Institute, MC 139-74, California Institute of

Technology, 1200 East California BouleVard, Pasadena, California 91125, Department of

Chemistry, UniVersity of LeuVen, Celestijnenlaan 200D, B-3001 LeuVen, Belgium, Laser Physics

Centre, Research School of Physics and Engineering, Australian National UniVersity,

Canberra, Australian Capital Territory 0200, Australia, and Institute of Physical and

Theoretical Chemistry, Wrocław UniVersity of Technology,

Wybrzez˙e Wyspian´skiego 27, 50-370 Wrocław, Poland

Received December 14, 2009; E-mail: b.coe@manchester.ac.uk

Abstract: We describe a series of nine new complex salts in which electron-rich Ru^IIor Fe^IIcenters are

connected via π-conjugated bridges to six electron-accepting N-methyl-/N-arylpyridinium groups. This work

builds upon our previous preliminary studies (Coe, B. J. J. Am. Chem. Soc. 2005, 127, 13399-13410; J.

Phys. Chem. A 2007, 111, 472-478), with the aims of achieving greatly enhanced NLO properties and

also combining large quadratic and cubic effects in potentially redox-switchable molecules. Characterization

has involved various techniques, including electronic absorption spectroscopy and cyclic voltammetry. The

complexes display intense, visible d f π* metal-to-ligand charge-transfer (MLCT) bands, and their π f π*

intraligand charge-transfer (ILCT) absorptions in the near-UV region show molar extinction coefﬁcients as

high as ca. 3.5 × 10⁵M^-1cm^-1. Molecular quadratic nonlinear optical (NLO) responses ꢀ have been

determined by using hyper-Rayleigh scattering at 800 and 1064 nm and also via Stark (electroabsorption)

spectroscopic studies. The directly and indirectly derived ꢀ values are very large, with the Stark-based

static ﬁrst hyperpolarizabilities ꢀ₀reaching as high as ca. 10^-27esu, and generally increase on extending

the π-conjugation and enhancing the electron-accepting strength of the ligands. Cubic NLO properties

have also been measured by using the Z-scan technique, revealing relatively high two-photon absorption

cross sections of up to 2500 GM at 750 nm.

Introduction

organometallic complexes are especially interesting in this

context. Such compounds offer unparalleled structural diversity

The promise of diverse applications, including optical data

processing and biological imaging, has stimulated many studies

with organic nonlinear optical (NLO) materials over recent

years.¹For example, NLO effects allow the manipulation of

laser light beams and may thus form the basis of all-optical

computing devices. Although purely organic compounds have

received most attention, transition metal coordination and

and scope for the creation of multifunctional optical materials,²

featuring for example redox-switchable NLO responses.³

Most NLO chromophores comprise relatively simple dipolar

electronic structures, but octupolar compounds have also at-

tracted considerable interest, especially for potential quadratic

(second-order) applications.⁴Octupolar molecules have zero net

ground-state dipole moments, but large dipole moment changes

accompanying intramolecular charge-transfer (ICT) excitations

can lead to substantial values of the ﬁrst hyperpolarizability ꢀ,

which governs quadratic NLO effects at the molecular level.

Molecules with large two-photon absorption (2PA) cross

sections σ₂are of major interest for a wide range of potential

applications, including optical power limiting, two-photon

upconversion lasing, 3D optical data storage, and photodynamic

cancer therapy.⁵The quantity σ₂is related to the imaginary part

of the second hyperpolarizability γ, and the latter is the origin

of cubic NLO responses. It is established that large γ and σ₂

values require extended conjugated π-systems, and recent studies

^†University of Manchester.

^‡California Institute of Technology.

^§University of Leuven.

^|Australian National University.

^⊥Wrocław University of Technology.

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9

3496 J. AM. CHEM. SOC. 2010, 132, 3496–3513

10.1021/ja910538s  2010 American Chemical Society

Tris-Chelate Metallochromophores

A R T I C L E S

have featured various branched structures, including octupolar

chromophores.⁶

esu.⁸However, other workers found that this ꢀ₀value is

overestimated due to contributions from 2PA-induced lumines-

cence,⁹and subsequent investigations have thus given a smaller

(albeit still large) ꢀ₀value of 380 × 10^-30esu for this complex.¹⁰

A number of other quadratic NLO studies have been carried

out with derivatives of [Ru^II(bpy)₃]²⁺and related complexes of

other metals.^2n,10,11Very recent reports concerning such chro-

mophores have also focused on their 2PA properties, from both

a largely experimental perspective¹²and purely theoretical

analyses.¹³

Zyss et al. ﬁrst highlighted the octupolar nature of D₃tris-

chelate transition metal complexes, using hyper-Rayleigh scat-

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to derive reasonably large static ﬁrst hyperpolarizabilities ꢀ₀of

ca. 50 × 10^-30esu for the salts [Ru^II(bpy)₃]Br₂(bpy ) 2,2′-

bipyridyl) and [Ru^II(phen)₃]Cl₂(phen ) 1,10-phenanthroline).⁷

An off-resonance ꢀ₀response is the quantity that is of most

relevance to quadratic NLO applications, because these normally

require the avoidance of any actual light absorption. Later HRS

studies on a [Ru^II(bpy)₃]²⁺derivative with six electron-donating

styryl substituents revealed a huge ꢀ₀value of 2200 × 10^-30

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9

J. AM. CHEM. SOC. VOL. 132, NO. 10, 2010 3497

A R T I C L E S

Coe et al.

nal,²¹and 4-[(E)-2-(4-pyridyl)vinyl]benzaldehyde²²were prepared

according to published procedures. All other reagents were obtained

commercially and used as supplied. Products were dried overnight

in a vacuum desiccator (CaSO₄) prior to characterization.

While [M^II(bpy)₃]²⁺(M ) Fe, Ru, Zn, etc.) complex units

themselves have octupolar ground states, they form dipolar ICT

excited states. Polarized HRS and Stark spectroscopic measure-

ments have shown that the ꢀ responses of [Ru^II(bpy)₃]²⁺

derivatives hexasubstituted with electron-donating styryl groups

are dominated by multiple degenerate dipolar intraligand charge-

transfer (ILCT) excitations, rather than an octupolar transition.¹⁴

These ILCT transitions are red-shifted by metal coordination

and are directionally opposed to the metal-to-ligand charge-

transfer (MLCT) excitations.^10,14Since all previous NLO studies

with this class of tris-chelate complexes have involved electron-

donor-substituted species,^2n,10–14we recently prepared a series

of [M^II(bpy)₃]²⁺(M ) Fe, Ru) derivatives bearing electron-

withdrawing pyridinium groups and carried out quadratic¹⁵and

cubic¹⁶NLO investigations. In these complexes, the MLCT

transitions are expected to dominate the NLO responses because

the ILCT processes, despite also being directed toward the

pyridinium units, occur at very short wavelengths (below about

330 nm). The present report concerns extended relatives of our

previously reported chromophores, with the aim being to achieve

greatly enhanced quadratic and cubic NLO properties. The

contributions of the ILCT transitions to the ꢀ responses become

more signiﬁcant in these new complexes and are even amenable

to assessment by Stark spectroscopy in two cases.

General Physical Measurements. ¹H NMR spectra were recorded

on a Bruker 400 MHz spectrometer, and all shifts are quoted with

respect to TMS. The ﬁne splitting of pyridyl or phenyl ring AA′BB′

patterns is ignored, and the signals are reported as simple doublets,

with J values referring to the two most intense peaks. Elemental

analyses were performed by the Microanalytical Laboratory,

University of Manchester, using a Carlo Erba EA1108 instrument.

UV-vis spectra were obtained by using a Shimadzu UV-2401 PC

spectrophotometer, and mass spectra were recorded by using

electron impact on a Micromass Trio 2000 or positive electrospray

on a Micromass Platform II spectrometer. Luminescence spectra

were recorded on a Gilden photonics ﬂuoroSENS ﬂuorimeter.

Cyclic voltammetric measurements were performed by using an

EG&G PAR Model 283 potentiostat/galvanostat. A single-compart-

ment cell was used with a silver/silver chloride reference electrode

(3 M NaCl, saturated AgCl) separated by a salt bridge from a 2

mm glassy-carbon-disk working electrode and Pt-wire auxiliary

electrode. Acetonitrile was freshly distilled (from CaH₂), and

[N(C₄H₉-n)₄]PF₆, as supplied from Fluka, was used as the supporting

electrolyte. Solutions containing ca. 10^-3M analyte (0.1 M

electrolyte) were deaerated by purging with N₂. All E_1/2values were

calculated from (E_pa+ E_pc)/2 at a scan rate of 200 mV s^-1

.

Synthesis of 4,4′-Bis[(E)-2-(4-pyridyl)vinyl]-2,2′-bipyridyl, bbpe.

Potassium tert-butoxide (624 mg, 5.56 mmol) was added to a stirred

solution of 4,4′-bis-[(diethoxyphosphinyl)methyl]-2,2′-bipyridyl

(1.00 g, 2.19 mmol) and 4-pyridinecarboxaldehyde (528 mg, 4.93

mmol) in tetrahydrofuran (40 mL). The reaction vessel was sealed

and protected from the light, and the contents were stirred at room

temperature for 2 h. Distilled water (60 mL) was added, and the

reaction mixture was stirred for a further 2 min. The cream-colored

solid was ﬁltered off, washed with copious amounts of water

followed by diethyl ether, and dried: yield 681 mg, 84%. ¹H NMR

δ_H(CDCl₃): 8.72 (2 H, d, J ) 5.1 Hz, C₅H₃N), 8.64 (4 H, d, J )

6.1 Hz, C₅H₄N), 8.60 (2 H, s, C₅H₃N), 7.45-7.41 (6 H, C₅H₃N +

C₅H₄N), 7.39 (2 H, d, J ) 16.5 Hz, CH), 7.32 (2 H, d, J ) 16.4

Hz, CH). Anal. Calcd for C₂₄H₁₈N₄·0.5H₂O: C, 77.61; H, 5.16; N,

15.08. Found: C, 77.80; H, 4.89; N, 15.05. EI-MS: m/z 363

([MH]⁺).

Experimental Section

Materials and Procedures. The compounds cis-Ru^IICl₂-

(DMSO)₄,¹⁷4,4′-bis[(diethoxyphosphinyl)methyl]-2,2′-bipyridyl,¹⁸

2,2′-bipyridyl-4,4′-dicarboxaldehyde,¹⁹N-phenyl-4-picolinium chlo-

ride hydrate ([Phpic⁺]Cl·1.25H₂O),²⁰(E)-3-(4-pyridyl)-2-prope-

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126, 034312-1–034312-13.

Synthesis of 4,4′-Bis[(E)-2-(N-methyl-4-pyridyl)vinyl]-2,2′-bipy-

ridyl Iodide, [Me₂bbpe²⁺]I₂. A mixture of bbpe ·0.5H₂O (303 mg,

0.816 mmol) and iodomethane (10 mL) in acetone (40 mL) was

heated at reﬂux in the dark for 4 h. The orange precipitate was

ﬁltered off, washed with a small amount of acetone followed by

copious amounts of CH₂Cl₂and then diethyl ether, and dried: yield

1

(12) (a) Girardot, C.; Lemercier, G.; Mulatier, J.-C.; Chauvin, J.; Baldeck,

P. L.; Andraud, C. Dalton Trans. 2007, 3421–3426. (b) Feuvrie, C.;

Maury, O.; Le Bozec, H.; Ledoux, I.; Morrall, J. P.; Dalton, G. T.;

Samoc, M.; Humphrey, M. G. J. Phys. Chem. A 2007, 111, 8980–

8985. (c) Boca, S. C.; Four, M.; Bonne, A.; van der Sanden, B.;

Astilean, S.; Baldeck, P. L.; Lemercier, G. Chem. Commun. 2009,

4590–4592.

454 mg, 84%. H NMR δ_H(CD₃SOCD₃): 8.98 (4 H, d, J ) 6.8

Hz, C₅H₄N), 8.86 (2 H, d, J ) 5.2 Hz, C₅H₃N), 8.74 (2 H, s,

C₅H₃N), 8.37 (4 H, d, J ) 6.8 Hz, C₅H₄N), 8.16 (2 H, d, J ) 16.4

Hz, CH), 7.91 (2 H, d, J ) 16.4 Hz, CH), 7.81 (2 H, dd, J ) 5.0,

1.4 Hz, C₅H₃N), 4.30 (6 H, s, Me). Anal. Calcd for

C₂₆H₂₄I₂N₄·0.7H₂O: C, 47.39; H, 3.89; N, 8.50. Found: C, 47.44;

H, 3.61; N, 8.45. ES-MS: m/z 519 ([M - I]⁺).

(13) (a) Liu, X.-J.; Feng, J.-K.; Ren, A.-M.; Cheng, H.; Zhou, X. J. Chem.

Phys. 2004, 120, 11493–11499. (b) Zhang, X.-B.; Feng, J.-K.; Ren,

A.-M. J. Phys. Chem. A 2007, 111, 1328–1338.

Synthesis of 4,4′-Bis[(E)-2-(N-methyl-4-pyridyl)vinyl]-2,2′-bipyr-

idyl Hexaﬂuorophosphate, [Me₂bbpe²⁺][PF₆]₂. [Me₂bbpe²⁺]I₂·

0.7H₂O (446 mg, 0.677 mmol) was dissolved in 2/1 methanol/water.

Slow addition of aqueous NH₄PF₆(0.6 M) followed by water (50

mL) gave a white precipitate, which was ﬁltered off, washed with

copious amounts of water and then diethyl ether, and dried: yield

(14) Vance, F. W.; Hupp, J. T. J. Am. Chem. Soc. 1999, 121, 4047–4053.

(15) Coe, B. J.; Harris, J. A.; Brunschwig, B. S.; Asselberghs, I.; Clays,

K.; Gar´ın, J.; Orduna, J. J. Am. Chem. Soc. 2005, 127, 13399–13410.

(16) Coe, B. J.; Samoc, M.; Samoc, A.; Zhu, L.-Y.; Yi, Y.-P.; Shuai, Z.-

G. J. Phys. Chem. A 2007, 111, 472–478.

(17) Evans, I. P.; Spencer, A.; Wilkinson, G. J. Chem. Soc., Dalton Trans.

1973, 204–209.

(18) Gillaizeau-Gauthier, I.; Odobel, F.; Alebbi, M.; Argazzi, R.; Costa,

E.; Bignozzi, C. A.; Qu, P.; Meyer, G. J. Inorg. Chem. 2001, 40, 6073–

6079.

(19) Maury, O.; Gue´gan, J.-P.; Renouard, T.; Hilton, A.; Dupau, P.; Sandon,

N.; Toupet, L.; Le Bozec, H. New J. Chem. 2001, 25, 1553–1566.

(20) Coe, B. J.; Harris, J. A.; Asselberghs, I.; Clays, K.; Olbrechts, G.;

Persoons, A.; Hupp, J. T.; Johnson, R. C.; Coles, S. J.; Hursthouse,

M. B.; Nakatani, K. AdV. Funct. Mater. 2002, 12, 110–116.

1

433 mg, 92%. H NMR δ_H(CD₃SOCD₃): 8.97 (4 H, d, J ) 6.6

Hz, C₅H₄N), 8.86 (2 H, d, J ) 5.0 Hz, C₅H₃N), 8.75 (2 H, s,

(21) Coe, B. J.; Jones, L. A.; Harris, J. A.; Brunschwig, B. S.; Asselberghs,

I.; Clays, K.; Persoons, A.; Gar´ın, J.; Orduna, J. J. Am. Chem. Soc.

2004, 126, 3880–3891.

(22) Ichimura, K.; Watanabe, S. J. Polym. Sci., Polym. Chem. 1982, 20,

1419–1432.

9

3498 J. AM. CHEM. SOC. VOL. 132, NO. 10, 2010

Tris-Chelate Metallochromophores

A R T I C L E S

C₅H₃N), 8.35 (4 H, d, J ) 6.5 Hz, C₅H₄N), 8.14 (2 H, d, J ) 16.4

Hz, CH), 7.90 (2 H, d, J ) 16.5 Hz, CH), 7.80 (2 H, dd, J ) 5.2,

1.4 Hz, C₅H₃N), 4.30 (6 H, s, Me). Anal. Calcd for

C₂₆H₂₄F₁₂N₄P₂·0.8H₂O: C, 44.81; H, 3.70; N, 8.04. Found: C, 44.79;

H, 3.44; N, 8.10. ES-MS: m/z 537 ([M - PF₆]⁺).

2.41 mmol) instead of 4-pyridinecarboxaldehyde. Distilled water

(60 mL) was added and a cream-colored solid obtained: yield 325

mg, 70%. ¹H NMR δ_H(CDCl₃): 8.67 (2 H, d, J ) 5.5 Hz, C₅H₃N),

8.59 (4 H, d, J ) 6.1 Hz, C₅H₄N), 8.49 (2 H, s, C₅H₃N), 7.36-7.25

(8 H, C₅H₃N + C₅H₄N + CH), 7.16 (2 H, dd, J ) 15.2, 10.8 Hz,

CH), 6.80 (2 H, d, J ) 15.4 Hz, CH), 6.72 (2 H, d, J ) 15.4 Hz,

CH). Anal. Calcd for C₂₈H₂₂N₄·0.5H₂O: C, 79.41; H, 5.47; N,

13.23. Found: C, 79.43; H, 5.57; N, 12.79. EI-MS: m/z 415 (M⁺).

Synthesis of 4,4′-Bis[(E,E)-4-(N-methyl-4-pyridyl)buta-1,

3-dienyl]-2,2′-bipyridyl Iodide, [Me₂bbpb²⁺]I₂. This compound was

prepared in a manner similar to that for [Me₂bbpe²⁺]I₂by using

bbpb ·0.5H₂O (200 mg, 0.472 mmol) instead of bbpe ·0.5H₂O and

acetone (50 mL). Further puriﬁcation was effected by precipitation

from 1/1 DMF/diethyl ether to give an orange solid: yield 224 mg,

65%. ¹H NMR δ_H(CD₃SOCD₃): 8.87 (4 H, d, J ) 6.9 Hz, C₅H₄N),

8.75 (2 H, d, J ) 5.1 Hz, C₅H₃N), 8.54 (2 H, s, C₅H₃N), 8.21 (4 H,

d, J ) 6.9 Hz, C₅H₄N), 7.86 (2 H, dd, J ) 15.4, 10.8 Hz, CH),

7.72 (2 H, dd, J ) 5.2, 1.6 Hz, C₅H₃N), 7.62 (2 H, dd, J ) 15.5,

10.8 Hz, CH), 7.19 (2 H, d, J ) 15.5 Hz, CH), 7.12 (2 H, d, J )

15.5 Hz, CH), 4.26 (6 H, s, Me). Anal. Calcd for

C₃₀H₂₈I₂N₄·1.9H₂O: C, 49.18; H, 4.38; N, 7.65. Found: C, 49.17;

H, 4.06; N, 7.56. ES-MS: m/z 571 ([M - I]⁺).

Synthesis of 4,4′-Bis[(E,E)-4-(N-methyl-4-pyridyl)buta-1,3-di-

enyl]-2,2′-bipyridyl Hexaﬂuorophosphate, [Me₂bbpb²⁺][PF₆]₂. This

compound was prepared in a manner similar to that for

[Me₂bbpe²⁺][PF₆]₂by using [Me₂bbpb²⁺]I₂·1.9H₂O (216 mg, 0.295

mmol) instead of [Me₂bbpe²⁺]I₂·0.7H₂O in DMF (30 mL) to afford

a bright yellow solid: yield 193 mg, 85%. ¹H NMR δ_H

(CD₃SOCD₃): 8.86 (4 H, d, J ) 6.9 Hz, C₅H₄N), 8.75 (2 H, d, J )

5.1 Hz, C₅H₃N), 8.55 (2 H, s, C₅H₃N), 8.20 (4 H, d, J ) 7.0 Hz,

C₅H₄N), 7.85 (2 H, dd, J ) 15.4, 10.7 Hz, CH), 7.71 (2 H, dd, J

) 5.4, 1.6 Hz, C₅H₃N), 7.62 (2 H, dd, J ) 15.5, 10.7 Hz, CH),

7.18 (2 H, d, J ) 15.5 Hz, CH), 7.11 (2 H, d, J ) 15.5 Hz, CH),

4.26 (6 H, s, Me). Anal. Calcd for C₃₀H₂₈F₁₂N₄P₂·1.8H₂O: C, 46.98;

H, 4.15; N, 7.31. Found: C, 47.00; H, 3.83; N, 7.28. ES-MS: m/z

589 ([M - PF₆]⁺).

Synthesis of 4,4′-Bis{(E)-2-[N-(2,4-dinitrophenyl)-4-pyridyl]

vinyl}-2,2′-bipyridyl Chloride, [(2,4-DNPh)₂bbpe²⁺]Cl₂. A mixture

of bbpe ·0.5H₂O (300 mg, 0.808 mmol) and 2,4-dinitrochloroben-

zene (1.81 g, 8.94 mmol) in ethanol (40 mL) was heated at reﬂux

for 72 h, producing a beige precipitate. The precipitate was ﬁltered

off, washed with a small amount of ethanol and then dichlo-

1

romethane and diethyl ether, and dried: yield 546 mg, 81%. H

NMR δ_H(CD₃SOCD₃): 9.38 (4 H, d, J ) 6.9 Hz, C₅H₄N), 9.15 (2

H, d, J ) 2.6 Hz, C₆H₃), 9.00 (2 H, dd, J ) 8.7, 2.5 Hz, C₆H₃),

8.92 (2 H, d, J ) 5.0 Hz, C₅H₃N), 8.83 (2 H, s, C₅H₃N), 8.67 (4 H,

d, J ) 6.9 Hz, C₅H₄N), 8.47-8.41 (4 H, C₆H₃+ CH), 8.11 (2 H,

d, J ) 16.4 Hz, CH), 7.89 (2 H, dd, J ) 5.0, 1.3 Hz, C₅H₃N).

Anal. Calcd for C₃₆H₂₄Cl₂N₈O₈·3.8H₂O: C, 51.72; H, 3.81; N,

13.40. Found: C, 52.14; H, 3.62; N, 12.90. ES-MS: m/z 731 ([M

- Cl]⁺), 348 ([M - 2Cl]²⁺).

Synthesis of 4,4′-Bis{(E)-2-[N-(2,4-dinitrophenyl)-4-pyridyl]

vinyl}-2,2′-bipyridyl Hexaﬂuorophosphate, [(2,4-DNPh)₂bbpe²⁺]

[PF₆]₂. [(2,4-DNPh)₂bbpe²⁺]Cl₂·3.8H₂O (536 mg, 0.641 mmol) was

dissolved in a 1/1 methanol/water mixture. Slow addition of aqueous

NH₄PF₆(0.6 M) gave a beige precipitate, which was ﬁltered off,

1

washed with water, and dried: yield 570 mg, 89%. H NMR δ_H

(CD₃SOCD₃): 9.34 (4 H, d, J ) 6.7 Hz, C₅H₄N), 9.15 (2 H, d, J )

2.5 Hz, C₆H₃), 9.00 (2 H, dd, J ) 8.7, 2.4 Hz, C₆H₃), 8.92 (2 H, d,

J ) 5.0 Hz, C₅H₃N), 8.83 (2 H, s, C₅H₃N), 8.63 (4 H, d, J ) 6.6

Hz, C₅H₄N), 8.43 (2 H, d, J ) 8.7 Hz, C₆H₃), 8.38 (2 H, d, J )

16.6 Hz, CH), 8.07 (2 H, d, J ) 16.4 Hz, CH), 7.87 (2 H, d, J )

5.3 Hz, C₅H₃N). Anal. Calcd for C₃₆H₂₄F₁₂N₈O₈P₂·H₂O: C, 43.04;

H, 2.61; N, 11.15. Found: C, 43.24; H, 2.31; N, 10.96. ES-MS:

m/z 841 ([M - PF₆]⁺), 348 ([M - 2PF₆]²⁺).

Synthesis of 4,4′-Bis[(E)-2-(N-phenyl-4-pyridyl)vinyl]-2,2′-bipyr-

idyl Chloride, [Ph₂bbpe²⁺]Cl₂. A mixture of 2,2′-bipyridyl-4,4′-

dicarboxaldehyde (350 mg, 1.65 mmol) and [Phpic⁺]Cl·1.25H₂O

(746 mg, 3.27 mmol) in acetic anhydride (20 mL) was heated at

reﬂux in the dark for 1.5 h. The resulting beige precipitate was

ﬁltered off, washed with a small amount of acetic anhydride

followed by acetone and then diethyl ether, and dried. Further

puriﬁcation was effected by reprecipitation from DMF/diethyl ether:

Synthesis of 4,4′-Bis((E,E)-2-{4-[2-(4-pyridyl)vinyl]phenyl}

vinyl)-2,2′-bipyridyl, bbpvb. This compound was prepared in a

manner similar to that for bbpb by using 4-[(E)-2-(4-pyridyl)vi-

nyl]benzaldehyde (515 mg, 2.45 mmol) instead of (E)-3-(4-pyridyl)-

2-propenal in tetrahydrofuran (30 mL). Distilled water (40 mL)

was added and a cream-colored solid obtained after an additional

wash with ethyl acetate: yield 325 mg, 51%. ¹H NMR δ_H

(CF₃CO₂D): 9.58 (2 H, d, J ) 6.4 Hz, C₅H₃N), 9.31 (2 H, s, C₅H₃N),

9.27 (4 H, d, J ) 6.7 Hz, C₅H₄N), 8.99 (2 H, d, J ) 6.4 Hz, C₅H₃N),

8.78 (4 H, d, J ) 6.8 Hz, C₅H₄N), 8.68 (2 H, d, J ) 16.2 Hz, CH),

8.52-8.45 (10 H, C₆H₄+ CH), 8.17 (2 H, d, J ) 16.1 Hz, CH),

8.07 (2 H, d, J ) 16.2 Hz, CH). Anal. Calcd for C₄₀H₃₀N₄·0.5H₂O:

C, 83.45; H, 5.43; N, 9.73. Found: C, 83.47; H, 5.20; N, 9.63. EI-

MS: m/z 567 (M⁺).

1

yield 540 mg, 49%. H NMR δ_H(CD₃SOCD₃): 9.37 (4 H, d, J )

6.4 Hz, C₅H₄N), 8.90 (2 H, d, J ) 5.0 Hz, C₅H₃N), 8.80 (2 H, s,

C₅H₃N), 8.56 (4 H, d, J ) 6.5 Hz, C₅H₄N), 8.37 (2 H, d, J ) 16.2

Hz, CH), 8.08 (2 H, d, J ) 16.4 Hz, CH), 7.95-7.92 (4 H, m, Ph),

7.87 (2 H, d, J ) 5.8 Hz, C₅H₃N), 7.80-7.72 (6 H, Ph). Anal.

Calcd for C₃₆H₂₈Cl₂N₄·4.5H₂O: C, 64.67; H, 5.58; N, 8.38. Found:

C, 64.71; H, 5.46; N, 8.49. ES-MS: m/z 551 ([M - Cl]⁺), 258 ([M

- 2Cl]²⁺).

Synthesisof4,4′-Bis((E,E)-2-{4-[2-(N-methyl-4-pyridyl)vinyl]phenyl}-

vinyl)-2,2′-bipyridyl Iodide, [Me₂bbpvb²⁺]I₂. This compound was

prepared in a manner similar to that for [Me₂bbpb²⁺]I₂by using

bbpvb ·0.5H₂O (200 mg, 0.347 mmol) instead of bbpb·0.5H₂O to

afford an orange solid: yield 447 mg, 99%. ¹H NMR δ_H

(CD₃SOCD₃): 8.87 (4 H, d, J ) 6.9 Hz, C₅H₄N), 8.73 (2 H, d, J )

5.1 Hz, C₅H₃N), 8.61 (2 H, s, C₅H₃N), 8.23 (4 H, d, J ) 6.9 Hz,

C₅H₄N), 8.04 (2 H, d, J ) 16.2 Hz, CH), 7.89-7.81 (8 H, m, C₆H₄),

7.73-7.68 (4 H, C₅H₃N + CH), 7.61-7.54 (4 H, CH), 4.26 (6 H,

s, Me). Anal. Calcd for C₄₂H₃₆I₂N₄·2.5H₂O: C, 56.33; H, 4.61; N,

6.26. Found: C, 56.31; H, 4.29; N, 6.17. ES-MS: m/z 723 ([M -

I]⁺).

Synthesis of 4,4′-Bis((E,E)-2-{4-[2-(N-methyl-4-pyridyl)vinyl]

phenyl}vinyl)-2,2′-bipyridyl Hexaﬂuorophosphate, [Me₂bbpvb²⁺]

[PF₆]₂. This compound was prepared in a manner similar to that

for [Me₂bbpe²⁺][PF₆]₂by using [Me₂bbpvb²⁺]I₂·2.5H₂O (435 mg,

0.486 mmol) instead of [Me₂bbpe²⁺]I₂·0.7H₂O in DMF (50 mL).

Further puriﬁcation was effected by precipitation from 1/1 DMF/

diethyl ether to afford a bright yellow solid: yield 342 mg, 75%.

Synthesis of 4,4′-Bis[(E)-2-(N-phenyl-4-pyridyl)vinyl]-2,2′-bipyr-

idyl Hexaﬂuorophosphate, [Ph₂bbpe²⁺][PF₆]₂. [Ph₂bbpe²⁺]Cl₂·

4.5H₂O (540 mg, 0.808 mmol) was dissolved in methanol. Slow

addition of aqueous NH₄PF₆(0.6 M) gave a beige precipitate, which

was ﬁltered off, washed with water and then diethyl ether, and dried.

Further puriﬁcation was effected by reprecipitation from DMF/

diethyl ether: yield 612 mg, 93%. ¹H NMR δ_H(CD₃SOCD₃): 9.35

(4 H, d, J ) 6.6 Hz, C₅H₄N), 8.90 (2 H, d, J ) 4.9 Hz, C₅H₃N),

8.81 (2 H, s, C₅H₃N), 8.53 (4 H, d, J ) 6.6 Hz, C₅H₄N), 8.34 (2 H,

d, J ) 16.7 Hz, CH), 8.05 (2 H, d, J ) 16.5 Hz, CH), 7.93-7.91

(4 H, m, Ph), 7.85 (2 H, d, J ) 4.4 Hz, C₅H₃N), 7.80-7.74 (6 H,

Ph). Anal. Calcd for C₃₆H₂₈F₁₂N₄P₂·0.6H₂O: C, 52.90; H, 3.60; N,

6.85. Found: C, 52.90; H, 3.16; N, 6.85. ES-MS: m/z 661 ([M -

PF₆]⁺).

Synthesis of 4,4′-Bis[(E,E)-4-(4-pyridyl)buta-1,3-dienyl]-2,

2′-bipyridyl, bbpb. This compound was prepared in a manner

similar to that for bbpe by using potassium tert-butoxide (312 mg,

2.78 mmol), 4,4′-bis[(diethoxyphosphinyl)methyl]-2,2′-bipyridyl

(500 mg, 1.10 mmol), and (E)-3-(4-pyridyl)-2-propenal (321 mg,

9

J. AM. CHEM. SOC. VOL. 132, NO. 10, 2010 3499

A R T I C L E S

Coe et al.

¹H NMR δ_H(CD₃SOCD₃): 8.86 (4 H, d, J ) 6.7 Hz, C₅H₄N), 8.73

(2 H, d, J ) 5.1 Hz, C₅H₃N), 8.61 (2 H, s, C₅H₃N), 8.22 (4 H, d,

J ) 6.8 Hz, C₅H₄N), 8.02 (2 H, d, J ) 16.3 Hz, CH), 7.88-7.81

(8 H, m, C₆H₄), 7.72-7.68 (4 H, C₅H₃N + CH), 7.60-7.54 (4 H,

CH), 4.26 (6 H, s, Me). Anal. Calcd for C₄₂H₃₆F₁₂N₄P₂·2.8H₂O:

C, 53.83; H, 4.47; N, 5.98. Found: C, 53.80; H, 3.94; N, 5.97. ES-

MS: m/z 741 ([M - PF₆]⁺).

Synthesis of [Fe^II(Me₂bbpe²⁺)₃][PF₆]₈, 8. Fe^II(BF₄)₂·6H₂O (20

mg, 0.059 mmol) was added to a solution of [Me₂bbpe²⁺]-

[PF₆]₂·0.8H₂O (129 mg, 0.185 mmol) in DMF (10 mL), and the

deep blue solution was stirred at room temperature for 2 h in the

dark. Addition of aqueous NH₄PF₆afforded a dark blue precipitate,

which was ﬁltered off, washed with water, and dried. Puriﬁcation

was effected as for 2 to afford a dark blue solid: yield 90 mg, 62%.

¹H NMR δ_H(CD₃CN): 8.90 (6 H, d, J ) 1.4 Hz, C₅H₃N), 8.59 (12

H, d, J ) 6.9 Hz, C₅H₄N), 8.14 (12 H, d, J ) 7.0 Hz, C₅H₄N),

7.89 (6 H, d, J ) 16.5 Hz, CH), 7.83 (6 H, d, J ) 16.5 Hz, CH),

7.64 (6 H, dd, J ) 6.1, 1.8 Hz, C₅H₃N), 7.57 (6 H, d, J ) 6.0 Hz,

C₅H₃N), 4.29 (18 H, s, Me). Anal. Calcd for

C₇₈H₇₂F₄₈FeN₁₂P₈·3.5H₂O: C, 38.14; H, 3.24; N, 6.84. Found: C,

38.13; H, 2.89; N, 6.80.

Synthesis of [Ru^II(Me₂bbpe²⁺)₃][PF₆]₈, 2. A mixture of cis-

Ru^IICl₂(DMSO)₄(33 mg, 0.068 mmol), silver(I) tosylate (38 mg,

0.136 mmol), and [Me₂bbpe²⁺][PF₆]₂·0.8H₂O (162 mg, 0.232

mmol) in degassed DMF (10 mL) was heated at reﬂux for 4 h under

Ar in the dark. The resulting red mixture was cooled to room

temperature, and the AgCl precipitate was ﬁltered off. Addition of

aqueous NH₄PF₆to the ﬁltrate produced a red precipitate, which

was ﬁltered off, washed with water, and dried. The crude product

was dissolved in acetonitrile solution, loaded onto a silica gel

column (230-400 mesh, 5 × 22 cm), and eluted with 0.1 M

NH₄PF₆in acetonitrile, giving a major dark red band. Fractions

were collected and compared by TLC. Similar fractions were

combined, reduced to dryness, and precipitated from acetone/

aqueous NH₄PF₆to afford a dark red-brown solid: 83 mg, 49%.

¹H NMR δ_H(CD₃CN): 8.88 (6 H, d, J ) 1.3 Hz, C₅H₃N), 8.59 (12

H, d, J ) 6.8 Hz, C₅H₄N), 8.13 (12 H, d, J ) 6.9 Hz, C₅H₄N),

7.90-7.79 (18 H, CH + C₅H₃N), 7.65 (6 H, dd, J ) 6.0, 1.7 Hz,

C₅H₃N), 4.27 (18 H, s, Me). Anal. Calcd for

C₇₈H₇₂F₄₈N₁₂P₈Ru·3.5H₂O: C, 37.45; H, 3.18; N, 6.72. Found: C,

37.44; H, 2.91; N, 6.73.

Synthesis of [Fe^II(Me₂bbpe²⁺)₃][BPh₄]_7.3[PF₆]_0.7, 8B. This com-

pound was prepared in a manner similar to that for 2B by using 8

(47 mg, 19.1 µmol) instead of 2 to afford a dark blue solid: yield

1

63 mg, 90%. H NMR δ_H(CD₃CN): 8.28 (6 H, s, C₅H₃N), 8.25

(12 H, d, J ) 6.8 Hz, C₅H₄N), 7.68 (12 H, d, J ) 6.8 Hz, C₅H₄N),

7.36 (6 H, d, J ) 16.4 Hz, CH), 7.25-7.19 (70.4 H, C₅H₃N + CH

+ Ph), 6.97 (6 H, dd, J ) 6.2, 1.5 Hz, C₅H₃N), 6.91-6.88 (58.4

H, m, Ph), 6.80-6.76 (29.2 H, m, Ph), 4.13 (18 H, s, Me). Anal.

Calcd for C_253.2H₂₁₈B_7.3F_4.2FeN₁₂P_0.7: C, 82.97; H, 6.00; N, 4.59.

Found: C, 82.89; H, 6.02; N, 4.47.

Synthesis of [Fe^II(Ph₂bbpe²⁺)₃][PF₆]₈, 10. This compound was

prepared and puriﬁed in a manner similar to that for 8 by using

[Ph₂bbpe²⁺][PF₆]₂·0.6H₂O (153 mg, 0.187 mmol) instead of

[Me₂bbpe²⁺][PF₆]₂·0.8H₂O to afford a dark blue solid: yield 121

mg, 73%. ¹H NMR δ_H(CD₃CN): 9.01 (6 H, d, J ) 1.4 Hz, C₅H₃N),

8.92 (12 H, d, J ) 7.1 Hz, C₅H₄N), 8.34 (12 H, d, J ) 7.1 Hz,

C₅H₄N), 8.05 (6 H, d, J ) 16.5 Hz, CH), 7.98 (6 H, d, J ) 16.5

Hz, CH), 7.77-7.71 (36 H, C₅H₃N + Ph), 7.65 (6 H, d, J ) 6.0

Hz, C₅H₃N). Anal. Calcd for C₁₀₈H₈₄F₄₈FeN₁₂P₈·2H₂O: C, 46.30;

H, 3.17; N, 6.00. Found: C, 46.27; H, 2.91; N, 5.98.

Synthesis of [Ru^II(Me₂bbpe²⁺)₃][BPh₄]₇PF₆, 2B. A portion of 2

(19 mg, 7.60 µmol) in acetone (2 mL) was treated with aqueous

NaBPh₄, and the dark red precipitate was ﬁltered off, washed with

1

water, and dried: yield 22 mg, 79%. H NMR δ_H(CD₃SOCD₃):

9.23 (6 H, s, C₅H₃N), 8.89 (12 H, d, J ) 6.4 Hz, C₅H₄N), 8.16 (12

H, d, J ) 6.0 Hz, C₅H₄N), 7.99 (6 H, d, J ) 16.4 Hz, CH), 7.93

(6 H, d, J ) 16.4 Hz, CH), 7.86 (6 H, d, J ) 6.0 Hz, C₅H₃N), 7.51

(6 H, d, J ) 6.0, C₅H₃N), 7.24-7.10 (56 H, m, Ph), 6.95-6.84

(56 H, m, Ph), 6.80-6.70 (28 H, m, Ph), 4.27 (18 H, s, Me). Anal.

Calcd for C₂₄₆H₂₁₂B₇F₆N₁₂PRu: C, 80.77; H, 5.84; N, 4.59. Found:

C, 80.70; H, 5.76; N, 4.29.

Synthesis of [Fe^II(Me₂bbpb²⁺)₃][PF₆]₈, 11. This compound was

prepared and puriﬁed in a manner similar to that for 8 by using

[Me₂bbpb²⁺][PF₆]₂·1.8H₂O (131 mg, 0.171 mmol) instead of

[Me₂bbpe²⁺][PF₆]₂·0.8H₂O to afford a dark blue solid: yield 85

mg, 57%. ¹H NMR δ_H(CD₃CN): 8.72 (6 H, d, J ) 1.2 Hz, C₅H₃N),

8.48 (12 H, d, J ) 6.8 Hz, C₅H₄N), 8.00 (12 H, d, J ) 6.8 Hz,

C₅H₄N), 7.71-7.61 (12 H, CH), 7.52 (6 H, dd, J ) 6.0, 1.5 Hz,

C₅H₃N), 7.43 (6 H, d, J ) 6.1 Hz, C₅H₃N), 7.20-7.13 (6 H, m,

CH), 7.09-7.02 (6 H, m, CH), 4.22 (18 H, s, Me). Anal. Calcd for

C₉₀H₈₄F₄₈FeN₁₂P₈·4.2H₂O: C, 41.18; H, 3.55; N, 6.40. Found: C,

41.17; H, 3.24; N, 6.26.

Synthesis of [Ru^II(Ph₂bbpe²⁺)₃][PF₆]₈, 4. This compound was

prepared and puriﬁed in a manner similar to that for 2 by using

[Ph₂bbpe²⁺][PF₆]₂·0.6H₂O (192 mg, 0.235 mmol) instead of

[Me₂bbpe²⁺][PF₆]₂·0.8H₂O to afford a dark orange-red solid: yield

1

124 mg, 63%. H NMR δ_H(CD₃CN): 9.00 (6 H, d, J ) 1.2 Hz,

C₅H₃N), 8.92 (12 H, d, J ) 7.1 Hz, C₅H₄N), 8.34 (12 H, d, J ) 7.1

Hz, C₅H₄N), 8.10-7.90 (18 H, CH + C₅H₃N), 7.77-7.72 (36 H,

C₅H₃N + Ph). Anal. Calcd for C₁₀₈H₈₄F₄₈N₁₂P₈Ru·4.3H₂O: C,

44.91; H, 3.23; N, 5.82. Found: C, 44.91; H, 2.94; N, 5.74.

Synthesis of [Ru^II(Me₂bbpb²⁺)₃][PF₆]₈, 5. This compound was

prepared and puriﬁed in a manner similar to that for 2 by using

[Me₂bbpb²⁺][PF₆]₂·1.8H₂O (175 mg, 0.228 mmol) instead of

[Me₂bbpe²⁺][PF₆]₂·0.8H₂O to afford a dark orange-red solid: yield

Synthesis of [Fe^II{(2,4-DNPh)₂bbpe²⁺}₃][PF₆]₈, 12. This com-

pound was prepared in a manner similar to that for 8 by using [(2,4-

DNPh)₂bbpe²⁺][PF₆]₂·H₂O (175 mg, 0.174 mmol) instead of

[Me₂bbpe²⁺][PF₆]₂·0.8H₂O. Puriﬁcation was achieved by careful

reprecipitation from acetone/ethanol to afford a dark blue solid:

1

yield 185 mg, 94%. H NMR δ_H(CD₃CN): 9.17 (6 H, d, J ) 2.5

1

92 mg, 51%. H NMR δ_H(CD₃CN): 8.70 (6 H, d, J ) 1.4 Hz,

Hz, C₆H₃), 9.03 (6 H, d, J ) 1.5 Hz, C₅H₃N), 8.85-8.82 (18 H,

C₆H₃+ C₅H₄N), 8.41 (12 H, d, J ) 7.0 Hz, C₅H₄N), 8.14-8.00

(12 H, C₆H₃+ CH), 8.01 (6 H, d, J ) 16.4 Hz, CH), 7.75 (6 H,

dd, J ) 5.8, 1.4 Hz, C₅H₃N), 7.69 (6 H, d, J ) 6.0 Hz, C₅H₃N).

Anal. Calcd for C₁₀₈H₇₂F₄₈FeN₂₄O₂₄P₈·4.2H₂O: C, 38.37; H, 2.40;

N, 9.94. Found: C, 38.35; H, 1.97; N, 9.88.

C₅H₃N), 8.48 (12 H, d, J ) 6.9 Hz, C₅H₄N), 7.99 (12 H, d, J ) 6.9

Hz, C₅H₄N), 7.74 (6 H, d, J ) 6.1 Hz, C₅H₃N), 7.70-7.59 (12 H,

CH), 7.54 (6 H, dd, J ) 6.1, 1.6 Hz, C₅H₃N), 7.16 (6 H, d, J )

14.8 Hz, CH), 7.05 (6 H, d, J ) 14.8 Hz, CH), 4.22 (18 H, s, Me).

Anal. Calcd for C₉₀H₈₄F₄₈N₁₂P₈Ru·4.3H₂O: C, 40.46; H, 3.49; N,

6.29. Found: C, 40.46; H, 3.01; N, 6.24.

Synthesis of [Fe^II{(2,4-DNPh)₂bbpe²⁺}₃][BPh₄]_6.55[PF₆]_1.45, 12B.

This compound was prepared in a manner similar to that for 2B by

using 12 (31 mg, 9.17 µmol) instead of 2 to afford a dark blue

solid: yield 25 mg, 61%. ¹H NMR δ_H(CD₃CN): 9.15 (6 H, d, J )

2.4 Hz, C₆H₃), 8.76 (6 H, dd, J ) 8.8, 2.4 Hz, C₆H₃), 8.37-8.32

(18 H, C₆H₃+ C₅H₄N), 7.87-7.82 (18 H, C₅H₄N + C₅H₃N), 7.52

(6 H, d, J ) 16.4 Hz, CH), 7.36-7.21 (64.4 H, C₆H₃N + CH +

Ph), 7.05 (6 H, dd, J ) 5.8, 1.6 Hz, C₅H₃N), 6.94-6.86

(52.4 H, Ph), 6.81-6.74 (26.2 H, Ph). Anal. Calcd for

C_265.2H₂₀₃B_6.55F_8.7FeN₂₄O₂₄P_1.45: C, 71.63; H, 4.60; N, 7.56. Found:

C, 71.63; H, 4.58; N, 7.05.

Synthesis of [Ru^II(Me₂bbpvb²⁺)₃][PF₆]₈, 6. This compound was

prepared and puriﬁed in a manner similar to that for 2 by using

cis-Ru^IICl₂(DMSO)₄(28 mg, 0.058 mmol), silver(I) tosylate (33

mg, 0.118 mmol), and [Me₂bbpvb²⁺][PF₆]₂·2.8H₂O (180 mg, 0.203

mmol) to afford a dark red solid: yield 134 mg, 75%. ¹H NMR δ_H

(CD₃CN): 8.80 (6 H, d, J ) 1.5 Hz, C₅H₃N), 8.47 (12 H, d, J )

7.0 Hz, C₅H₄N), 8.02 (12 H, d, J ) 7.0 Hz, C₅H₄N), 7.86-7.77

(42 H, C₅H₃N + C₆H₄+ CH), 7.57 (6 H, dd, J ) 6.1, 1.5 Hz,

C₅H₃N), 7.48-7.40 (12 H, CH), 4.21 (18 H, s, Me). Anal. Calcd

for C₁₂₆H₁₀₈F₄₈N₁₂P₈Ru·2.3H₂O: C, 48.94; H, 3.67; N, 5.44. Found:

C, 48.93; H, 3.56; N, 5.28.

9

3500 J. AM. CHEM. SOC. VOL. 132, NO. 10, 2010

Tris-Chelate Metallochromophores

A R T I C L E S

Table 1. Crystallographic Data and Reﬁnement Details for the Salt

[Ph₂bbpe²⁺][PF₆]₂·2DMF

switched Nd:YAG laser (Quanta-Ray PRO, 8 ns pulses, 7 mJ, 10

Hz). ꢀ values were determined by using the electric-ﬁeld-induced

second harmonic generation ꢀ_zzz,1064for p-nitroaniline (29 × 10^-30

esu in acetonitrile)³⁰as an external reference. The different nature

of the reference tensor component for p-nitroaniline has been taken

into account in deriving the octupolar ꢀ_xxxvalues for the compounds

studied. The apparatus and experimental procedures used for the

femtosecond HRS studies were exactly as described previously.³¹

These measurements were also carried out in acetonitrile, and the

reference compound was crystal violet (octupolar ꢀ_xxx,800) 327 ×

10^-30esu in acetonitrile; from the value of 338 × 10^-30esu in

methanol, corrected for local ﬁeld factors at optical frequencies).

For all measurements, dilute solutions (10^-5-10^-6M) were used

to ensure a linear dependence of I_2ω/I_ω²on concentration, precluding

the need for Lambert-Beer correction factors. The absence of

demodulation at 800 nm, i.e. constant values of ꢀ versus frequency,

conﬁrmed that no luminescence contributions to the HRS signals

were present at 400 nm, consistent with the observation of

luminescence well above this wavelength (for the Ru compounds)

or no luminescence at all (for the Fe compounds). The reported

formula

M

C₄₂H₄₂F₁₂N₆O₂P₂

952.76

cryst syst

space group

a, Å

b, Å

c, Å

monoclinic

P2₁/c

9.1252(13)

17.637(2)

13.683(2)

109.478(5)

2076.1(5)

2

ꢀ, deg

V, Å³

Z

T, K

100(2)

0.205

µ, mm^-1

cryst size, mm³

cryst appearance

no. of rﬂns collected

no. of indep rﬂns (R_int)

no. of rﬂns with I > 2σ(I)

goodness of ﬁt on F²

ﬁnal R1, wR2 (I > 2σ(I))^a

(all data)

0.30 × 0.25 × 0.20

pale brown prism

11 919

4694 (0.0315)

3259

1.123

0.0561, 0.1595

0.0802, 0.1736

0.853, -0.569

peak and hole, e Å^-3

ꢀ

_xxx,800values are the averages taken from measurements at different

amplitude modulation frequencies.

a

2

The structure was reﬁned on F_ousing all data; the values of R1 are

Stark Spectroscopy. The Stark apparatus, experimental methods,

and data collection procedure were as previously reported,³²except

that a Xe arc lamp was used as the light source in the place of a W

ﬁlament bulb. The Stark spectrum for each compound was measured

at least twice. To ﬁt the Stark data, the absorption (ε/ν vs ν)

spectrum was modeled with a sum of Gaussian curves that

reproduce the data and separate the peaks. The ﬁrst and second

derivatives of the Gaussian curves were then used to ﬁt the Stark

spectra with Liptay’s equation.³³The dipole moment change, ∆µ₁₂

) µ_e- µ_g, where µ_eand µ_gare the respective excited- and ground-

state dipole moments, associated with each of the optical transitions

considered in the ﬁt was then calculated from the coefﬁcient of

the second-derivative component. Butyronitrile was used as the

glassing medium, for which the local ﬁeld correction f_intis estimated

as 1.33.³²A two-state analysis of the ICT transitions gives

given for comparison with older reﬁnements based on F_owith a typical

threshold of F_o> 4σ(F_o).

Synthesis of [Fe^II(Me₂bbpvb²⁺)₃][PF₆]₈, 13. This compound was

prepared and puriﬁed in a manner similar to that for 8 by using

[Me₂bbpvb²⁺][PF₆]₂·2.8H₂O (131 mg, 0.140 mmol) instead of

[Me₂bbpe²⁺][PF₆]₂·0.8H₂O. A ﬁnal reprecipitation from acetonitrile/

1

diethyl ether afforded a dark blue solid: yield 107 mg, 75%. H

NMR δ_H(CD₃CN): 8.83 (6 H, d, J ) 1.4 Hz, C₅H₃N), 8.47 (12 H,

d, J ) 6.9 Hz, C₅H₄N), 8.02 (12 H, d, J ) 7.0 Hz, C₅H₄N),

7.86-7.81 (36 H, C₅H₃N + C₆H₄+ CH), 7.56 (6 H, dd, J ) 6.1,

1.5 Hz, C₅H₃N), 7.50-7.40 (18 H, CH), 4.21 (18 H, s, Me). Anal.

Calcd for C₁₂₆H₁₀₈F₄₈FeN₁₂P₈·3.4H₂O: C, 49.34; H, 3.77; N, 5.48.

Found: C, 49.32; H, 3.55; N, 5.52.

X-ray Crystallography. Crystals of the proligand salt

[Ph₂bbpe²⁺][PF₆]₂·2DMF were obtained by slow diffusion of

diethyl ether vapor into a DMF solution. Data were collected on

an Oxford Diffraction XCalibur 2 X-ray diffractometer and

processed (including absorption correction) using the Oxford

Diffraction CrysAlis RED software package,²³before ﬁnal ins and

hkl ﬁles were prepared using the SHELXTL program suite.²⁴The

structure was solved by direct methods using SIR-92²⁵via WinGX²⁶

2

∆µ_ab) ∆µ₁₂+ 4µ₁₂

(1)

where ∆µ_abis the dipole moment change between the diabatic states

and ∆µ₁₂is the observed (adiabatic) dipole moment change. The

value of the transition dipole moment µ₁₂can be determined from

the oscillator strength f_osof the transition by

2

and reﬁned by full-matrix least squares on all F_ovalues using

SHELXL-97²⁷via SHELXTL. All non-hydrogen atoms were

reﬁned anisotropically, and hydrogen atoms were included in

idealized positions using the riding model, with thermal parameters

1.2 times those of aryl parent carbon atoms and 1.5 times those of

methyl parent carbons. The asymmetric unit contains half of one

(28) (a) Terhune, R. W.; Maker, P. D.; Savage, C. M. Phys. ReV. Lett.

1965, 14, 681–684. (b) Clays, K.; Persoons, A. Phys. ReV. Lett. 1991,

66, 2980–2983. (c) Clays, K.; Persoons, A. ReV. Sci. Instrum. 1992,

63, 3285–3289. (d) Hendrickx, E.; Clays, K.; Persoons, A. Acc. Chem.

Res. 1998, 31, 675–683.

(29) Houbrechts, S.; Clays, K.; Persoons, A.; Pikramenou, Z.; Lehn, J.-M.

Chem. Phys. Lett. 1996, 258, 485–489.

(30) Sta¨helin, M.; Burland, D. M.; Rice, J. E. Chem. Phys. Lett. 1992, 191,

245–250.

Ph₂bbpe²⁺dication, one PF₆anion, and one DMF molecule. All

-

other calculations were carried out using the SHELXTL package.²⁴

Crystallographic data and reﬁnement details are presented in

Table 1.

(31) (a) Olbrechts, G.; Strobbe, R.; Clays, K.; Persoons, A. ReV. Sci.

Instrum. 1998, 69, 2233–2244. (b) Olbrechts, G.; Wostyn, K.; Clays,

K.; Persoons, A. Opt. Lett. 1999, 24, 403–405. (c) Clays, K.; Wostyn,

K.; Olbrechts, G.; Persoons, A.; Watanabe, A.; Nogi, K.; Duan, X.-

M.; Okada, S.; Oikawa, H.; Nakanishi, H.; Vogel, H.; Beljonne, D.;

Bre´das, J.-L. J. Opt. Soc. Am. B 2000, 17, 256–265. (d) Franz, E.;

Harper, E. C.; Coe, B. J.; Zahradnik, P.; Clays, K.; Asselberghs, I.

Proc. SPIE-Int. Soc. Opt. Eng. 2008, 6999, 699923-1–699923-11.

(32) (a) Shin, Y. K.; Brunschwig, B. S.; Creutz, C.; Sutin, N. J. Phys. Chem.

1996, 100, 8157–8169. (b) Coe, B. J.; Harris, J. A.; Brunschwig, B. S.

J. Phys. Chem. A 2002, 106, 897–905.

(33) (a) Liptay, W. In Excited States; Lim, E. C., Ed.; Academic Press:

New York, 1974; Vol. 1, pp 129-229. (b) Bublitz, G. U.; Boxer,

S. G. Annu. ReV. Phys. Chem. 1997, 48, 213–242. (c) Vance, F. W.;

Williams, R. D.; Hupp, J. T. Int. ReV. Phys. Chem. 1998, 17, 307–

329. (d) Brunschwig, B. S.; Creutz, C.; Sutin, N. Coord. Chem. ReV.

1998, 177, 61–79.

Hyper-Rayleigh Scattering. Details of the hyper-Rayleigh

scattering (HRS) experiment have been discussed elsewhere,²⁸and

the experimental procedure used for the nanosecond measurements

was as previously described.²⁹The latter studies were performed

by using the 1064 nm fundamental of an injection-seeded, Q-

(23) CrysAlis RED (Version 1.171.32.4); Oxford Diffraction Ltd., Yarnton,

Oxfordshire, U.K., 2006.

(24) SHELXTL (Version 6.10); Bruker AXS Inc., Madison, WI, 2000.

(25) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.; Burla,

M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27, 435–

435.

(26) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837–838.

(27) Sheldrick, G. M. SHELXL 97, Programs for Crystal Structure Analysis

(Release 97-2); University of Go¨ttingen, Go¨ttingen, Germany, 1997.

9

J. AM. CHEM. SOC. VOL. 132, NO. 10, 2010 3501

A R T I C L E S

Coe et al.

1/2

imaginary parts of the phase shift on concentration as described

elsewhere.³⁵The data were calibrated against the nonlinearity of

fused silica, which was taken to be n₂) 2.78 × 10^-16cm²W^-1at

750 nm.³⁵The nonlinear absorption was also expressed in terms

of the values of σ₂calculated as shown previously.³⁵

f_os

|µ₁₂| )

(2)

-5

(

)

1.08 × 10

E

max

where E_maxis the energy of the ICT maximum (in wavenumbers)

and µ₁₂is in eÅ. The latter is converted into Debye units upon

multiplying by 4.803. The degree of delocalization c_b²and electronic

coupling matrix element H_abfor the diabatic states are given by

Results and Discussion

Synthetic Studies. The new 4,4′-disubstituted bpy derivatives

bbpe, bbpb, and bbpvb were prepared in good yields via

Wadsworth-Emmons reactions of 4,4′-bis-[(diethoxyphosphi-

nyl)methyl]-2,2′-bipyridyl with the appropriate aldehyde in THF

at room temperature using potassium tert-butoxide as the base

(Scheme 1a). Although the related compound 4-methyl-4′-[(E)-

2-(4-pyridyl)vinyl]-2,2′-bipyridyl is already known,³⁶perhaps

surprisingly, to our knowledge bbpe has not been reported

previously. Dimethylation of bbpe, bbpb, and bbpvb with

iodomethane in acetone proceeded smoothly to afford the salts

[Me₂bbpe²⁺]I₂, [Me₂bbpb²⁺]I₂, and [Me₂bbpvb²⁺]I₂, with no

evidence for methylation of the bpy N atoms. The salt [(2,4-

DNPh)₂bbpe²⁺]Cl₂was prepared in high yield via reaction of

bbpe with an excess of 2,4-dinitrochlorobenzene in ethanol under

reﬂux (Scheme 1a), as previously described for the preparation

of [(2,4-DNPh)₂qpy²⁺]Cl₂(qpy ) 2,2′:4,4′′:4′,4′′′-quaterpy-

ridyl).¹⁵We have previously used a Zincke-type reaction with

aniline to convert (2,4-DNPh)₂qpy²⁺into Ph₂qpy²⁺.³⁷However,

2

1/2

∆µ₁₂

2

1

2

c_b

)

1 -

(3)

2

(

)

[

]

∆µ₁₂+ 4µ₁₂

E_max(µ₁₂)

|H_ab| )

(4)

|

∆µ_ab

If the hyperpolarizability ꢀ₀tensor has only nonzero elements along

the ICT direction, then this quantity is given by

3∆µ₁₂(µ₁₂)²

ꢀ₀)

(5)

2

(E_max

)

A relative error of (20% is estimated for the ꢀ₀values derived

from the Stark data and using eq 5, while experimental errors of

(10% are estimated for µ₁₂, ∆µ₁₂, and ∆µ_ab, (15% for H_ab, and

2

(50% for c_b.

Z-Scan Measurements. All compounds were investigated as

DMF solutions placed in 1 mm path length glass cells. The

measurements were carried out at 750 nm (1.652 eV). This

wavelength was chosen in such a way as to provide the optimum

possible enhancement of the NLO properties of the compounds

while avoiding excessive one-photon absorption. The system used

for the measurements used a Clark-MXR regenerative ampliﬁer

operating at 250 Hz and generating ca. 800 µJ pulses at 775 nm,

which were used to pump a Light Conversion TOPAS traveling

wave optical parametric ampliﬁer (OPA). The wavelength for the

present measurements was obtained by doubling the OPA signal.

A standard Z-scan measurement system was used, the beam from

the OPA being ﬁrst attenuated and spatially ﬁltered by a pair of

apertures and then focused with a lens to form a w₀≈ 60 µm spot

at the focal plane: i.e., z ) 0. The cell was mounted on a traveling

stage which was moved from z ) -40 to z ) 40 mm. The beam

transmitted through the sample was probed with a beam splitter to

provide an “open aperture” signal and with an iris aperture in the

far ﬁeld to provide the “closed aperture” signal simultaneously.

The Z-scans were analyzed using home-written software imple-

menting the calculations as presented by Sheikh-Bahae et al.³⁴

By comparing the shapes of computed curves with the experi-

mental ones, we obtained the values of the nonlinear phase shift

attempts at achieving

a

similar reaction of [(2,4-

DNPh)₂bbpe²⁺]Cl₂under various different conditions failed to

yield any trace of [Ph₂bbpe²⁺]Cl₂. We therefore prepared the

latter compound in good yield by a Knoevenagel-type reaction

of [Phpic⁺]Cl with 2,2′-bipyridyl-4,4′-dicarboxaldehyde using

acetic anhydride as the solvent and dehydration catalyst (Scheme

1b). All of the new proligand halide salts were readily converted

into their corresponding hexaﬂuorophosphate salts by anion

metathesis with aqueous NH₄PF₆.

The Ru^IIcomplex salts 2 and 4-6 (Figure 1) were prepared

via a previously reported procedure¹⁵involving reactions of the

17

precursor cis-Ru^IICl₂(DMSO)₄

with [Me₂bbpe²⁺][PF₆]₂,

[Me₂bbpb²⁺][PF₆]₂, [Me₂bbpvb²⁺][PF₆]₂, or [Ph₂bbpe²⁺][PF₆]₂.

Yields of ca. 50-65% were obtained after column chromato-

graphic puriﬁcation. Attempts to isolate the compound [Ru^II{(2,4-

DNPh)₂bbpe²⁺}₃][PF₆]₈were unfortunately unsuccessful, prob-

ably due to decomposition of the strongly electron-deﬁcient

pyridinium units under the relatively harsh reaction conditions

used (DMF under reﬂux). However, the use of milder conditions

(DMF at room temperature) and the precursor Fe^II(BF₄)₂·6H₂O

allowed the synthesis of [Fe^II{(2,4-DNPh)₂bbpe²⁺}₃][PF₆]₈(12)

as well as the other related Fe^IIcomplex salts 8, 10, 11, and 13

(Figure 1). The last four compounds were puriﬁed by using

column chromatography and isolated in yields of ca. 55-75%,

while 12 was obtained pure in over 90% yield without any need

for chromatographic puriﬁcation.

ˆ

∆Φ₀()Re(∆Φ₀)) and the T factor deﬁned here according to the

equation

∧

Im(∆Φ₀)

T ) 4π

(6)

∧

Re(∆Φ₀)

The expected E conﬁguration of the ethenyl units in all of

the proligands and complex salts is conﬁrmed by the ¹H NMR

coupling constants J in the range ca. 15-17 Hz in all cases

(except where signal overlap prevents these from being mea-

sured). The alternative Z conﬁguration would give J values of

ˆ

where ∆Φ₀stands for the complex phase shift. All measurements

were performed in the range of intensities at which the phase shifts

were below ca. 1 rad. The Z-scans were measured for three

concentrations of each compound in DMF (the highest concentration

being typically about 4 wt %) as well as for the solvent itself, and

the real and imaginary components of the γ response of the solute

were determined assuming linear dependences of the real and

(35) Samoc, M.; Samoc, A.; Dalton, G. T.; Cifuentes, M. P.; Humphrey,

M. G.; Fleitz, P. A. In Multiphoton Processes in Organics and Their

Application; Rau, I., Kajzar, F., Eds.; Old City Publishing: Philadel-

phia, in press.

(36) Kim, D.; Shin, E. J. Bull. Korean Chem. Soc. 2003, 24, 1490–1494.

(37) Coe, B. J.; Harris, J. A.; Jones, L. A.; Brunschwig, B. S.; Song, K.;

Clays, K.; Gar´ın, J.; Orduna, J.; Coles, S. J.; Hursthouse, M. B. J. Am.

Chem. Soc. 2005, 127, 4845–4859.

(34) (a) Sheik-Bahae, M.; Said, A. A.; van Stryland, E. W. Opt. Lett. 1989,

14, 955–957. (b) Sheik-Bahae, M.; Said, A. A.; Wei, T. H.; Hagan,

D. J.; van Stryland, E. W. IEEE J. Quantum Electron. 1990, 26, 760–

769. (c) van Stryland, E. W.; Sheik-Bahae, M. Opt. Eng. 1998, 60,

655–692.

9

3502 J. AM. CHEM. SOC. VOL. 132, NO. 10, 2010

Tris-Chelate Metallochromophores

A R T I C L E S

Scheme 1. Syntheses of the New Proligand Salts [Me₂bbpe²⁺][PF₆]₂, [Me₂bbpb²⁺][PF₆]₂, [Me₂bbpvb²⁺][PF₆]₂, [(2,4-DNPh)₂bbpe²⁺][PF₆]₂, and

[Ph₂bbpe²⁺][PF₆]₂, Together with Their Precursors bbpe, bbpb, and bbpvb^a

^aLegend: (i) KO^tBu/THF; (ii) MeI/Me₂CO, then NH₄PF₆; (iii) EtOH under reﬂux, then NH₄PF₆; (iv) Ac₂O under reﬂux, then NH₄PF₆.

about 11 Hz and also doublets shifted to relatively higher ﬁelds.

We have observed no evidence for thermal isomerization/

photoisomerization of these linkages in any of the compounds

studied. It is worth noting that the pyridinium-substituted “arms”

of the complexes will adopt various conformations due to

rotations about the C-C single bonds. The spectroscopic and

other measurements clearly probe the averaged properties of

all possible structures that exist in solutions or frozen glasses.

Because the CdC bonds are held in a ﬁxed E conﬁguration,

the overall directionality of the ICT processes will not be greatly

variable, but some parameters such as dipole moments may

show some conformational dependency. However, given that

all of the samples were studied under the same experimental

conditions, comparisons between the data obtained are still

expected to be valid.

analogues (denoted 2B, 8B, and 12B) in order to give increased

solubilities in the butyronitrile glassing medium. However, both

the ¹H NMR spectra and CHN analyses show that anion

metathesis is not complete, with as much as ca. 1.5 equiv of

PF₆being retained in the products; this observation is unsur-

prising given the unusually large number of anions associated

with each complex cation.

Electronic Spectroscopy Studies. The electronic absorption

spectra of the new complex salts 2, 4-6, 8, and 10-13 have

been measured in acetonitrile, and the data are presented in Table

2. These spectra feature intense intraligand π f π* absorptions

in the UV region, together with intense, broad d(M^II) f π*(L^A)

(M ) Ru/Fe, L^A) N-substituted bpy derivative) visible MLCT

bands, with two maxima in each case. The energy separation

between the two MLCT maxima lies in the range of 0.45-0.48

eV for the Ru^IIcomplexes and 0.48-0.58 eV for their Fe^II

analogues. The data for only the MLCT bands are collected in

-

For the purposes of Stark spectroscopy (see below), complex

salts 2, 8, and 12 were converted into their tetraphenylborate

9

J. AM. CHEM. SOC. VOL. 132, NO. 10, 2010 3503

A R T I C L E S

Coe et al.

Figure 1. Chemical structures of the new complex salts investigated, together with the previously reported 1, 3, 7, and 9.¹⁵

Table 3, together with those for the previously reported related

compounds 1, 3, 7, and 9¹⁵for comparison purposes. Repre-

sentative UV-vis spectra of the complex salts 1, 2, 4-6, 10,

and 13 are shown in Figures 2 and 3.

strongly coupled π-conjugated systems in 2 and 4-6. The

MLCT ꢀ values quoted for 6 and 13 are strongly enhanced by

overlap with the ILCT bands (Figures 2 and 3).

As noted previously for the qpy-based chromophores,¹⁵the

MLCT bands of the Fe^IIcomplexes are somewhat less intense

and red-shifted in comparison with those of their Ru^IIcounter-

parts (Figure 3). On moving from 1 to 7, the energy decrease

for the LE band is ca. 0.39 eV and that for the HE band is ca.

0.33 eV. The corresponding changes for the pair 3 and 9 are

0.38 and 0.29 eV. For the new complexes, the shift of the LE

absorption (-0.35 eV in all cases) on replacing Ru with Fe is

slightly smaller than those for the analogous qpy-based chro-

mophores. The shift for the HE band appears to vary over a

range of ca. 0.25-0.30 eV, but this may not really be the case,

due to the uncertainties in the E_maxvalues. In contrast to the

behavior of the MLCT absorptions, the intraligand bands are

more intense in the Fe^IIcomplexes in comparison with their

Ru^IIanalogues. In keeping with our previous studies on vari-

ous chromophores with N-arylpyridinium electron acceptor

groups,^15,20,37replacing a methyl with a phenyl substituent

causes the MLCT bands to red-shift in every case; these

decreases in energy for the HE bands are generally about twice

those of the LE absorptions (Table 3). On moving from 10 to

its (2,4-DNPh)₂bbpe²⁺counterpart in 12, further small red shifts

are observed.

Considering the MLCT absorptions of the Ru^IIcomplexes 1,

2, and 5 (Figure 2), the lower energy (LE) band shows a steady

red shift as the conjugation within the ligands is extended, with

sequential energy differences of 0.10 and 0.03 eV (Table 3).

The energy of the higher energy (HE) band decreases by 0.25

eV on moving from 1 to 2 but then remains constant in 5,

although it should be noted that the error limits on the quoted

HE maxima are large due to strong overlap with the LE band.

The band intensities also increase steadily and substantially on

moving along this series, due to the increasing extent of

π-coupling. Comparison of the data for the N-Ph derivatives 3

and 4 reveals a similar pattern, with red shifts of 0.09 and 0.16

eV for the LE and HE bands, respectively. A concomitant ca.

2-fold enhancement in ꢀ is also observed. The Me₂bbpvb²⁺

-

containing complex in 6 shows only one discernible MLCT

maximum (but with a high-energy shoulder; Figure 2) that lies

to high energy of those of all the other new Ru^IIchromophores,

but to slightly lower energy in comparison with the qpy-based

species 1 and 3. A major difference between the latter and the

new complexes is the increased intensities of the ILCT bands;

for 1 and 3, the lowest energy such absorptions are observed at

324 nm (ꢀ ) 55.0 × 10³M^-1cm^-1) and 328 nm (ꢀ ) 65.7 ×

10³M^-1cm^-1), respectively.¹⁵As for the MLCT bands, this

change is attributable to the presence of extended and more

Taken in the context of the vast literature on transition-metal

tris-chelate complexes of R-diimine ligands, our new chro-

mophores, and especially the Me₂bbpvb²⁺-containing species

9

3504 J. AM. CHEM. SOC. VOL. 132, NO. 10, 2010

Tris-Chelate Metallochromophores

A R T I C L E S

Table 2. UV-Vis Absorption and Electrochemical Data for the New Complex Salts [M^II(L^A)₃][PF₆]₈in Acetonitrile

E_1/2, V vs Ag/AgCl (∆E_p, mV)^b

salt (M/L^A)

λ

_max, nm^a(ꢀ, 10³M^-1cm^-1

)

E_max, eV

assignment

M^III/II

L^Areduction

2 (Ru/Me₂bbpe²⁺

4 (Ru/Ph₂bbpe²⁺

5 (Ru/Me₂bbpb²⁺

)

506 (58.9)

428 (40.5)

314 (159.4)

514 (75.6)

434 (49.2)

329 (206.5)

513 (80.1)

428 (61.6)

351 (215.4)

255 (61.1)

501 (106.1)

385 (275.0)

307 (61.5)

247 (88.2)

591 (40.9)

468 (26.9)

317 (190.0)

221 (60.2)

601 (50.7)

485 (31.7)

336 (241.9)

600 (56.6)

475 (38.5)

358 (274.7)

251 (61.9)

611 (50.0)

495 (29.2)

333 (228.1)

226 (159.0)

2.45

2.90

3.95

2.41

2.86

3.77

2.42

2.90

3.53

4.86

2.48

3.22

4.04

5.02

2.10

2.65

3.91

2.21

2.06

2.56

3.69

2.07

2.61

3.46

4.94

2.03

2.51

3.72

5.49

d f π*

π f π*

d f π*

π f π*

d f π*

π f π*

d f π*

π f π*

d f π*

π f π*

d f π*

π f π*

d f π*

π f π*

d f π*

π f π*

1.43 (90)

-0.67 (190)

-0.99^c

)

1.42 (80)

1.32 (80)

-0.52^c

-0.94 (130)

)

-0.67^c

-1.13^c

6 (Ru/Me₂bbpvb²⁺

)

1.23 (90)

1.19 (80)

-0.83^c

-0.99^c

-1.78^c

8 (Fe/Me₂bbpe²⁺

)

-0.67 (180)

-1.10^c

10 (Fe/Ph₂bbpe²⁺

)

1.19 (90)

1.08 (90)

-0.54^c

-0.96 (120)

11 (Fe/Me₂bbpb²⁺

)

-0.71^c

-1.18^c

12 (Fe/(2,4-DNPh)₂bbpe²⁺

)

1.21 (100)

0.97 (80)

-0.30^c

-0.98^c

-1.31^c

-1.39^c

-1.53^c

-0.87^c

-1.05^c

-1.84^c

13 (Fe/Me₂bbpvb²⁺

)

581 (61.5)

458 (67.9)

385 (349.8)

246 (93.8)

2.13

2.71

3.22

5.04

d f π*

π f π*

b

^aSolutions ca. 3-8 × 10^-5M. Measured in solutions ca. 10^-3M in analyte and 0.1 M in [N(C₄H₉-n)₄]PF₆at a 2 mm glassy-carbon-disk working

c

electrode with a scan rate of 200 mV s^-1. Ferrocene was used as the internal reference: E_1/2) 0.43 V, ∆E_p) 80-90 mV. E_pcfor an irreversible

reduction process.

6 and 13, display unusually intense ICT absorptions. The

underivatized complexes [M^II(bpy)₃]²⁺give MLCT ꢀ values of

ca. 1.3 × 10⁴M^-1cm^-1for M ) Ru (λ_max452 nm in

acetonitrile)³⁸and ca. 0.9 × 10⁴M^-1cm^-1for M ) Fe (λ_max

522 nm in water).³⁹In comparison with the latter, the LE MLCT

band of 13 is approximately 7 times more intense (Table 2).

Comparisons with more closely related species substituted with

extended π-conjugated systems are of more immediate interest.

Le Bozec and colleagues have reported a number of such

species, and the most intense absorption band apparently

reported is for a Zn^IIcomplex of an azobenzene-derivatized bpy

ligand.^11eThus, replacing the outer vinyl units in Me₂bbpvb²⁺

with NdN and swapping the N-methylpyridinium groups for

-N(ⁿBu)₂gives an ꢀ value of 2.46 × 10⁵M^-1cm^-1for the

ILCT band (λ_max496 nm in dichloromethane); this intensity is

slightly smaller than that observed for 6 but only about 70% of

that for 13. Indeed, to our knowledge the ILCT band for 13 is

the most intense single absorption feature yet reported for a

mononuclear complex of this type.

reported qpy-based species 1, 3, 7, and 9¹⁵in Table 3. In general,

these compounds give better deﬁned electrochemistry when

using a glassy-carbon working electrode rather than a platinum

disk;¹⁵therefore, only data obtained by using the former are

quoted here. Representative cyclic voltammograms of the

complex salts 4 and 10 are shown in Figure 4.

All of the complexes show reversible or quasi-reversible

M^III/II(M ) Ru, Fe) oxidation waves, together with reversible,

quasi-reversible, and/or irreversible ligand-based reductions. The

most obvious trend evident in these data is that the E_1/2(Fe^III/II

)

values are lower than the corresponding potentials for their Ru

analogues by 210-260 mV (Figure 4), a consequence of the

higher electron density at the Fe^IIcenters (Table 3).¹⁵This

difference is also reﬂected in the red shifts observed for the

MLCT bands (see above). For a given conjugated system in

L^A, the E_1/2(M^III/II) values are not signiﬁcantly affected by the

nature of the N substituent, as observed for example in the series

8, 10, and 12. However, extending the conjugation within the

ligands does affect the potentials. For the series 1, 2, 5, and 6,

E_1/2(Ru^III/II) decreases by respective increments of 170, 110, and

90 mV, while for the series 7, 8, 11, and 13 the sequential

changes in E_1/2(Fe^III/II) are 200, 110, and 110 mV. This trend is

attributable to the cumulative electron-donating inﬂuence of the

ethylene substituents, and it is apparent that the (E,E)-1,4-

bis(vinyl)phenylene unit is more strongly donating than the

(E,E)-1,3-butadienyl fragment. By way of comparison, inserting

an (E)-vinyl unit into the N-methyl-4,4′-bipyridinium ligand in

Electrochemical Studies. The new complex salts 2, 4-6, 8,

and 10-13 have been studied by cyclic voltammetry in

acetonitrile, and the results are presented in Table 2. Selected

data are also collected together with those for the previously

(38) Juris, A.; Balzani, V.; Belser, P.; von Zelewsky, A. HelV. Chim. Acta

1981, 64, 2175–2182.

(39) Ford-Smith, M. H.; Sutin, N. J. Am. Chem. Soc. 1961, 83, 1830–

1834.

9

J. AM. CHEM. SOC. VOL. 132, NO. 10, 2010 3505

A R T I C L E S

Coe et al.

Table 3. MLCT Absorption and Selected Electrochemical Data for Complex Salts 1-13 in Acetonitrile

E_1/2or E, V vs Ag/AgCl^b

L^Areduction

salt (M/L^A)

λ

_max, nm^a(ꢀ, 10³M^-1cm^-1

)

E_max, eV

M^III/II

c

1 (Ru/Me₂qpy²⁺

)

486 (37.3)

394 (26.4)

506 (58.9)

428 (40.5)

496 (37.3)

410 (26.1)

514 (75.6)

434 (49.2)

513 (80.1)

428 (61.6)

501 (106.1)

574 (26.0)

440 (18.8)

591 (40.9)

468 (26.9)

584 (32.5)

454 (22.0)

601 (50.7)

485 (31.7)

600 (56.6)

475 (38.5)

611 (50.0)

495 (29.2)

581 (61.5)

458 (67.9)

2.55

3.15

2.45

2.90

2.50

3.02

2.41

2.86

2.42

2.90

2.48

2.16

2.82

2.10

2.65

2.12

2.73

2.06

2.56

2.07

2.61

2.03

2.51

2.13

2.71

1.60

-0.70

-0.67

-0.53

-0.52^d

-0.67^d

2 (Ru/Me₂bbpe²⁺

)

1.43

1.61

1.42

1.32

c

3 (Ru/Ph₂qpy²⁺

)

4 (Ru/Ph₂bbpe²⁺

)

5 (Ru/Me₂bbpb²⁺

)

6 (Ru/Me₂bbpvb²⁺

)

1.23

1.39

-0.83^d

-0.72^d

c

7 (Fe/Me₂qpy²⁺

)

8 (Fe/Me₂bbpe²⁺

)

1.19

1.39

1.19

1.08

1.21

0.97

-0.67

c

9 (Fe/Ph₂qpy²⁺

)

-0.51^d

-0.54^d

-0.71^d

-0.30^d

-0.87^d

10 (Fe/Ph₂bbpe²⁺

11 (Fe/Me₂bbpb²⁺

12 (Fe/(2,4-DNPh)₂bbpe²⁺

13 (Fe/Me₂bbpvb²⁺

)

b

^aSolutions ca. 3-8 × 10^-5M. Measured in solutions ca. 10^-3M in analyte and 0.1 M in [N(C₄H₉-n)₄]PF₆at a 2 mm glassy-carbon-disk working

d

electrode with a scan rate of 200 mV s^-1. Ferrocene was used as the internal reference: E_1/2) 0.43 V. ^cData taken from ref 15. E_pcfor an irreversible

reduction process.

Figure 2. Electronic absorption spectra of 1 (blue), 2 (red), 5 (gold), and

6 (green) at 293 K in acetonitrile.

Figure 4. Cyclic voltammograms of 4 (blue) and 10 (red) in acetonitrile

at 200 mV s^-1at a glassy-carbon working electrode (the arrow indicates

the direction of the initial scans).

1,4-bis(vinyl)phenylene bridge gives smaller additional changes

of 0-30 mV.^21,40These decreases in the E_1/2(M^III/II) values

correlate with the steady red shifts observed in the LE MLCT

bands (see above), except for the Me₂bbpvb²⁺compounds.

The often irreversible nature of the ligand-based reduction

processes precludes further extensive discussion. However, it

is apparent that the nature of the metal center does not

signiﬁcantly affect the potentials. Also, the ﬁrst reduction wave

is markedly sensitive to the N substituent, with all of the

N-phenyl complexes showing higher potentials in comparison

with their N-methyl analogues. A total anodic shift of 370 mV

is observed moving along the series 8, 10, and 12. This trend

reﬂects the increasing electron-accepting ability of the pyri-

Figure 3. Electronic absorption spectra of 4 (blue), 6 (green), 10 (red),

and 13 (gold) at 293 K in acetonitrile.

(40) Coe, B. J.; Foxon, S. P.; Harper, E. C.; Raftery, J.; Shaw, R.; Swanson,

C. A.; Asselberghs, I.; Clays, K.; Brunschwig, B. S.; Fitch, A. G.

Inorg. Chem. 2009, 48, 1370–1379.

1D dipolar complexes causes E_1/2(M^III/II) to decrease by ca.

30-50 mV, while moving to a (E,E)-1,3-butadienyl or a (E,E)-

9

3506 J. AM. CHEM. SOC. VOL. 132, NO. 10, 2010

Tris-Chelate Metallochromophores

A R T I C L E S

Figure 5. Representation of the molecular structure of the salt [Ph₂bbpe²⁺][PF₆]₂·2DMF, with H atoms omitted for clarity (50% probability ellipsoids).

-

While the Ph₂bbpe²⁺cation has been completed by generating symmetry-equivalent atoms, only the single PF₆anion and DMF molecule present in the

asymmetric unit are shown.

sured in acetonitrile solutions by using the HRS technique^28,29,31

Table 4. Selected Interatomic Distances (Å) and Angles (deg) for

the Salt [Ph₂bbpe²⁺][PF₆]₂·2DMF

with a 800 nm Ti³⁺:sapphire laser, and the results are collected

N(1)-C(4)

N(1)-C(1)

C(3)-C(4)

C(1)-C(2)

C(3)-C(5)

C(2)-C(5)

C(5)-C(6)

C(6)-C(7)

C(7)-C(8)

1.336(3)

1.333(3)

1.380(3)

1.373(3)

1.391(3)

1.385(3)

1.454(3)

1.325(3)

1.449(3)

C(8)-C(9)

1.392(3)

1.394(3)

1.356(3)

1.349(3)

1.358(3)

1.354(3)

1.436(3)

1.485(4)

in Table 5, together with the previously published data for 1,

3, 7, and 9.¹⁵This wavelength was chosen originally because

the latter complexes do not absorb strongly at either 800 nm or

at the second harmonic (SH) of 400 nm, their MLCT bands

falling conveniently between these wavelengths. However,

several of the new, extended chromophores do show intense

ILCT bands near 400 nm (Table 2, Figures 2 and 3), and the

results obtained for these salts are hence strongly affected by

resonance enhancement. In order to obtain additional data, we

have therefore also used a 1064 nm Nd³⁺:YAG laser to study

these compounds, and the results are included in Table 5. A

beneﬁt of using a 800 or 1064 nm laser is the avoidance of any

contributions due to two-photon luminescence. All of the Ru^II

complexes emit at wavelengths above 670 nm (Table 5), with

negligible emission below about 600 nm. In contrast, the Fe^II

complexes are predictably nonemissive at room temperature.

Within the series 1, 2, and 5, the λ_emvalues red-shift steadily,

consistent with the trend in the MLCT absorption maxima.

However, the emission intensity is signiﬁcantly weaker for 5

in comparison with those for 1 and 2. Comparison of the λ_em

data for 3 and 4 also reveals a red shift, while the emission

band for 6 falls between those of 2 and 4. The experimentally

determined ꢀ values for each operating wavelength are based

on the assumption of a single tensor component, ꢀ_xxx, and are

derived from the total HRS intensity 〈ꢀ_HRS²〉 by using the

equation

C(8)-C(11)

C(9)-C(10)

C(11)-C(12)

N(2)-C(10)

N(2)-C(12)

N(2)-C(13)

C(4)-C(4)A

C(5)-C(6)-C(7)

123.1(2)

C(6)-C(7)-C(8)

127.4(2)

dinium groups, as also manifested in the MLCT data (see above)

and in previously reported related compounds.^15,20,37The

relatively large currents for the cathodic peaks in comparison

with those for the metal-based waves (Figure 4) are consistent

with multielectron processes attributable to simultaneous reduc-

tions of all three L^Aligands.

Crystallographic Study. Although we have unfortunately been

unable to grow diffraction-quality crystals of any of the new

complex salts, a single-crystal X-ray structure has been obtained

for the proligand salt [Ph₂bbpe²⁺][PF₆]₂·2DMF. A representation

of the molecular structure is shown in Figure 5, and selected

interatomic distances and angles are presented in Table 4. We

have also determined structures of the related compounds

[Ph₂qpy²⁺][PF₆]₂·Me₂CO³⁷and [Phbpe⁺]Cl·HCl·H₂O.⁴¹

As expected on the basis of steric considerations, the

Ph₂bbpe²⁺cation adopts a transoid conﬁguration with a crystal-

lographic center of symmetry in the middle of the C-C bond

between the rings of the bpy group. The same applies to the

related cation in [Ph₂qpy²⁺][PF₆]₂·Me₂CO.³⁷The π-conjugated

bbpe unit in [Ph₂bbpe²⁺][PF₆]₂· 2DMF is close to planar,

with a dihedral angle of 7.9° between the two pyridyl rings;

this compares with a corresponding angle of 15.7° in

[Phbpe⁺]Cl·HCl·H₂O.⁴¹The dihedral angle between the quat-

ernized pyridyl and attached phenyl rings in the latter is 53.4°,⁴¹

somewhat larger than that observed in [Ph₂bbpe²⁺][PF₆]₂·2DMF

(44.5°). Such twists are attributable in part to steric interactions

between the ortho hydrogen atoms of the two rings. All of the

other geometric parameters are closely similar for both

[Ph₂bbpe²⁺][PF₆]₂·2DMF and [Phbpe⁺]Cl·HCl·H₂O.⁴¹

Hyper-Rayleigh Scattering and Luminescence Studies. The

ꢀ values of the complex salts 2, 4-6, 8, and 10-13 were mea-

8

35

16

105

8

21

2

〈ꢀ_HRS²〉 )

+

ꢀ_xxx

)

ꢀ_xxx

(7)

(

)

(

)

While the effects of resonance preclude a detailed analysis

of the HRS results, some trends are apparent. First, extending

the ligand conjugated systems leads to steadily increasing ꢀ

values in all instances except for the N-phenylated pair 3 and 4

when studied at 800 nm (Table 5). It is well established that ꢀ

responses normally increase as conjugated systems are ex-

tended,¹although the very large values obtained for 2 and 4-6

can be attributed in large part to resonance enhancement. The

second trend that emerges from the collected data is that the

Ru-based chromophores give larger ꢀ values than their Fe

analogues in all cases except for the Me₂bbpe²⁺-containing pair

2 and 8 when studied at 800 nm. However, the ꢀ_xxx,1064value

for 2 is about 3 times that for 8. Comparison within the Fe-

based series 8, 10, and 12 does not reveal the steady increases

in ꢀ_xxx,800that might be expected on the basis of the decreasing

(41) Coe, B. J.; Foxon, S. P.; Harper, E. C.; Helliwell, M.; Raftery, J.;

Swanson, C. A.; Brunschwig, B. S.; Clays, K.; Franz, E.; Gar´ın, J.;

Orduna, J.; Horton, P. N.; Hursthouse, M. B. J. Am. Chem. Soc. 2010,

132, 1706–1723.

9

J. AM. CHEM. SOC. VOL. 132, NO. 10, 2010 3507

A R T I C L E S

Coe et al.

Table 5. MLCT Absorption, Luminescence, and HRS Data for Complex Salts 1-13 in Acetonitrile

b

,

c

salt (M/L^A)

λ

_max, nm

λ

_em, nm^a

ꢀ_xxx,800

10^-30esu

ꢀ_xxx,1064,

10^-30esu

d

1 (Ru/Me₂qpy²⁺

)

486, 394

506, 428

496, 410

514, 434

513, 428

501

574, 440

591, 468

584, 454

601, 485

600, 475

611, 495

581, 458

671

712

685

724

764^f

718

g

170 ( 21

190 ( 35

270 ( 35

170 ( 30

1200 ( 400

1500 ( 300

78 ( 10

320 ( 110

80 ( 10

90 ( 30

e

2 (Ru/Me₂bbpe²⁺

)

850 ( 160

d

3 (Ru/Ph₂qpy²⁺

)

e

4 (Ru/Ph₂bbpe²⁺

5 (Ru/Me₂bbpb²⁺

)

1200 ( 600

3100 ( 700

5600 ( 1300

e

6 (Ru/Me₂bbpvb²⁺

)

d

7 (Fe/Me₂qpy²⁺

)

8 (Fe/Me₂bbpe²⁺

)

275 ( 110

d

9 (Fe/Ph₂qpy²⁺

)

e

10 (Fe/Ph₂bbpe²⁺

11 (Fe/Me₂bbpb²⁺

12 (Fe/(2,4-DNPh)₂bbpe²⁺

13 (Fe/Me₂bbpvb²⁺

)

480 ( 80

340 ( 200

400 ( 140

700 ( 200

)

340 ( 180

145 ( 40

660 ( 130

)

g

)

^aEmission maximum upon excitation into the MLCT bands (λ_ex480 nm for 2 and 5, 514 nm for 4, and 510 nm for 6), with ca. 10^-5M solutions;

b

dilution produced no changes in the λ_emvalues. First hyperpolarizability measured by using a 800 nm Ti³⁺:sapphire laser. ^cFirst hyperpolarizability

measured by using a 1064 nm Nd³⁺:YAG laser. The quoted cgs units (esu) can be converted into SI units (C³m³J^-2) by dividing by a factor of 2.693

× 10²⁰. ^dData taken from ref 15. ^eNot measured. ^fRelatively weak signal. ^gNot observed.

MLCT energies or previous studies with related 1D complexes.⁴²

In contrast, 8 does show the smallest ꢀ_xxx,1064value of these

three compounds.

For the MLCT bands, the E_maxvalues for the two most intense

Gaussian curves ﬁtted to the absorption spectra at 77 K in

butyronitrile are generally decreased in comparison with the data

recorded in acetonitrile solutions (Table 3). In almost every

instance, the data obtained at 77 K show the same trends as

those observed at room temperature: i.e., the red-shifting effects

arising from extending the conjugated systems, replacing Ru

with Fe, or replacing N-methyl with N-phenyl/2,4-dinitrophenyl

substituents (see above). In order to analyze the f_osand µ₁₂data,

it is appropriate to consider the totals for the various compo-

nents, since the superposition of these gives the overall

absorption proﬁles. The total f_osand µ₁₂values measured at 77

K show trends that generally correlate with those observed in

the ꢀ values in acetonitrile solutions (Table 3). Thus, the MLCT

intensities increase when the conjugated systems are extended

or when a N-methyl is replaced with a N-phenyl/2,4-dinitro-

phenyl substituent. Also, the Fe complexes show lower total f_os

and µ₁₂values in comparison with their Ru analogues. This latter

observation also applies to the combined MLCT and ILCT data

for 6 and 13.

Stark Spectroscopic Studies. Complex salts 2B, 4-6, 8B, 10,

11, 12B, and 13 have been studied by Stark spectroscopy³³in

butyronitrile glasses at 77 K, and the results are presented in

Table 6, together with the previously published data for 1, 3, 7,

and 9.¹⁵Gaussian ﬁtting of the MLCT absorption spectra using

three or four curves was necessary to successfully model the

Stark data. It is likely that the ILCT transitions in the extended

chromophores, especially 5, 6, 11, and 13, will contribute

substantially to their NLO responses. However, these bands

could be analyzed only for 6 and 13 because the short-

wavelength operational cutoff of our Stark spectrometer is about

350 nm. For both 6 and 13, the entire ICT region was treated

as one. In the case of 6, strong overlap between the MLCT and

ILCT bands means that it is not possible to fully resolve the

two types of transition and a total of eight curves was used. In

contrast, for 13 there is a relatively good degree of separation

between the MLCT and ILCT bands; three curves were then

used for the former and four for the latter, with some overlap

even so. The resulting data for 6 and 13 are included in Table

6. Representative absorption spectra and electroabsorption

spectra in the MLCT region for the complex salts 2B, 5, and

8B are shown in Figure 6, while the full ICT region spectra for

for 6 and 13 are shown in Figure 7. The MLCT region spectra

for all of the other new complex salts are provided as Supporting

Information (Figures S1 and S2).

As noted previously for 1 and 3, for each of the Ru complexes

a relatively low intensity Gaussian curve was placed to low

energy in order to ﬁt the Stark signal satisfactorily; these curves

may correspond with spin-forbidden transitions to triplet MLCT

excited states.⁴³For 1, 3, 7, and 9, only two of the three Gaussian

curves used to ﬁt the transitions to singlet excited states

contribute signiﬁcantly to the observed Stark signals; therefore,

the data obtained for the other functions are not quoted.

However, for all of the new complex salts the latter are

signiﬁcant contributors; thus, the corresponding data are included

in Table 6.

The derived adiabatic and diabatic dipole moment changes

(∆µ₁₂and ∆µ_ab, respectively) for the MLCT bands cover a broad

range of ca. 5-34 D, with the largest being measured for one

of the Gaussian components for 13. For the purpose of analyzing

and comparing these parameters, it is reasonable to consider

average values because the ﬁtted Gaussian components represent

hypothetical ICT transitions that are codirectional but not

additive in terms of their dipole moment changes. Both ∆µ₁₂

and ∆µ_abgenerally increase in a logical fashion as the conjugated

systems are extended: for example, on moving along the Ru

series 1 f 2B f 5. Except for the pair 1 and 3, a pattern of

increasing ∆µ₁₂and ∆µ_abon replacing N-methyl with N-phenyl

substituents is also observed, and steady increases occur on

moving along the Fe series 8B f 10 f 12B. Overall, there is

no clear trend in the dipole moment changes on varying the

metal center. Since they derive directly from these data, the

average delocalized and localized electron-transfer distances (r₁₂

and r_ab, respectively) show the same trends, with r_abexceeding

5 Å in several instances and being as high as 7 Å for 13.

2

Although the parameters c_band H_abmight be expected to

(42) (a) Coe, B. J.; Harris, J. A.; Harrington, L. J.; Jeffery, J. C.; Rees,

L. H.; Houbrechts, S.; Persoons, A. Inorg. Chem. 1998, 37, 3391–

3399. (b) Coe, B. J.; Harries, J. L.; Harris, J. A.; Brunschwig, B. S.;

Coles, S. J.; Light, M. E.; Hursthouse, M. B. Dalton Trans. 2004,

2935–2942.

decrease steadily with extension of the conjugated systems, the

data obtained do not provide convincing evidence for such

behavior, especially since the estimated errors on the c_b²values

are very large.

(43) Kober, E. M.; Meyer, T. J. Inorg. Chem. 1982, 21, 3967–3977.

9

3508 J. AM. CHEM. SOC. VOL. 132, NO. 10, 2010

Tris-Chelate Metallochromophores

A R T I C L E S

Table 6. ICT Absorption and Stark Spectroscopic Data for Complex Salts 1-13

a

,

k

ν_max

,

λ_max

,

E_max

eV

µ₁₂,^b∆µ₁₂,^c∆µ_ab,^dr₁₂,^er_ab,^f

H_ab,^h

ꢀ₀,ⁱ

∑ꢀ₀,

ꢀ_EN,

g

j

salt (M/L^A)

f_os

c_b²

N

cm^-1

nm

D

Å

10³cm^-110^-30esu

10^-30esu

10^-32esu

l

1 (Ru/Me₂qpy²⁺

)

17 359 576 2.15 0.002 0.5 12.9 12.9 2.7 2.7 0.00

0.7

7.4

9.4

2.8

6.5

7.1

5.9

3.2

6.4

8.6

4.0

6.0

6.1

2.8

6.6

6.5

5.3

2.7

7.9

6.8

8.8

11.8

12.7

13.8

7.1

8.1

4.9

6.4

4.4

6.5

7.1

5.2

6.2

5.1

5.5

6.8

5.3

4.1

4.8

3.7

6.9

6.3

4.2

11.3

11.1

11.7

12.5

1

26

41

37

98

30

26

4

34

68

72 11.1

84 24.8

20 149 496 2.50 0.17

25 240 396 3.13 0.39

17 421 574 2.16 0.07

19 115 523 2.37 0.32

21 132 473 2.62 0.19

23 309 429 2.89 0.16

16 998 589 2.11 0.02

19 901 502 2.47 0.16

24 553 407 3.04 0.51

17 664 566 2.19 0.27

18 712 534 2.32 0.39

20 164 496 2.50 0.32

22 100 452 2.74 0.52

17 180 582 2.13 0.12

18 712 534 2.32 0.56

20 486 488 2.54 0.40

22 422 446 2.78 0.21

17 451 573 2.16 0.12

4.2

7.9 11.6 1.6 2.4 0.16

5.8 10.3 15.5 2.2 3.2 0.17

2.9 17.4 18.4 3.6 3.8 0.02

6.0 13.0 17.7 2.7 3.7 0.13

4.3

3.8 12.9 14.9 2.7 3.1 0.07

1.4 7.0 7.5 1.5 1.6 0.04

4.2 10.0 13.1 2.1 2.7 0.12

6.7 13.5 19.0 2.8 4.0 0.14

5.7 22.8 25.4 4.8 5.3 0.05

6.7 15.9 20.7 3.3 4.3 0.12

5.8 15.7 19.5 3.3 4.1 0.10

7.1 21.2 25.5 4.4 5.3 0.08

3.7 20.6 21.9 4.3 4.6 0.03

8.0 15.8 22.4 3.3 4.7 0.14

6.5 14.7 19.6 3.1 4.1 0.12

4.5 16.6 18.8 3.5 3.9 0.06

3.8 22.9 24.1 4.8 5.0 0.02

2B (Ru/Me₂bbpe²⁺

)

191

9.6 13.0 2.0 2.7 0.13

l

3 (Ru/Ph₂qpy²⁺

)

114

586

108 10.2

120 44.6

76

4 (Ru/Ph₂bbpe²⁺

)

181

146

101

158

73

217

111

50

5 (Ru/Me₂bbpb²⁺

)

451

96 47.9

6 (Ru/Me₂bbpvb²⁺

)

81

1078

132 71.1

19 144 522 2.37 1.10 11.1 15.4 27.0 3.2 5.6 0.21

20 978 477 2.60 0.49 7.1 16.4 21.7 3.4 4.5 0.12

22 896 437 2.84 1.28 10.9 18.1 28.4 3.8 5.9 0.18

393

141

313

53

41

56

0

30

56

44

24 396 410 3.02 1.00

25 326 395 3.14 0.68

26 239 381 3.25 1.21

27 602 362 3.42 0.57

17 225 581 2.14 0.16

22 531 444 2.79 0.35

16 292 614 2.02 0.11

16 534 605 2.05 0.26

20 486 488 2.54 0.18

16 950 590 2.10 0.16

21 675 461 2.69 0.37

16 050 623 1.99 0.19

16 534 605 2.05 0.43

19 680 508 2.44 0.41

16 131 620 2.00 0.21

16 696 599 2.07 0.45

19 922 502 2.47 0.47

15 808 633 1.96 0.11

16 292 614 2.02 0.28

19 438 514 2.41 0.29

16 569 604 2.05 0.42

17 833 561 2.21 0.16

20 675 484 2.56 0.46

23 818 420 2.95 0.86

9.4

7.6

9.9

6.6

4.4

4.7 19.4 1.0 4.0 0.38

6.0 16.3 1.3 3.4 0.31

5.1 20.5 1.1 4.3 0.37

0.0 13.3 0.0 2.8 0.50

6.1 10.6 1.3 2.2 0.22

l

7 (Fe/Me₂qpy²⁺

)

86

72 14.1

84 25.6

5.8 11.2 16.1 2.3 3.4 0.15

3.9 10.1 12.7 2.1 2.7 0.10

5.8

4.3 18.5 20.4 3.9 4.3 0.05

4.5 7.4 11.6 1.5 2.4 0.18

6.1 13.9 18.4 2.9 3.8 0.12

5.1 11.6 15.4 2.4 3.2 0.12

7.4 13.2 19.8 2.8 4.1 0.17

6.7 21.9 25.6 4.5 5.3 0.07

5.3 11.2 15.4 2.3 2.4 0.13

7.6 11.0 18.8 2.3 3.9 0.20

7.1 22.5 26.6 4.7 5.5 0.08

3.8 12.6 14.8 2.6 3.1 0.07

6.1 16.8 20.8 3.5 4.3 0.10

5.7 27.6 29.9 5.8 6.2 0.04

7.4 10.0 17.8 2.1 3.7 0.22

8B (Fe/Me₂bbpe²⁺

)

197

9.7 15.1 2.0 3.2 0.08

91

62

39

82

l

9 (Fe/Ph₂qpy²⁺

)

121

477

108 10.8

120 36.3

10 (Fe/Ph₂bbpe²⁺

11 (Fe/Me₂bbpb²⁺

12B (Fe/(2,4-DNPh)₂bbpe²⁺

13 (Fe/Me₂bbpvb²⁺

)

87

201

189

90

173

217

56

180

150

41

259

60

)

480

416

96 51.0

)

132 27.4

)

836 (450)^m132 55.1 (29.7)^m

4.4

6.9 30.7 33.7 6.4 7.0 0.04

8.8 5.8 18.5 1.2 3.9 0.34

24 635 406 3.05 1.44 11.2 10.9 24.9 2.3 5.2 0.28

8.8 12.5 1.8 2.6 0.15

170

101

55

25 561 391 3.17 1.18

26 759 374 3.32 0.97

9.9

8.8

8.9 21.7 1.8 4.5 0.30

6.7 18.8 1.4 3.9 0.32

^aMaxima for Gaussian functions used to ﬁt to spectral data recorded in butyronitrile glass at 77 K. ^bCalculated using eq 2; f_osobtained from (4.60 ×

10^-9M cm²)ꢀ_max× fw_1/2, where ꢀ_maxis the maximal molar extinction coefﬁcient and fw_1/2is the full width at half height (in wavenumbers). ^cCalculated

from f_int∆µ₁₂using f_int) 1.33. ^dCalculated from eq 1. ^eDelocalized electron-transfer distance calculated from ∆µ₁₂/e. ^fEffective (localized)

electron-transfer distance calculated from ∆µ_ab/e. ^gCalculated from eq 3. ^hCalculated from eq 4. ⁱCalculated from eq 5. ^jThe total number of π-bonding

k

electrons (excluding metal d-electrons) in the complex cation. Electron-normalized off-resonant ﬁrst hyperpolarizability, deﬁned as ∑ꢀ₀/N^3/2

.

^{50 l}Data taken

from ref 15. ^mThe total contribution associated with (largely) only the MLCT transitions is in brackets for purposes of comparison with the other

compounds.

As in our previous studies,¹⁵we have used the standard two-

state model (eq 5)⁴⁴to estimate static ﬁrst hyperpolarizabilities

from the Stark data for the transitions to dipolar ICT excited

states. Although metal tris-chelate complexes have octupolar

ground-state structures,⁷their NLO responses are clearly as-

sociated largely with dipolar ICT transitions. In this context, it

is also important to note that other two-state descriptions are

also available, which would alter the Stark-derived ꢀ₀values

quoted here by a constant factor of 0.5 or 2.⁴⁵These data may

hence be used primarily as a means to reveal structure-activity

relationships involving all contributing ICT transitions without

any interference from variable resonance effects, rather than

necessarily revealing the absolute magnitudes of ꢀ₀. Even so,

it is remarkable that our previous studies with dipolar Ru^II

ammine complexes demonstrate that using the two-state equation

(5) with a prefactor of 3 (the “perturbation series convention”)⁴⁵

(44) (a) Oudar, J. L.; Chemla, D. S. J. Chem. Phys. 1977, 66, 2664–2668.

(b) Oudar, J. L. J. Chem. Phys. 1977, 67, 446–457.

(45) Willetts, A.; Rice, J. E.; Burland, D. M.; Shelton, D. P. J. Chem. Phys.

1992, 97, 7590–7599.

9

J. AM. CHEM. SOC. VOL. 132, NO. 10, 2010 3509

A R T I C L E S

Coe et al.

Figure 6. MLCT and Stark spectra with calculated ﬁts for complex salts 2B, 5, and 8B in an external electric ﬁeld of 4.74 × 10⁷V m^-1: (top panels)

absorption spectra illustrating Gaussian curves used in data ﬁtting; (bottom panels) electroabsorption spectra, with experimental data shown in blue and ﬁts

in green according to the Liptay equation.³³

contributions are included. Large increases in ∑ꢀ₀also ac-

company the insertion of (E)-vinyl bridges into the N-phenyl

chromophores, i.e. on moving from 3 to 4 and from 9 to 10.

The second trend observed in the ∑ꢀ₀values is that these

increase substantially (ca. 1.5-3 times) on replacing N-methyl

with more strongly electron-accepting N-phenyl substituents.

Both of these trends evident in the ∑ꢀ₀values for 1-13 reﬂect

those we have observed previously when analyzing the MLCT

bands for related 1D metallochromophores.^{21,32b,40,42b}Although

the N-2,4-dinitrophenyl substituent is an even more powerful

electron acceptor according to the MLCT energies and ligand-

based reduction potentials (see above), the total ꢀ₀response

derived for 12B is possibly slightly smaller than that for 10.

This result can be attributed largely to the decreased MLCT

transition intensities caused by diminished π-orbital overlap due

to twisting of the 2,4-DNPh group with respect to the rest of

Figure 7. ICT and Stark spectra with calculated ﬁts for complex salts 6

the conjugated systems; this effect is especially evident in the

and 13 in an external electric ﬁeld of 5.36 × 10⁷V m^-1: (top panels)

f_osand µ₁₂values measured at 77 K (Table 6). It is, however,

worth noting that our previous Stark studies have shown that

replacing N-Ph with 2,4-DNPh substituents leads to increases

in ꢀ₀, in both metal complexes^42band purely organic chro-

mophores.⁴⁷Also in contrast with the HRS results (see above),

the estimated ∑ꢀ₀values for 1-13 taken as a whole do not

provide any clear indication that the nature of the metal center

substantially affects the NLO responses. Although ∑ꢀ₀appears

to decrease considerably on replacing Ru with Fe in two

instances (pairs 4/10 and 6/13), the differences are within the

experimental error limits.

It is not appropriate to attempt direct, quantitative comparisons

between HRS and Stark-derived ꢀ values, because of the

importance of resonance effects in HRS measurements and the

potential uncertainty about the prefactor in eq 5 (see above).

The challenges inherent in obtaining reliable ꢀ₀data, especially

from HRS studies, have been discussed previously.⁴⁸Unfortu-

nately, it is not possible to use this technique to derive anything

absorption spectra illustrating Gaussian curves used in data ﬁtting; (bottom

panels) electroabsorption spectra, with experimental data shown in blue

and ﬁts in green according to the Liptay equation.³³

often gives excellent quantitative agreement with the ꢀ₀values

determined via HRS with a 1064 nm laser.^32b

We have shown in several previous studies that while using

deconvolution is required to give satisfactory ﬁts to the Stark

data, the total [ꢀ₀]_ICTvalues that arise from such treatment are

generally very similar to those obtained without deconvolution.^41,46

These precedents conﬁrm the reliability of this indirect approach

to deriving ꢀ₀responses. While it is probably of little value to

consider the data for the individual Gaussian components in

detail due to the uncertainties involved in the spectral decon-

volution, the resulting total ꢀ₀responses do merit further

comments. First, sequential and dramatic increases are observed

on extending the conjugated systems in moving along the Ru

series 1 f 2B f 5 or the Fe series 7 f 8B f 11, with the

Me₂bbpb²⁺-containing chromophores having estimated total

MLCT-associated ꢀ₀values about 6 times larger than those of

their previously reported qpy-based counterparts. Moving from

11 to 13 does not change ∑ꢀ₀signiﬁcantly when considering

only the MLCT-based contributions, but the total ꢀ₀response

derived for 13 is increased almost 2-fold when the ILCT

(47) Coe, B. J.; Harris, J. A.; Asselberghs, I.; Wostyn, K.; Clays, K.;

Persoons, A.; Brunschwig, B. S.; Coles, S. J.; Gelbrich, T.; Light,

M. E.; Hursthouse, M. B.; Nakatani, K. AdV. Funct. Mater. 2003, 13,

347–357.

(48) Selected examples: (a) Woodford, J. N.; Wang, C. H.; Jen, A. K.-Y.

Chem. Phys. 2001, 271, 137–143. (b) Di Bella, S. New J. Chem. 2002,

26, 495–497. (c) Tai, O. Y.-H.; Wang, C. H.; Ma, H.; Jen, A. K.-Y.

J. Chem. Phys. 2004, 121, 6086–6092. (d) Campo, J.; Wenseleers,

W.; Goovaerts, E.; Szablewski, M.; Cross, G. H. J. Phys. Chem. C

2008, 112, 287–296.

(46) Coe, B. J.; Foxon, S. P.; Harper, E. C.; Harris, J. A.; Helliwell, M.;

Raftery, J.; Asselberghs, I.; Clays, K.; Franz, E.; Brunschwig, B. S.;

Fitch, A. G. Dyes Pigments 2009, 82, 171–186.

9

3510 J. AM. CHEM. SOC. VOL. 132, NO. 10, 2010

Tris-Chelate Metallochromophores

A R T I C L E S

Table 7. Cubic NLO Parameters for Complex Salts 1-13 at 750 nm

salt (M/L^A)

γ

_real, 10^-36esu

γ

_imag, 10^-36esu

|γ|, 10^-36esu

σ₂, GM^a

b

1 (Ru/Me₂qpy²⁺

)

-4300 ( 600

-3000 ( 1500

-5800 ( 1500

-9000 ( 1000

-12000 ( 2000

-24000 ( 6000

-7400 ( 800

-1700 ( 1000

-6400 ( 1500

-11000 ( 2000

-13000 ( 4000

-21000 ( 2000

-18000 ( 3000

220 ( 30

4300 ( 600

4000 ( 1500

5800 ( 1500

9900 ( 1000

13100 ( 2000

25600 ( 6000

7400 ( 800

2100 ( 1000

6400 ( 1500

11200 ( 2000

14100 ( 4000

21000 ( 2000

18700 ( 3000

62 ( 8

720 ( 90

120 ( 25

1200 ( 50

1500 ( 170

2500 ( 250

13 ( 4

350 ( 40

3.6 ( 2

600 ( 100

1500 ( 200

265 ( 30

1400 ( 100

2 (Ru/Me₂bbpe²⁺

)

2600 ( 300

420 ( 100

4200 ( 200

5200 ( 600

8900 ( 900

48 ( 15

b

3 (Ru/Ph₂qpy²⁺

)

4 (Ru/Ph₂bbpe²⁺

5 (Ru/Me₂bbpb²⁺

)

6 (Ru/Me₂bbpvb²⁺

)

b

7 (Fe/Me₂qpy²⁺

)

8 (Fe/Me₂bbpe²⁺

)

1200 ( 150

13 ( 6

2200 ( 400

5500 ( 700

950 ( 120

5100 ( 300

b

9 (Fe/Ph₂qpy²⁺

)

10 (Fe/Ph₂bbpe²⁺

11 (Fe/Me₂bbpb²⁺

12 (Fe/(2,4-DNPh)₂bbpe²⁺

13 (Fe/Me₂bbpvb²⁺

)

^a1 GM ) 10^-50cm⁴s photon^-1

.

^bData taken from ref 16.

more than resonance-enhanced ꢀ values for compounds that

display more than one ICT band. Therefore, the Stark-derived

ꢀ₀data we report for 1-13 give the most accurate description

of the (relative) NLO responses of this series of chromophores.

However, it should also be emphasized that the ꢀ₀values for

most of the new complex chromophores are only lower limits,

since they do not account for contributions associated with the

ILCT transitions. In Ru^IIammine complexes of monodentate

ligands related to Me₂bbpb²⁺, the ILCT processes account for

ca. 10-20% of the total estimated ꢀ₀values.^21,41Given that

the tris-chelate derivatives contain six individual π-conjugated

“arms”, the total ILCT contributions to ꢀ₀will be larger when

compared with related 1D or 2D systems. Nevertheless, the

MLCT-associated responses achieved of ca. (400-600) × 10^-30

esu for 4, 5, 10, and 11 are among the largest (off-resonance)

quadratic NLO responses determined for metal-containing

chromophores. Furthermore, the ∑ꢀ₀values for 6 and 13 are

possibly the highest yet recorded. These results serve to highlight

the utility of the Stark-based approach, which is the only method

by which such data for chromophores with relatively compli-

cated ICT spectra may be obtained. In order to set these results

in context, it is worth considering data obtained by applying

the Stark-based approach to a benchmark chromophore, under

the same experimental conditions and using eq 5. The salt (E)-

4′-(dimethylamino)-N-methyl-4-stilbazolium (DAS) tosylate⁴⁹

is the ﬁrst organic NLO material to become commercialized

(for THz wave generation via nonlinear frequency mixing). We

have derived previously a ꢀ₀value of 236 × 10^-30esu for

[DAS]PF₆,⁴⁷substantially smaller than most of those determined

for the new compounds (Table 6).

Figure 8. Examples of open-aperture Z-scans on 11 in DMF solutions at

750 nm. Full lines denote theoretically computed curves.

ﬁrst hyperpolarizabilities ꢀ_EN,

⁵⁰which are derived by consider-

ing the total number of π-bonding electrons. We have therefore

calculated ꢀ_ENvalues for 1-13, and these are included in Table

6. These data conﬁrm substantial enhancements in the NLO

response on moving along the series 1 f 2B f 5 and 7 f 8B

f 11. Replacing R ) Me with R ) Ph gives lower ꢀ_ENvalues

for the qpy-based species but higher values for their bbpe-based

counterparts. The series 8B f 10 f 12B shows an increase in

ꢀ_ENon moving from R ) Me to R ) Ph, but then a decrease

for the 2,4-DNPh-containing complex. The application of this

approach to the Stark-derived ꢀ₀values for stilbazolium and

closely related organic chromophores^46,47also indicates that the

ꢀ_ENresponses of N-Ph species are larger than those of their

2,4-DNPh analogues.⁵¹

Two-Photon Absorption Studies. The new complex salts 2,

4-6, 8, and 10-13 were investigated in DMF solutions by using

the Z-scan technique^34,35at a laser wavelength of 750 nm, to

afford the real and imaginary parts of γ together with the 2PA

cross sections σ₂. The results are collected in Table 7, along

with the data published previously for 1, 3, 7, and 9.¹⁶Figure

8 shows an example set of open-aperture Z-scans on 11 showing

the concentration dependence of the dip due to 2PA. The curves

are shifted due to the presence of some one-photon absorption

in solutions of 11 at 750 nm.

It is well established that hyperpolarizabilities generally

increase with the size of the π-conjugated system.¹In attempts

to assess the extent to which quadratic NLO responses are also

inﬂuenced by other factors and to probe their fundamental

quantum limits, recent studies with purely organic dipolar

chromophores have included electron-normalized off-resonant

(49) Selected examples: (a) Kawase, K.; Mizuno, M.; Sohma, S.; Takahashi,

H.; Taniuchi, T.; Urata, Y.; Wada, S.; Tashiro, H.; Ito, H. Opt. Lett.

1999, 24, 1065–1067. (b) Kawase, K.; Hatanaka, T.; Takahashi, H.;

Nakamura, K.; Taniuchi, T.; Ito, H. Opt. Lett. 2000, 25, 1714–1716.

(c) Taniuchi, T.; Okada, S.; Nakanishi, H. Appl. Phys. Lett. 2004, 95,

5984–5988. (d) Taniuchi, T.; Ikeda, S.; Okada, S.; Nakanishi, H. Jpn.

J. Appl. Phys. 2005, 44, L652–L654. (e) Schneider, A.; Neis, M.;

Stillhart, M.; Ruiz, B.; Khan, R. U. A.; Gu¨nter, P. J. Opt. Soc. Am. B

2006, 23, 1822–1835. (f) Schneider, A.; Stillhart, M.; Gu¨nter, P. Opt.

Express 2006, 14, 5376–5384. (g) Yang, Z.; Mutter, L.; Stillhart, M.;

Ruiz, B.; Aravazhi, S.; Jazbinsek, M.; Schneider, A.; Gramlich, V.;

Gu¨nter, P. AdV. Funct. Mater. 2007, 17, 2018–2023.

All of the complex salts 1-13 show relatively strong negative

refractive nonlinearity, as indicated by their γ_realvalues (Table

(50) (a) Kuzyk, M. G. Phys. ReV. Lett. 2000, 85, 1218–1221. (b) Kuzyk,

M. G. Phys. ReV. Lett. 2003, 90, 039902. (c) Tripathy, K.; Pe´rez

Moreno, J.; Kuzyk, M. G.; Coe, B. J.; Clays, K.; Myers Kelley, A.

J. Chem. Phys. 2004, 121, 7932–7945.

9

J. AM. CHEM. SOC. VOL. 132, NO. 10, 2010 3511

A R T I C L E S

Coe et al.

Figure 9. Chemical structures of some complex chromophores investigated for 2PA purposes previously.¹²

7). Such behavior is typical for molecules probed at wavelengths

in the vicinity of strong one- or two-photon absorptions. Several

trends are evident in the 2PA data. (i) Extending the conjugated

systems of the ligands leads to large increases in activity, as

expected on the basis of previous reports concerning purely

organic chromophores. On moving along the Ru series 1 f 2

f 5, a total σ₂enhancement of ca. 24-fold is observed, while

the corresponding increase for the Fe series 7 f 8 f 11 is ca.

115-fold. (ii) Replacing R ) Me with R ) Ph causes an

approximate doubling of σ₂in three out of four instances (with

9 providing the exception). However, the 2PA activity of the

2,4-DNPh derivative 12 is only about half that of 10. (iii) The

previously reported Fe compounds 7 and 9 show only very small

σ₂values, while their Ru analogues 1 and 3 are much more

efﬁcient two-photon absorbers.¹⁶However, the data for the new

chromophores show that the effect of the metal center becomes

less signiﬁcant as the ligand π-systems are extended. Hence,

the Ru bbpe-based species 2 and 4 have σ₂values about twice

those of their Fe counterparts 8 and 10. The Me₂bbpb²⁺

complexes in 5 and 11 have indistinguishable 2PA efﬁciencies,

while that of 6 is substantially larger than that of 13.

and ILCT transitions (the latter involving strong amino donors

as in 14), as opposed to codirectional MLCT and ILCT

processes (the latter involving weak pyridyl donors as in 2).

However, it is probably inappropriate to speculate further

without more extensive studies involving variations in the Z-scan

measurement wavelength. Despite the relatively impressive σ₂

values for these new and related complexes, other studies with

Cu^I-, Cd^II-, or Zn^II-based systems have afforded considerably

larger activities, exceeding even 10⁴GM.⁵²A similarly huge

σ₂value has recently been reported for a dendrimeric nona-

nuclear Ru^IIσ-acetylide complex,⁵³and appropriately substituted

Zn^IIporphyrin complexes, with their extensive π-systems, are

also among the most active 2PA chromophores known.⁵⁴

Conclusions

We have synthesized several new bpy-based proligands and

used these to extend the family of pyridinium-substituted tris-

chelate complexes of Ru^IIor Fe^II. As the conjugated systems

extend, the presence of intense, low-energy ILCT bands

becomes more apparent in the electronic absorption spectra. The

Our new complex chromophores clearly show much larger

σ₂values in comparison with their previously reported coun-

terparts, but it is also of interest to draw some comparisons with

data obtained for other species. Girardot et al. have used two-

photon excited luminescence measurements to determine a σ₂

value of 90 GM (at 750 nm) for a derivative of [Ru^II(phen)₃]²⁺

with six ﬂuorenyl substituents (14; Figure 9).^12aThe relatively

low efﬁciency in this case is probably attributable to limited

π-conjugation within the ligands due to twisting about the

interannular bonds. Nonetheless, a complex related to 14, with

σ₂) 40 GM at 740 nm, has recently been studied in vitro as a

singlet oxygen generator for potential applications in photody-

namic cancer therapy.^12cSome of us have also used the Z-scan

approach to derive much larger σ₂values of 2200 ( 300 and

1900 ( 300 GM (at 765 nm) for the complexes 15 and 16

(Figure 9), respectively,^12bwhich are of magnitude similar to

that reported here for 6 (Table 7). It is interesting to note that

although 15 contains shorter conjugated systems, its 2PA activity

is close to that measured for 6. Comparison with the σ₂value

of 720 GM for 2 indicates that replacing an electron-accepting

N-methylpyridinium group with a di-n-butylamino donor en-

hances the 2PA activity approximately 3-fold. It therefore

appears that such tris-chelate complexes give larger 2PA effects

when these are associated with directionally opposed MLCT

(51) The ꢀ_ENvalues (10^-32esu) for (E)-4′-(dimethylamino)-N-phenyl-4-

stilbazolium hexaﬂuorophosphate and its 2,4-dinitrophenyl counterpart

are 322.0 and 307.9, respectively. For the corresponding (E,E)-1,3-

butadienyl-containing compounds, the respective values are 602.8 and

532.5. Changing the electron donor to a julolidinyl group gives a

similar pattern in the ꢀ_ENvalues (10^-32esu): 420.4 (R ) Ph) and 398.0

(R ) 2,4-DNPh) for (E)-N-R-4-[2-(2,3,6,7-tetrahydro-1H,5H-py-

rido[3,2,1-ij]quinolin-9-yl)vinyl]pyridinium hexaﬂuorophosphate and

680.3 (R ) Ph) and 663.0 (R ) 2,4-DNPh) for the corresponding

(E,E)-1,3-butadienyl compounds. Note also that the ꢀ_ENvalues for

these pseudo-1D organic species are about 1 order of magnitude larger

than those derived for the 3D tris-chelate complexes (Table 6).

(52) (a) Das, S.; Nag, A.; Goswami, D.; Bharadwaj, P. K. J. Am. Chem.

Soc. 2006, 128, 402–403. (b) Jana, A.; Jang, S. Y.; Shin, J.-Y.; Kumar

De, A.; Goswami, D.; Kim, D.; Bharadwaj, P. K. Chem. Eur. J. 2008,

14, 10628–10638.

(53) Roberts, R. L.; Schwich, T.; Corkery, T. C.; Cifuentes, M. P.; Green,

K. A.; Farmer, J. D.; Low, P. J.; Marder, T. B.; Samoc, M.; Humphrey,

M. G. AdV. Mater. 2009, 21, 2318–2322.

(54) (a) Drobizhev, M.; Stepanenko, Y.; Rebane, A.; Wilson, C. J.; Screen,

T. E. O.; Anderson, H. L. J. Am. Chem. Soc. 2006, 128, 12432–12433.

(b) Kim, K. S.; Noh, S. B.; Katsuda, T.; Ito, S.; Osuka, A.; Kim, D.-

H. Chem. Commun. 2007, 2479–2481. (c) Collini, E.; Mazzucato, S.;

Zerbetto, M.; Ferrante, C.; Bozio, R.; Pizzotti, M.; Tessore, F.; Ugo,

R. Chem. Phys. Lett. 2008, 454, 70–74. (d) Odom, S. A.; et al. J. Am.

Chem. Soc. 2009, 131, 7510–7511. (e) Webster, S.; Odom, S. A.;

Padilha, L. A.; Przhonska, O. V.; Peceli, D.; Hu, H.-H.; Nootz, G.;

Kachkovski, A. D.; Matichak, J.; Barlow, S.; Anderson, H. L.; Marder,

S. R.; Hagan, D. J.; Van Stryland, E. W. J. Phys. Chem. B 2009, 113,

14854–14867.

9

3512 J. AM. CHEM. SOC. VOL. 132, NO. 10, 2010

Tris-Chelate Metallochromophores

A R T I C L E S

MLCT bands show red shifts on extending the π-conjugation,

increasing the electron acceptor strength of the pyridinium

groups, and/or replacing Ru with Fe. Cyclic voltammetry reveals

reversible M^III/IIwaves, but generally irreversible ligand-based

reductions. The ꢀ responses determined by via HRS measure-

ments at 800 and 1064 nm are very large (up to 5600 × 10^-30

esu), but are strongly enhanced by resonance. Stark spectro-

scopic studies on the MLCT bands lead to estimated ꢀ₀values

as high as ca. 600 × 10^-30esu in one instance, which represent

lower limits because the contributions associated with the ILCT

bands are not taken into account in most cases. For the two

compounds where the ILCT contributions are also amenable to

Stark analysis, potentially record ꢀ₀values as high as ca. 10^-27

esu are determined. These indirectly derived NLO responses

generally increase on extending the π-conjugation and increasing

the electron-accepting strength of the ligands, and are much

larger than those determined previously for related complexes.¹⁵

However, no clear superiority of either Ru or Fe is evident.

The measured dipole moment changes can be as large as ca. 34

D and show the expected dependence on conjugation path

length. Z-scan measurements at 750 nm reveal high σ₂values

of up to 2500 GM; these are slightly larger than or similar to

those measured for related electron-donor-substituted com-

plexes,^12bbut are as much as an order of magnitude higher than

the activities we have determined previously for pyridinium

derivatives.¹⁶The new chromophores reported are therefore

relatively rare examples of species that combine unusually high

quadratic and cubic NLO effects with potentially redox-

switchable transition metal centers.

Acknowledgment. We thank the EPSRC for support (grants

EP/D070732 and EP/E000738) and also the Fund for Scientiﬁc

Research-Flanders (FWO-V, G.0312.08), the University of Leuven

(GOA/2006/3), the NSF (grant CHE-0802907, ‘Powering the Planet:

an NSF Center for Chemical Innovation’) and the Foundation for

Polish Science. I.A. is a postdoctoral fellow of the FWO-V and

M.S. is a Laureate of the FNP Welcome programme.

Supporting Information Available: Crystallographic informa-

tion in CIF format; Figures S1-S3; complete ref 54d (PDF).

This material is available free of charge via the Internet at http://

pubs.acs.org.

JA910538S

9

J. AM. CHEM. SOC. VOL. 132, NO. 10, 2010 3513

Article Doi

Combining very large quadratic and cubic nonlinear optical responses in extended, Tris-chelate metallochromophores with Six π-conjugated pyridinium substituents

DOI: 10.1021/ja910538s

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Article abstract of DOI:10.1021/ja910538s

Full text of DOI:10.1021/ja910538s

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