Quadratic and cubic nonlinear optical properties of salts of diquat-based chromophores with diphenylamino substituents

Article abstract of DOI:10.1021/jp106473e

A series of chromophoric salts has been prepared in which 4-(diphenylamino)phenyl (Dpap) electron donor groups are connected to electron-accepting diquaternized 2,2′-bipyridyl (diquat) units. The main aim is to combine large quadratic and cubic nonlinear

Full text of DOI:10.1021/jp106473e

12028
J. Phys. Chem. A 2010, 114, 12028–12041
Quadratic and Cubic Nonlinear Optical Properties of Salts of Diquat-Based Chromophores
with Diphenylamino Substituents
Benjamin J. Coe,*,† John Fielden,† Simon P. Foxon,† Madeleine Helliwell,†
Bruce S. Brunschwig,‡ Inge Asselberghs,§ Koen Clays,§ Joanna Olesiak,|
Katarzyna Matczyszyn,| 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, United States, Department of Chemistry, UniVersity of LeuVen,
Celestijnenlaan 200D, B-3001 LeuVen, Belgium, Institute of Physical and Theoretical Chemistry, Wrocław
UniVersity of Technology, Wybrzez˙e Wyspian´skiego 27, 50-370 Wrocław, Poland and Laser Physics Centre,
Research School of Physics and Engineering, Australian National UniVersity, Canberra ACT 0200, Australia
ReceiVed: July 13, 2010
A series of chromophoric salts has been prepared in which 4-(diphenylamino)phenyl (Dpap) electron donor
groups are connected to electron-accepting diquaternized 2,2′-bipyridyl (diquat) units. The main aim is to
combine large quadratic and cubic nonlinear optical (NLO) effects in potentially redox-switchable molecules
with 2D structures. The chromophores have been characterized as their PF6- salts by using various techniques
including electronic absorption spectroscopy and cyclic voltammetry. The visible absorption spectra are
dominated by intense π f π* intramolecular charge-transfer (ICT) bands, and all of the compounds show
two reversible or quasireversible diquat-based reductions and partially reversible Dpap oxidations. Single
crystal X-ray structures have been obtained for one salt and for the precursor compound (E)-4-
(diphenylamino)cinnamaldehyde, both of which adopt centrosymmetric space groups. First hyperpolarizabilities
ꢀ have been measured by using hyper-Rayleigh scattering (HRS) with a 800 nm laser, and Stark
(electroabsorption) spectroscopy of the ICT bands affords estimated static first hyperpolarizabilities ꢀ0. The
directly and indirectly derived ꢀ values are large and generally increased substantially for the bis-Dpap
derivatives when compared with their monosubstituted analogues. Polarized HRS studies show that the NLO
responses of the disubstituted species are dominated by “off-diagonal” ꢀzyy components. Lengthening the
diquaternizing alkyl unit lowers the electron-acceptor strength and therefore increases the ICT energies and
decreases the E1/2 values for diquat reduction. However, compensating increases in the ICT intensity prevent
significant decreases in the Stark-based ꢀ0 responses. Cubic NLO properties have been measured by using
the Z-scan technique over a wavelength range of 520-1600 nm, revealing relatively high two-photon absorption
cross-sections of up to 730 GM at 620 nm for one of the disubstituted chromophores.
1. Introduction
From this perspective, special attention has been given to
compounds such as (E)-4′-(dimethylamino)-N-methyl-4-stilba-
zolium tosylate (DAST)8 and related species.9 Notably, crystals
of DAST have recently been commercialized for THz wave
generation via nonlinear frequency mixing,10 promising ap-
plications including security scanning, biomedical analysis, and
space communications.11
Organic compounds containing pyridinium units are of
longstanding interest from a diverse range of perspectives,
including applications in electronic/optical devices.1 Such
compounds are generally synthetically readily accessible, highly
stable, and often combine intense absorptions in the visible
region (thus dye-like behavior) with reversible electrochemical
properties. Just some of the recent interesting studies with such
compounds have included observations of aggregation-induced
emission,2 excited-state inclusion into zeolite channels,3 mul-
tielectron redox,4 ferroelectricity in organic-inorganic hybrid
materials,5 and fluorescent DNA probes for live-cell imaging.6
However, one of the areas that has proven most fruitful for
exploitation of pyridinium salts is that of nonlinear optical
(NLO) effects, which is of relevance to various applications
such as advanced telecommunications and biological imaging.7
Molecular NLO phenomena are divided into two major
classes. So-called quadratic (second-order) behavior arises from
the first hyperpolarizability ꢀ that translates into the susceptibil-
ity ꢁ(2) in bulk materials. In order to achieve nonzero values of
both ꢀ and ꢁ(2), the structure must be noncentrosymmetric at
the molecular and macroscopic levels. Second harmonic genera-
tion (SHG) and linear electro-optic behavior are the most widely
studied quadratic NLO effects. The other main class of property,
cubic (third-order) NLO behavior, is governed by the second
hyperpolarizability γ. The bulk counterpart of γ is ꢁ(3), and while
this parameter has nonzero values for any symmetry, its
magnitude is affected by the macroscopic structure of a material.
Potentially useful for applications including optical power
limiting, 3D optical data storage, and photodynamic cancer
therapy, two-photon absorption (2PA) derives from the γ
* To whom correspondence should be addressed. Fax: 44 161-275-4598.
E-mail: b.coe@manchester.ac.uk.
† University of Manchester.
‡ California Institute of Technology.
§ University of Leuven.
| Wrocław University of Technology.
# Australian National University.
10.1021/jp106473e  2010 American Chemical Society
Published on Web 10/26/2010

Nonlinear Optical Properties of Chromophoric Salts
J. Phys. Chem. A, Vol. 114, No. 45, 2010 12029
response.12 Molecules with large ꢀ values typically feature
π-electron donor (π-ED) and acceptor (π-EA) groups linked
via a polarizable π-system, the π-EA function being the
pyridinium rings in compounds like DAST. In principle, cubic
NLO effects can arise from any material, but the primary
requirement for a large γ value is simply an extended π-system.
Hyperpolarizabilities are third- (ꢀ) or fourth-rank (γ) tensors
that can have a number of nonzero components. Although most
NLO molecules are simple 1D dipoles, multidimensional species
including 2D dipoles and octupoles are also highly interesting.13,14
Such chromophores can offer significant potential advantages
over their 1D counterparts, including enhanced NLO responses
without undesirable losses of visible transparency. In V-shaped
species, having more than one significant component of ꢀ can
prevent reabsorption of SHG that can be polarized perpendicular
to the π f π* intramolecular charge-transfer (ICT) transition
dipole moment (µ12); phase-matching between the fundamental
and harmonic waves may also be facilitated.13a
We have previously studied salts of chromophores containing
diquaternized 2,2′-bipyridyl (diquat, DQ2+) units.15 Except for
one brief theoretical report relating to 2PA,16 DQ2+ groups had
not been used for NLO purposes previously. Such tricyclic
moieties are stronger π-EAs than simple pyridinium groups,
allow the creation of 2D dipoles and show reversible reductions
that may allow redox-switching of the optical properties. Our
previous reports concerned only the quadratic NLO properties
of chromophores with 4-(dimethylamino)phenyl (Dmap) or
julolidinyl (Jd) π-ED groups, showing very large and strongly
2D ꢀ responses.15 Here we describe a series of closely related
compounds that contain instead 4-(diphenylamino)phenyl (Dpap)
groups, including measurements of both quadratic and cubic
NLO effects. We chose to focus on these triarylamine derivatives
for 2PA studies because they contain larger π-systems when
compared with their Dmap or Jd analogues.
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. Aceto-
nitrile was freshly distilled (from CaH2), and [N(C4H9-n)4]PF6,
as supplied from Fluka, was used as the supporting electrolyte.
Solutions containing ca. 10-3 M analyte (0.1 M electrolyte) were
deaerated by purging with N2. All E1/2 values were calculated
from (Epa + Epc)/2 at a scan rate of 200 mV s-1
.
2.3. Synthesis of (E)-4-(Diphenylamino)cinnamaldehyde,
Dpaca. 4-(Diphenylamino)benzaldehyde (500 mg, 1.83 mmol)
and 1,3-dioxan-2-yl-tributylphosphonium bromide (3.0 mL, 1.2
M, 3.60 mmol) were dissolved in anhydrous THF (30 mL).
Sodium hydride (125 mg, 5.21 mmol) was added with a catalytic
amount of 18-crown-6, and the suspension was stirred for 22 h
at room temperature. Water (25 mL) was added to the mixture,
and the mixture was extracted with diethyl ether (3 × 25 mL).
The extracts were combined and concentrated under vacuum
to afford a yellow oil. This material was dissolved in THF (25
mL), 10% hydrochloric acid (25 mL) was added, and the
solution was stirred for 1 h at room temperature. The solution
was extracted with dichloromethane (3 × 25 mL), and the
extracts were combined and concentrated under vacuum to give
a yellow solid that was purified via column chromatography
on silica gel with dichloromethane as eluant. The major yellow
band was collected, evaporated to dryness under vacuum and
dried to give a golden powder: 507 mg, 91%; δH (500 MHz,
CDCl3) 9.64 (1 H, d, J ) 8.0 Hz, CHO), 7.41-7.37 (3 H, CH
+ C6H4), 7.33-7.30 (4 H, Ph), 7.15-7.11 (6 H, Ph), 7.01 (2
H, d, J ) 8.5 Hz, C6H4), 6.59 (1 H, dd, J ) 15.8, 7.8 Hz, CH).
Anal. Calcd. (%) for C21H17NO·0.2H2O: C, 83.25; H, 5.79; N,
4.62. Found: C, 83.54; H, 5.92; N, 4.52. m/z ) 300.0 (MH+).
Note: the duration of the column chromatography affects the
isolated yield, which is optimized by using a relatively short
silica path length (ca. 8-10 cm) and a rapid elution rate. The
loss of product observed when allowing the material to remain
on the silca gel for prolonged periods may be attributable to
facile aerial oxidation to the corresponding carboxylic acid that
is expected to have an especially high affinity for oxide surfaces.
2.4. Synthesis of 4-[(E)-2-(4-Diphenylaminophenyl)ethe-
nyl]-4′-methyl-2,2′-bipyridyl (1). Potassium tert-butoxide (117
mg, 1.043 mmol) was added to a solution of 4-[(diethoxyphos-
phinyl)methyl]-4′-methyl-2,2′-bipyridyl (240 mg, 0.749 mmol)
and 4-(diphenylamino)benzaldehyde (205 mg, 0.750 mmol)
stirred in THF (20 mL), resulting in near instantaneous
production of a yellow precipitate. After stirring at room
temperature for 5 h, water (20 mL) was added to the mixture,
and the THF was removed under vacuum. The yellow product
was filtered off, washed with cold water, and dried: 293 mg,
88%; δH (500 MHz, CDCl3) 8.62 (1 H, d, J ) 5.4 Hz, C5H3N),
8.58 (1 H, d, J ) 4.7 Hz, C5H3N), 8.49 (1 H, s, C5H3N), 8.27
(1 H, s, C5H3N), 7.44-7.38 (3 H, C6H4 + CH), 7.35 (1 H, d,
J ) 5.0 Hz, C5H3N), 7.29 (4 H, t, J ) 7.4 Hz, Ph), 7.17-7.13
(5 H, C5H3N + Ph), 7.09-7.05 (4 H, C6H4 + Ph), 7.01 (1 H,
2. Experimental Section
2.1. Materials and Procedures. Dichloromethane and ac-
etonitrile were dried over CaH2 and distilled under argon. All
other reagents and solvents were obtained as ACS grade from
Sigma Aldrich, Alfa Aesar, or Fisher Scientific and were used
as supplied. The compounds 4-[(diethoxyphosphinyl)methyl]-
4′-methyl-2,2′-bipyridyl,17 4,4′-bis-[(diethoxyphosphinyl)methyl]-
2,2′-bipyridyl,18 1,2-bis(triflyloxy)ethane,19 and 1,3-bis(trifly-
loxy)propane19 were synthesized by following previously
published methods. Products were dried at room temperature
overnight in a vacuum desiccator (CaSO4) prior to characteriza-
tion.
2.2. General Physical Measurements. 1H NMR spectra
were recorded on Bruker UltraShield 500, AV-400, or DPX-
300 spectrometers, 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. Abbreviations
used: ax ) axial; eq ) equatorial. 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
+electrospray on a Micromass Platform II spectrometer. The
IR spectrum of Dpaca was obtained by using the pure solid on
a Varian Bio-Rad Excalibur series spectrometer. Cyclic volta-
mmetric measurements were performed by using an EG&G PAR
model 283, Autolab PGStat 100 or Ivium CompactStat poten-
tiostat/galvanostat. A single-compartment cell was used with a
d, J ) 16.4 Hz, CH), 2.38 (3 H, s, Me). ν(CdO) 1664s cm-1
.
Anal. Calcd (%) for C31H25N3 ·0.25H2O: C, 83.85; H, 5.79; N,
9.46. Found: C, 84.21; H, 5.73; N, 9.11. m/z ) 440.2 (MH+).
2.5. Synthesis of 4-[(E,E)-4-(4-Diphenylaminophenyl)buta-
1,3-dienyl]-4′-methyl-2,2′-bipyridyl (2). This compound was
prepared in a manner similar to 1 by using potassium tert-
butoxide (93 mg, 0.829 mmol), 4-[(diethoxyphosphinyl)methyl]-
4′-methyl-2,2′-bipyridyl (190 mg, 0.593 mmol), and Dpaca (178
mg, 0.593 mmol) in place of 4-(diphenylamino)benzaldehyde.
The product was obtained as a golden solid: 222 mg, 78%; δH
(500 MHz, CD3CN + CF3CO2D) 8.64 (1 H, d, J ) 5.4 Hz,

12030 J. Phys. Chem. A, Vol. 114, No. 45, 2010
Coe et al.
C5H3N), 8.56 (1 H, d, J ) 6.0 Hz, C5H3N), 8.44 (1 H, s, C5H3N),
8.33 (1 H, s, C5H3N), 7.79 (1 H, d, J ) 6.3 Hz, C5H3N), 7.73-
7.66 (1 H, m, CH), 7.61 (1 H, d, J ) 6.0 Hz, C5H3N), 7.47 (2
H, d, J ) 7.9 Hz, C6H4), 7.34 (4 H, t, J ) 7.9 Hz, Ph),
7.15-7.09 (6 H, Ph), 7.08-7.00 (2 H, 2CH), 6.96 (2 H, d, J )
7.9 Hz, C6H4), 6.80 (1 H, d, J ) 15.1 Hz, CH), 2.59 (3 H, s,
Me). Anal. Calcd (%) for C33H27N3 ·0.7H2O: C, 82.88; H, 5.99;
N, 8.79. Found: C, 82.93; H, 5.88; N, 8.44. m/z ) 466.2 (MH+).
2.6. Synthesis of 4,4′-Bis-[(E)-2-(4-diphenylaminophe-
nyl)ethenyl]-2,2′-bipyridyl (3). This compound was prepared
in a manner similar to 1 by using potassium tert-butoxide (312
mg, 2.78 mmol), 4,4′-bis-[(diethoxyphosphinyl)methyl]-2,2′-
bipyridyl (500 mg, 1.10 mmol) in place of 4-[(diethoxyphos-
phinyl)methyl]-4′-methyl-2,2′-bipyridyl, and 4-(diphenylami-
no)benzaldehyde (599 mg, 2.19 mmol) to afford a golden solid:
760 mg, 97%; δH (500 MHz, CDCl3) 8.65 (2 H, d, J ) 5.4 Hz,
C5H3N), 8.52 (2 H, s, C5H3N), 7.44 (4 H, d, J ) 8.5 Hz, 2C6H4),
7.42 (2 H, d, J ) 16.1 Hz, 2CH), 7.38 (2 H, d, J ) 5.0 Hz,
C5H3N), 7.29 (8 H, t, J ) 7.9 Hz, Ph), 7.14 (8 H, d, J ) 7.6
Hz, Ph), 7.10-7.06 (8 H, 2C6H4 + Ph), 7.02 (2 H, d, J ) 16.1
Hz, 2CH). Anal. Calcd (%) for C50H38N4 ·H2O: C, 84.24; H,
5.66; N, 7.86. Found: C, 84.37; H, 5.75; N, 7.55.
2.7. Synthesis of 4,4′-Bis-[(E,E)-4-(4-diphenylaminophe-
nyl)buta-1,3-dienyl]-2,2′-bipyridyl (4). This compound was
prepared in a manner similar to 3 by using Dpaca (656 mg,
2.19 mmol) in place of 4-(diphenylamino)benzaldehyde. The
mixture was washed with a small amount of methanol, and a
golden solid was obtained: 737 mg, 88%; δH (500 MHz, CDCl3)
8.63 (2 H, d, J ) 5.4 Hz, C5H3N), 8.44 (2 H, s, C5H3N), 7.35
(4 H, d, J ) 8.5 Hz, 2C6H4), 7.32-7.27 (12 H, Ph + 2CH +
C3H5N), 7.13 (8 H, d, J ) 8.2 Hz, Ph), 7.08-7.02 (8 H, 2C6H4
+ Ph), 6.92-6.85 (2 H, m, 2CH), 6.76 (2 H, d, J ) 16.1 Hz,
2CH), 6.64 (2 H, J ) 15.5 Hz, 2CH). Anal. Calcd (%) for
C54H42N4 ·H2O: C, 84.79; H, 5.80; N, 7.32. Found: C, 84.83;
H, 5.70; N, 7.36.
2.8. Synthesis of 2-Methyl-11-[(E)-2-(4-diphenylaminophe-
nyl)ethenyl]-6,7-dihydro-dipyrido[1,2-a:2′,1′-c]pyrazinediium
Hexafluorophosphate (5). To a solution of 1·0.25H2O (164
mg, 0.369 mmol) in chloroform (20 mL) was added 1,2-bis(triflyl-
oxy)ethane (242 mg, 0.742 mmol) in dry dichloromethane (2
mL). The reaction was stirred at room temperature for 48 h,
during which time the solution color changed to deep purple
and a dark precipitate formed. The volume was reduced to ca.
5 mL under vacuum, and diethyl ether was added to the mixture
to ensure complete precipitation of the crude triflate salt, which
was then filtered off and washed with diethyl ether. This solid
was dissolved in methanol, and aqueous NH4PF6 was added to
precipitate the product, which was then filtered off and washed
with water. The dark purple product was purified by reprecipi-
tation from acetonitrile/diethyl ether, filtered off, washed with
diethyl ether, and dried: 175 mg, 62%; δH (500 MHz, CD3CN)
8.71 (1 H, d, J ) 6.3 Hz, C5H3N), 8.69 (1 H, s, C5H3N), 8.67
(1 H, s, C5H3N), 8.56 (1 H, d, J ) 6.3 Hz, C5H3N), 8.07 (1 H,
d, J ) 6.6 Hz, C5H3N), 8.05-8.01 (2 H, C5H3N + CH), 7.63
(2 H, d, J ) 8.8 Hz, C6H4), 7.40 (4 H, t, J ) 7.5 Hz, Ph),
7.28-7.18 (7 H, CH + Ph), 7.00 (2 H, d, J ) 8.8 Hz, C6H4),
5.00-4.97 (2 H, m, CH2), 4.91-4.88 (2 H, m, CH2), 2.83 (3
H, s, Me). Anal. Calcd (%) for C33H29F12N3P2 ·0.5H2O: C, 51.70;
H, 3.94; N, 5.48. Found: C, 51.70; H, 3.63; N, 5.45. m/z )
612.4 ([M - PF6]+), 234.1 ([M - 2PF6]2+).
mg, 0.209 mmol) in place of 1·0.25H2O with 1,2-bis(triflyl-
oxy)ethane (140 mg, 0.430 mmol). Purification of the crude
product (103 mg) was effected by column chromatography on
silica gel, eluting with 48:6:1 acetone/water/saturated aqueous
KNO3. The major purple band was collected, and the addition
of aqueous NH4PF6 afforded a precipitate that was filtered off,
washed with water, and dried. Further purification was achieved
by reprecipitation from acetonitrile/diethyl ether to afford a dark
purple solid: 35 mg, 21%; δH (500 MHz, CD3CN) 8.72 (1 H,
d, J ) 6.3 Hz, C5H3N), 8.69 (1 H, s, C5H3N), 8.60 (1 H, s,
C5H3N), 8.55 (1 H, d, J ) 6.6 Hz, C5H3N), 8.07 (1 H, d, J )
6.3 Hz, C5H3N), 8.00 (1 H, d, J ) 6.6 Hz, C5H3N), 7.94-7.88
(1 H, m, CH), 7.51 (2 H, d, J ) 8.8 Hz, C6H4), 7.36 (4 H, t, J
) 8.0 Hz, Ph), 7.18-7.11 (8 H, 2CH + Ph), 6.96 (2 H, d, J )
8.8 Hz, C6H4), 6.85 (1 H, d, J ) 15.5 Hz, CH), 4.99-4.96 (2
H, m, CH2), 4.90-4.87 (2 H, m, CH2), 2.80 (3 H, s, Me). Anal.
Calcd (%) for C35H31F12N3P2: C, 53.65; H, 3.99; N, 5.36. Found:
C, 53.49; H, 3.58; N, 5.20. m/z ) 638.3 ([M - PF6]+), 246.9
([M - 2PF6]2+).
2.10. Synthesis of 2-Methyl-12-[(E)-2-(4-diphenylami-
nophenyl)ethenyl]-7,8-dihydro-6H-dipyrido[1,2-a:2′,1′-c]-
[1,4]diazepinium Hexafluorophosphate (7). This compound
was prepared and purified in manner similar to 5 by using
1·0.25H2O (100 mg, 0.225 mmol) with 1,3-bis(triflyloxy)pro-
pane (155 mg, 0.456 mmol) in place of 1,2-bis(triflyloxy)ethane
to afford a dark purple solid: 90 mg, 52%; δH (300 MHz,
CD3CN) 8.76 (1 H, d, J ) 6.4 Hz, C5H3N), 8.59 (1 H, d, J )
6.6 Hz, C5H3N), 8.23 (1 H, s, C5H3N), 8.21 (1 H, s, C5H3N),
8.13-8.05 (2 H, C5H3N), 7.94 (1 H, d, J ) 16.0 Hz, CH), 7.61
(2 H, d, J ) 8.7 Hz, C6H4), 7.40 (4 H, t, J ) 7.2 Hz, Ph), 7.26
(1 H, d, J ) 15.4 Hz, CH), 7.22-7.17 (6 H, Ph), 6.98 (2 H, d,
J ) 8.7 Hz, C6H4), 4.83-4.73 (1 H, m, NCH2eq), 4.72-4.62 (1
H, m, NCH2eq), 4.42-4.29 (1 H, m, NCH2ax), 4.23-4.11 (1 H,
m, NCH2ax), 2.79-2.68 (5 H, Me + CH2). Anal. Calcd (%) for
C34H31F12N3P2: C, 52.93; H, 4.05; N, 5.45. Found: C, 52.68; H,
3.69; N, 5.22. m/z ) 626.3 ([M - PF6]+), 240.9 ([M - 2PF6]2+).
2.11. Synthesis of 2-Methyl-12-[(E,E)-4-(4-diphenylami-
nophenyl)buta-1,3-dienyl]-7,8-dihydro-6H-dipyrido[1,2-a:
2′,1′-c]-[1,4]diazepinium Hexafluorophosphate (8). This com-
pound was prepared and purified in manner similar to 7 by using
2·0.7H2O (100 mg, 0.209 mmol) in place of 1·0.25H2O with
1,3-bis(triflyloxy)propane (146 mg, 0.429 mmol) to afford a dark
purple solid: 92 mg, 55%; δH (400 MHz, CD3CN) 8.75 (1 H,
d, J ) 6.3 Hz, C5H3N), 8.57 (1 H, d, J ) 6.6 Hz, C5H3N), 8.20
(1 H, s, C5H3N), 8.15 (1 H, s, C5H3N), 8.11 (1 H, d, J ) 6.3
Hz, C5H3N), 8.02 (1 H, d, J ) 6.6 Hz, C5H3N), 7.85-7.77 (1
H, m, CH), 7.49 (2 H, d, J ) 8.8 Hz, C6H4), 7.36 (4 H, t, J )
7.4 Hz, Ph), 7.18-7.10 (8 H, 2CH + Ph), 6.96 (2 H, d, J ) 8.8
Hz, C6H4), 6.84 (1 H, d, J ) 15.4 Hz, CH), 4.82-4.73 (1 H,
m, NCH2eq), 4.70-4.62 (1 H, m, NCH2eq), 4.39-4.28 (1 H, m,
NCH2ax), 4.22-4.11 (1 H, m, NCH2ax), 2.79-2.68 (5 H, Me +
CH2). Anal. Calcd (%) for C36H33F12N3P2: C, 54.21; H, 4.17;
N, 5.27. Found: C, 54.58; H, 4.28; N, 5.23. m/z ) 652.3 ([M -
PF6]+), 253.9 ([M - 2PF6]2+).
2.12. Synthesis of 2,11-Bis[(E)-2-(4-diphenylaminophe-
nyl)ethenyl]-6,7-dihydro-dipyrido[1,2-a:2′,1′-c]pyrazinediium
Hexafluorophosphate (9). This compound was prepared and
purified in manner similar to 5 by using 3·H2O (100 mg, 0.140
mmol) in place of 1·0.25H2O with 1,3-bis(triflyloxy)ethane (94
mg, 0.288 mmol). A dark purple solid was obtained: 76 mg,
53%; δH (500 MHz, CD3CN) 8.84 (2 H, s, C5H3N), 8.53 (2 H,
d, J ) 6.0 Hz, C5H3N), 8.11 (2 H, d, J ) 15.7 Hz, 2CH), 8.00
(2 H, d, J ) 6.0 Hz, C5H3N), 7.63 (4 H, d, J ) 8.8 Hz, 2C6H4),
7.40 (8 H, t, J ) 7.8 Hz, Ph), 7.29-7.17 (14 H, 2CH + Ph),
2.9. Synthesis of 2-Methyl-11-[(E,E)-4-(4-diphenylami-
nophenyl)buta-1,3-dienyl]-6,7-dihydro-dipyrido[1,2-a:2′,1′-
c]pyrazinediium Hexafluorophosphate (6). This compound
was prepared in manner similar to 5 by using 2·0.7H2O (100

Nonlinear Optical Properties of Chromophoric Salts
J. Phys. Chem. A, Vol. 114, No. 45, 2010 12031
TABLE 1: Crystallographic Data and Refinement Details
for Dpaca and the Salt 8 ·0.5Et2O·0.25MeCN
6.99 (4 H, d, J ) 8.8 Hz, 2C6H4), 4.87 (4 H, s, 2CH2). Anal.
Calcd (%) for C52H42F12N4P2 ·H2O: C, 60.59; H, 4.30; N, 5.43.
Found: C, 60.44; H, 4.13; N, 5.45. m/z ) 867.5 ([M - PF6]+),
361.6 ([M - 2PF6]2+).
Dpaca
8·0.5Et2O·0.25MeCN
formula
M
crystal system
space group
a, Å
b, Å
c, Å
R, deg
ꢀ, deg
γ, deg
V, Å3
C21H17NO
C38.5H38.75F12N3.25O0.5P2
844.92
2.13. Synthesis of 2,11-Bis[(E,E)-4-(4-diphenylaminophe-
nyl)buta-1,3-dienyl]-6,7-dihydro-dipyrido[1,2-a:2′,1′-c]pyra-
zinediium Hexafluorophosphate (10). This compound was
prepared in manner similar to 5 by using 4·H2O (108 mg, 0.141
mmol) in place of 1·0.25H2O with 1,2-bis(triflyloxy)ethane (94
mg, 0.288 mmol). Purification of the crude product (82 mg)
was effected as for 6 to afford a dark purple solid: 42 mg, 27%;
δH (500 MHz, CD3CN) 8.65 (2 H, s, C5H3N), 8.52 (2 H, d, J )
6.3 Hz, C5H3N), 7.99 (2 H, d, J ) 6.6 Hz, C5H3N), 7.96-7.90
(2 H, m, 2CH), 7.52 (4 H, d, J ) 8.5 Hz, 2C6H4), 7.36 (8 H, t,
J ) 7.7 Hz, Ph), 7.19-7.11 (16 H, 4CH + Ph), 6.97 (4 H, d,
J ) 8.5 Hz, 2C6H4), 6.86 (2 H, d, J ) 15.5 Hz, 2CH), 4.85 (4
H, s, 2CH2). Anal. Calcd (%) for C56H46F12N3P2 ·1.9H2O: C,
61.19; H, 4.57; N, 5.10. Found: C, 61.21; H, 4.31; N, 5.07. m/z
) 919.6 ([M - PF6]+), 387.6 ([M - 2PF6]2+).
2.14. Synthesis of 2,12-Bis[(E)-2-(4-diphenylaminophe-
nyl)ethenyl]-7,8-dihydro-6H-dipyrido[1,2-a:2′,1′-c]-[1,4]diaz-
epinium Hexafluorophosphate (11). This compound was
prepared and purified in a manner similar to 5 by using 3·H2O
(100 mg, 0.140 mmol) in place of 1·0.25H2O and 1,3-
bis(triflyloxy)propane (94 mg, 0.276 mmol) in place of 1,2-
bis(triflyloxy)ethane. A dark purple solid was obtained: 63 mg,
43%; δH (500 MHz, CD3CN) 8.58 (2 H, d, J ) 6.6 Hz, C5H3N),
8.31 (2 H, s, C5H3N), 8.06 (2 H, d, J ) 6.0 Hz, C5H3N), 7.95
(2 H, d, J ) 16.4 Hz, 2CH), 7.61 (4 H, d, J ) 8.5 Hz, 2C6H4),
7.39 (8 H, t, J ) 7.7 Hz, Ph), 7.27 (2 H, d, J ) 15.8 Hz, 2CH),
7.23-7.15 (12 H, Ph), 6.98 (4 H, d, J ) 8.5 Hz, 2C6H4),
4.71-4.64 (2 H, m, NCH2eq), 4.31-4.22 (2 H, m, NCH2ax),
2.74-2.67 (2 H, m, CH2). Anal. Calcd (%) for C53H44-
F12N4P2 ·0.7H2O: C, 61.24; H, 4.40; N, 5.39. Found: C, 61.22;
H, 4.20; N, 5.52. m/z ) 881.9 ([M - PF6]+), 368.5 ([M -
2PF6]2+).
2.15. Synthesis of 2,12-Bis[(E,E)-4-(4-diphenylaminophe-
nyl)buta-1,3-dienyl]-7,8-dihydro-6H-dipyrido[1,2-a:2′,1′-c]-
[1,4]diazepinium Hexafluorophosphate (12). This compound
was prepared and purified in manner similar to 11 by using
4·H2O (108 mg, 0.141 mmol) in place of 3·H2O with
1,3-bis(triflyloxy)propane (98 mg, 0.288 mmol) to afford a dark
purple solid: 82 mg, 53%; δH (500 MHz, CD3CN) 8.57 (2 H,
d, J ) 6.9 Hz, C5H3N), 8.24 (2 H, s, C5H3N), 8.01 (2 H, d, J )
6.6 Hz, C5H3N), 7.86-7.80 (2 H, m, 2CH), 7.49 (4 H, d, J )
8.5 Hz, 2C6H4), 7.36 (8 H, t, J ) 7.7 Hz, Ph), 7.18-7.10 (16
H, Ph + 4CH), 6.96 (4 H, d, J ) 8.5 Hz, 2C6H4), 6.85 (2 H, d,
J ) 15.8 Hz, 2CH), 4.69-4.66 (2 H, m, NCH2eq), 4.29-4.20
(2 H, m, NCH2ax), 2.74-2.65 (2 H, m, CH2). Anal. Calcd (%)
for C57H48F12N4P2 ·0.8H2O: C, 62.61; H, 4.57; N, 5.12. Found:
C, 62.62; H, 4.56; N, 5.14. m/z ) 933.8 ([M - PF6]+), 394.7
([M - 2PF6]2+).
2.16. X-ray Crystallography. Crystals of the precursor
compound Dpaca were obtained by slow evaporation of a
dichloromethane solution at room temperature. Crystals of the
salt 8·0.5Et2O·0.25MeCN were obtained by diffusion of diethyl
ether vapor into an acetonitrile solution at room temperature,
with the diffusion rate slowed by using a lid spiked with small
holes. Data were collected on Oxford Diffraction XCalibur 2
or Bruker SMART APEX CCD X-ray diffractometers using
MoKR radiation (λ ) 0.71073 Å), and processed by using the
Oxford Diffraction CrysAlis RED20 or Bruker SAINT21 software
packages.
299.36
monoclinic
P21/c
8.966(1)
18.422(2)
10.485(2)
90.00
116.327(9)
90.00
1552.1(3)
4
triclinic
j
P1
8.065(2)
13.810(3)
20.684(5)
106.539(4)
99.424(4)
96.092(4)
2150.1(9)
2
Z
Dcalcd (Mg m-3
T, K
)
1.281
100(2)
0.078
1.305
100(2)
0.185
µ, mm-1
crystal size, mm3
crystal appearance
reflections collected
0.20 × 0.12 × 0.10 0.65 × 0.10 × 0.02
yellow prism
6708
brown needle
8262
independent reflections (Rint) 2727 (0.0436)
3865 (0.1129)
19.78 (99.5%)
2157
θ
max, deg (completeness)
25.08 (98.4%)
1533
0.901
0.0362, 0.0476
0.0796, 0.0503
0.157, -0.167
reflections with I > 2σ(I)
goodness-of-fit on F2
final R1, wR2 [I > 2σ(I)]
(all data)
1.070
0.0958, 0.2297
0.1690, 0.2637
0.676, -0.285
peak and hole (e Å-3
)
The structures were solved by direct methods using SIR-9222
via WinGX,23 or SHELXS-97,24 and refined by full-matrix least-
squares on all F02 data using SHELXL-97.25 All non-hydrogen
atoms were refined anisotropically, except for the partially
occupied solvent atoms in 8 · 0.5Et2O · 0.25MeCN. Hydrogen
atoms were included in idealized positions by using the riding
model, with thermal parameters of 1.2 times those of aromatic
parent carbon atoms, and 1.5 times those of methyl parent
carbons. For 8·0.5Et2O·0.25MeCN, restraints were applied to
the geometry of the solvent molecules and the data were cut at
1.05 Å since they were very weak beyond this resolution. All
other calculations were carried out by using the SHELXTL
package.26 Crystallographic data and refinement details are
presented in Table 1.
2.17. Hyper-Rayleigh Scattering. General details of the
hyper-Rayleigh scattering (HRS) experiment have been dis-
cussed elsewhere,27 and the experimental procedure and data
analysis protocol for the femtosecond measurements used in
this study were as previously described.28 Measurements were
carried out in acetonitrile, with crystal violet as an external
octupolar reference (ꢀxxx,800 ) 338 × 10-30 esu in methanol),28a
and local field correction factors at optical frequencies are
applied to correct for the difference in solvent. All measurements
were performed by using the 800 nm fundamental of a
regenerative mode-locked Ti3+:sapphire laser (Spectra Physics,
model Tsunami, 100 fs pulses, 1 W, 80 MHz). Dilute solutions
(10-4-10-6 M) were used to ensure linear dependences of I2ω/
Iω2 on solute concentration, precluding the need for Lambert-
Beer correction factors. The absence of frequency demodulation
of the observed hyperpolarizability, that is, constant values of
ꢀ versus amplitude modulation frequency, showed that no
fluorescence contributions to the HRS signals were present at
400 nm. This situation may indicate: (i) a lack of fluorescence,
(ii) spectral filtering out of fluorescence, or (iii) the fluorescence
lifetime is too short for its demodulation to be observed within
the bandwidth of the instrument. The reported ꢀ values are the
averages taken from measurements at different amplitude
modulation frequencies. HRS depolarization ratios F29 were
determined at 800 nm in acetonitrile according to a published
methodology.30 Although the precision of individual depolar-

12032 J. Phys. Chem. A, Vol. 114, No. 45, 2010
Coe et al.
ization measurements is high (fitting errors of ca. ( 0.02),
multiple measurements on the crystal violet reference and on
some of the samples indicated significant variability in the F
values obtained. Hence, the errors for the whole data set have
been estimated based on the variability of ca. ( 15% observed
in the values obtained for the reference.
2.19. Z-Scan Measurements. All compounds were inves-
tigated as DMF solutions placed in 1 mm path length glass cells.
Measurements33 were recorded by using a Clark MXR CPA-
2001 Ti3+:sapphire amplifier pumping a tunable Light Conver-
sion TOPAS optical parametric amplifier (150 fs pulses,
operated at 250 Hz repetition rate). The experiments were carried
out at a number of wavelengths in the range 520-1600 nm. To
obtain the relevant wavelengths, the optical parametric amplifier
was appropriately tuned and one of the following modes of
output was selected: idler-pump mixing, signal doubling, idler
doubling, or the signal. The unwanted components of the
TOPAS output were discarded through the use of polarizing
optics, color glass filters, and spatial filtering. Other experimental
details and the data processing procedure were as described
previously.34 The real and imaginary parts of γ of the solutes
were computed assuming additivity of the nonlinear contribu-
tions of the solvent and the solute and the applicability of the
Lorentz local field approximation. The errors on the γ values
were estimated from the assessed accuracies of the parameters
for the fitting of Z-scans for the solutions and the corresponding
scans for the solvent. The nonlinear absorption, related to the
imaginary part of γ, was also expressed in terms of the values
of the two-photon absorption cross-section σ2 calculated as
shown previously.34
2.18. Stark Spectroscopy. The Stark apparatus, experimental
methods and data collection procedure were as previously
reported,31 except 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. Satisfactory fits of the
Stark data for the salts 5-8 were obtained by using the observed
absorption (ε/ν vs ν) spectra, but for 9-12 these spectra were
modeled with a sum of three 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.32 The dipole-moment change ∆µ12
)
µe - µg (where µe and µg are the excited and ground state dipole
moments, respectively, 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 fint is estimated as 1.33.31 A two-state analysis of the
ICT transitions gives
∆µ2ab ) ∆µ212 + 4µ212
(1)
3. Results and Discussion
3.1. Synthesis and Characterization. The new DQ2+-based
chromophores (Figure 1) were prepared and isolated as their
PF6 salts 5-12 following reactions between the appropriate
where ∆µab is the dipole-moment change between the diabatic
states, and ∆µ12 is the observed (adiabatic) dipole-moment
change. The value of µ12 can be determined from the oscillator
strength fos of the transition by
-
2,2′-bipyridyl (bpy) derivative and 1,2-bis(triflyloxy)ethane or
1,3-bis(triflyloxy)propane.19 The required precursors 1-4 (Fig-
ure 1) were prepared in high yields via Wadsworth-Emmons
reactions between 4-(diphenylamino)benzaldehyde or Dpaca and
4-[(diethoxyphosphinyl)methyl]-4′-methyl-2,2′-bipyridyl17 or 4,
4′-bis-[(diethoxyphosphinyl)methyl]-2,2′-bipyridyl.18 A recently
published synthesis of Dpaca involves a Vilsmeier reaction of
4-(diphenylamino)styrene,35 with a yield ca. 10% lower than
that we have achieved by using a Wittig oxopropenylation with
(1,3-dioxolan-2-yl-methyl)tributyl phosphonium bromide.36 Nickel
et al. have reported using a method similar to ours, but with
only 1.1 equivalents of the Wittig reagent and potassium tert-
butoxide as the base instead of sodium hydride, resulting in a
yield of 72%.37
1/2
fos
|µ12| )
(2)
-5
(
)
1.08 × 10 Emax
where Emax is the energy of the ICT maximum (in wavenumbers)
and µ12 is in eÅ. The latter is converted into Debye units upon
multiplying by 4.803. The degree of delocalization cb and
2
electronic coupling matrix element Hab for the diabatic states
are given by
∆µ212
1/2
The compound 4,4′-bis[(E)-2-(4-diphenylaminophenyl)ethe-
nyl]-2,2′-bipyridyl (3) has been prepared previously via a PPh3-
derived Wittig reagent, but the isolated yield was only 50%.38
Although we have previously used Knoevenagel-type condensa-
tions to synthesize related compounds containing Dmap or Jd
π-ED groups,15 such an approach is ineffective for the Dpap-
substituted chromophores in 5-12. Most of the products were
isolated in pure form simply via reprecipitation, with yields in
the range ca. 40-60%. However, column chromatographic
purification was required in order to obtain satisfactory results
for 6 and 10, returning yields about half those for the other
1
2
c2b ) 1 -
(3)
(4)
2
2
(
)
[
]
∆µ12 + 4µ12
Emax(µ12)
|Hab| )
|
|
∆µab
If the hyperpolarizability tensor ꢀ0 has only nonzero elements
along the CT direction, then this quantity is given by
3∆µ12(µ12)2
1
salts. All of the new compounds show diagnostic H NMR
ꢀ0 )
(5)
spectra, and CHN elemental analyses provide further confirma-
tion of identity and purity. In addition, informative +electrospray
mass spectra were obtained, with the exceptions of the precursor
compounds 3 and 4.
3.2. Electronic Spectroscopy Studies. The UV-vis absorp-
tion spectra of salts 5-12 have been measured in acetonitrile
and the results are presented in Table 2. These spectra are
dominated by intense, low-energy bands in the visible region
attributable to π f π* intramolecular charge-transfer (ICT)
2
(Emax
)
A relative error of (20% is estimated for the ꢀ0 values
derived from the Stark data and using eq 5, while experimental
errors of (10% are estimated for µ12, ∆µ12 and ∆µab, (15%
2
for Hab and (50% for cb . Note that the (20% uncertainty for
the ꢀ0 values is merely statistical and does not account for any
errors introduced by two-state extrapolation.

Nonlinear Optical Properties of Chromophoric Salts
J. Phys. Chem. A, Vol. 114, No. 45, 2010 12033
Figure 1. Chemical structures of the precursor compounds and DQ2+ derivatives prepared; the latter were isolated and studied as their PF6- salts.
Shorthand: EDQ2+ ) ethylene diquat; PDQ2+ ) propylene diquat.
TABLE 2: UV-Vis Absorption and Electrochemical Data for Compounds 1-12 in Acetonitrilea
E, V vs Ag-AgCl (∆Ep, mV)b
a
compound
λ
max, nm (ε, 103 M-1 cm-1
)
Emax (eV)
assignment
oxidations
reductions
1
389 (31.6)
290 (30.9)
404 (37.2)
302 (32.1)
395 (66.0)
299 (49.3)
410 (79.8)
316 (61.6)
551 (35.7)
394 (12.4)
302 (28.5)
568 (37.7)
396 (20.8)
305 (31.6)
528 (42.0)
332 (14.0)
287 (27.9)
545 (41.6)
377 (17.5)
292 (24.4)
551 (59.1)
426 (30.5)
298 (42.0)
571 (52.0)
426 (34.2)
306 (37.9)
527 (62.1)
339 (25.2)
297 (41.4)
542 (55.9)
382 (32.1)
302 (43.2)
3.19
4.28
3.07
4.11
3.14
4.15
3.02
3.92
2.25
3.15
4.11
2.18
3.13
4.07
2.35
3.73
4.32
2.28
3.29
4.25
2.25
2.91
4.16
2.17
2.91
4.05
2.35
3.66
4.18
2.29
3.25
4.11
ICT
π f π*
ICT
π f π*
ICT
π f π*
ICT
2
3
4
5
π f π*
ICT-1
ICT-2
π f π*
ICT-1
ICT-2
π f π*
ICT-1
π f π*
π f π*
ICT-1
ICT-2
π f π*
ICT-1
ICT-2
π f π*
ICT-1
ICT-2
π f π*
ICT-1
π f π*
π f π*
ICT-1
ICT-2
π f π*
1.05 (120)
1.00c
-0.39 (90)
-0.82 (90)
6
7
-0.38 (80)
-0.81 (80)
1.08 (100)
0.99 (100)
1.03 (150)
0.94 (140)
1.07 (130)
0.98 (140)
-0.55 (110)
-0.86 (100)
-1.26d
-0.53 (110)
-0.84 (100)
-1.20d
-0.38 (80)
-0.73 (80)
8
9
10
11
12
-0.33 (80)
-0.70 (90)
-0.51 (120)
-0.78 (110)
-0.48 (110)
-0.76 (120)
a Solutions ca. 2-3 × 10-5 M. b Solutions ca. 10-3 M in analyte and 0.1 M in [N(C4H9-n)4]PF6 at a 2 mm disk glassy carbon working
electrode with a scan rate of 200 mV s-1. E1/2 values are quoted unless stated otherwise. Ferrocene internal reference E1/2 ) 0.46 V, ∆Ep )
c
d
80-90 mV. Epa for an irreversible oxidation process. Epc for an irreversible reduction process.
excitations from the Dpap π-ED groups to the DQ2+ acceptors,
denoted ICT-1. Weaker bands ascribed to other π f π*
transitions with little or no ICT character are also observed at
higher energies. Bands with intermediate energies are (somewhat
arbitrarily) denoted as being ICT in nature (i.e., ICT-2) if their
maxima lie at wavelengths below about 380 nm, but of course
the extent of charge-transfer character in a π f π* transition is
not simply a predictable function of its energy. Representative
UV-vis spectra of 6, 9, 10, and 11 are shown in Figure 2.
The ICT energies Emax of the neutral precursor compounds
decrease both on extending the π-conjugated bridges (by 0.12
eV) and on moving from a monosubstituted (MS) species to its

12034 J. Phys. Chem. A, Vol. 114, No. 45, 2010
Coe et al.
Figure 2. UV-vis absorption spectra of 6 (red), 9 (green), 10 (orange),
and 11 (blue) at 293 K in acetonitrile.
Figure 3. Cyclic voltammograms for the salts 6 (blue) and 8 (red)
recorded at 200 mV s-1 in acetonitrile with a glassy carbon working
electrode (Fc ) ferrocene). The arrow indicates the direction of the
initial scans.
disubstituted (DS) counterpart (by 0.05 eV). The Emax values
of the other new chromophores display trends that we have
observed previously in both DQ2+-based chromophores15b and
simpler monocationic pseudolinear species.39 Thus, moving from
an EDQ2+ chromophore to its PDQ2+ analogue causes blue-
shifts of the ICT bands (as shown for 9 and 11 in Figure 2); for
the dominant ICT-1 bands, the increase in Emax is almost
constant at ca. 0.1 eV, but generally rather larger and more
variable shifts are observed for the ICT-2 bands. This trend is
due to the weaker electron-accepting strength of a PDQ2+ vs a
EDQ2+ group, as shown electrochemically.15b,40 On moving from
a MS compound to its DS counterpart, Emax[ICT-1] remains
essentially constant (as shown for 6 and 10 in Figure 2). In
contrast, the ICT-2 bands show significant red shifts of ca. 0.2
eV for the EDQ2+ chromophores and smaller decreases (ca. 0.05
eV) for the PDQ2+ species. Notably, our previous studies with
Dmap or Jd-substituted diquats revealed small increases in
Emax[ICT-1] on moving from MS to DS chromophores.15b
Moving from n ) 1 to 2 causes Emax[ICT-1] to decrease by
0.06-0.08 eV.
A π-ED strength order of Dpap < Dmap < Jd has been derived
by Kwon et al,41 by using density functional theory (DFT)-
calculated bond distances and 13C NMR chemical shifts. This
order is supported by the ICT energies of 5-12, which are
higher than those of the Dmap/Jd-substituted diquats we reported
previously.15b For the ICT-1 band, respective red-shifts of
0.03-0.11 and 0.23-0.36 eV are observed on replacing Dpap
with Dmap or Jd. The accompanying decreases in Emax[ICT-2]
are even larger (as much as ca. 0.6 eV) for the EDQ2+ species,
but we did not assign such bands in the Dmap/Jd-substituted
PDQ2+ chromophores.15b
Several trends are also observed in the molar extinction
coefficients ε of the lowest energy ICT bands. For the neutral
precursor compounds, moving from n ) 1 to 2 increases ε by
ca. 20%, while the ε values for the DS species are about twice
those of their MS counterparts. This latter trend is also found
with the DQ2+ chromophores, but the increases in intensity are
somewhat less than 2-fold (as shown for 6 and 10 in Figure 2).
Small increases in ε are evident on moving from an EDQ2+
chromophore to its PDQ2+ analogue (as shown for 9 and 11 in
Figure 2), contrasting with the more substantial changes
accompanying this structural modification in Dmap/Jd-substi-
tuted diquats.15b Perhaps surprisingly, the ε values of these new
DQ2+ species are not consistently affected by extending the
π-conjugated bridges, showing either no significant change or
small decreases.
are presented in Table 2. Representative voltammograms for 6
and 8 are shown in Figure 3. For each compound, waves with
relatively large peak currents in the region ca. 1-1.1 V versus
Ag-AgCl are attributable to oxidation of the Dpap units.
Although our previous studies with Dmap/Jd-substituted diquats
revealed only irreversible oxidations,15b return waves are
observed for the Dpap-containing PDQ2+ species, and 8 even
shows a quasireversible oxidation (although ipc is somewhat
smaller than ipa; Figure 3). In keeping with the ICT energies
(see above), the potentials for Dpap oxidation are always higher
than those of their Dmap or Jd analogues;15b and the latter π-ED
groups are hence easier to oxidize by ca. 0.1-0.2 and ca.
0.3-0.4 V, respectively. It is however notable that DFT-derived
and photoelectron spectroscopy-measured ionization potentials
follow the order Dmap > Jd > Dpap, implying that Dpap should
be the most easily oxidized of these three π-ED units.41
Moving from an EDQ2+ chromophore to its PDQ2+ analogue
does not affect the potentials of the oxidation processes
significantly. The differences between the related MS and DS
chromophores are also negligible, and both of these observations
also apply to Dmap/Jd-substituted diquats.15b Moving from
n ) 1 to 2 causes Epa or E1/2 to decrease by ca. 100 mV,
attributable to the increased electron-donating ability of (E,E)-
buta-1,3-dienylene vs ethenylene units. The fact that somewhat
larger corresponding changes of 150-200 mV are observed for
the Dmap/Jd-substituted species15b indicates that this effect is
more significant when stronger terminal π-EDs are present.
Each of salts 5-12 also shows two reversible or quasire-
versible one-electron reduction waves (Figure 3), and a third,
irreversible wave below -1 V versus Ag-AgCl is detectable
for 7 and 8. The E1/2 values for these reductions are slightly
higher (by up to 140 mV) than those of the Dmap/Jd-substituted
species,15b indicating that the DQ2+ units in the new compounds
are reduced more easily. This trend is consistent with the less
strongly electron-donating Dpap group(s) causing the diquats
to be more electron-deficient when compared with those
previously reported. Moving from n ) 1 to 2 causes no change
or very small increases in the E1/2 values, while slightly larger
increases (up to 110 mV for the second wave) are observed on
moving from a MS compound to its DS analogue. The
reductions become a little more favorable due to more effective
electron delocalization in the larger π-conjugated systems.
Previous electrochemical studies40 (and the ICT energies
3.3. Electrochemical Studies. We have investigated salts
5-12 by using cyclic voltammetry in acetonitrile, and the results

Nonlinear Optical Properties of Chromophoric Salts
J. Phys. Chem. A, Vol. 114, No. 45, 2010 12035
the latter with PDQ2+ hence causes the E1/2 values to decrease
(Figure 3). The observed cathodic shifts are larger for the first
wave (130-160 mV) than for the second one (30-60 mV).
All of these trends are also observed for the related Dmap/Jd-
substituted species.15b
It is worth highlighting that the presence of reversible
electrochemical behavior means that the new DQ2+ derivatives
have potential for redox-switching of optical properties over
multiple states. Redox-switching of NLO responses has been
achieved previously with a range of transition metal com-
plexes,42 but not yet in any purely organic molecules.
3.4. Crystallographic Studies. This study is focused pri-
marily on molecular electronic/optical properties, but bulk NLO
properties that depend on macroscopic structures are relevant
for potential applications. Although the new DQ2+ salts contain-
ing Dpap groups do not appear to crystallize as readily as their
Dmap/Jd counterparts (for which nine single crystal X-ray
structures have been obtained),15b we have solved the structure
of the salt 8·0.5Et2O·0.25MeCN, together with that of its
precursor aldehyde, Dpaca. Representations of the molecular
structures are shown in Figures 4 and 5, and selected geometric
parameters are collected in Table 3.
Figure 4. Representation of the molecular structure of Dpaca (50%
probability ellipsoids).
As for the acetone solvate of the analogous Dmap-containing
compound,15b the salt 8·0.5Et2O·0.25MeCN crystallizes in the
j
centrosymmetric space group P1 and is therefore not expected
to show bulk quadratic NLO effects. The geometric parameters
of the cation in 8 are also generally similar to those of its Dmap
counterpart 8′,15b with the only notable exception being the N-C
bond distance between the phenylene ring and the amino group.
In 8′, this distance is 1.366(3) Å, apparently ca. 0.05 Å shorter
than that in 8 (1.413(15)). This observation is consistent with
the stronger relative π-ED ability of the Dmap unit when
compared with Dpap because it indicates that the amino group
in the former is more strongly conjugated with the phenylene
ring. Data obtained from other structural studies support this
conclusion.43 The dihedral angle between the two pyridyl rings
in 8·0.5Et2O·0.25MeCN is ca. 52.4°, within the range found
in other related structures.15b,44
Figure 5. Representation of the molecular structure of the salt
8·0.5Et2O·0.25MeCN, with the solvent molecules removed for clarity
(50% probability ellipsoids).
discussed above) show that a PDQ2+ unit is a weaker electron
acceptor when compared with an EDQ2+ group, and replacing
TABLE 3: Selected Interatomic Distances and Angles for Dpaca and the Salt 8 ·0.5Et2O·0.25MeCN
distance (Å)
distance (Å)
angle (deg)
Dpaca
N1-C7
N1-C10
N1-C16
C6-C7
C7-C8
C8-C9
C4-C9
1.406(2)
1.437(2)
1.411(2)
1.389(2)
1.394(2)
1.375(2)
1.384(2)
C4-C5
C5-C6
C3-C4
C2-C3
C1-C2
O1-C1
1.396(2)
1.384(2)
1.448(2)
1.328(2)
1.447(2)
1.216(2)
C2-C3-C4
C1-C2-C3
O1-C1-C2
129.43(16)
119.73(16)
126.44(16)
8·0.5Et2O·0.25MeCN
1.354(13)
N1-C13
N1-C7
1.413(15)
1.453(16)
1.465(16)
1.402(17)
1.366(15)
1.390(16)
1.401(16)
1.377(16)
1.371(16)
1.457(16)
1.336(15)
1.438(16)
1.374(15)
1.436(15)
1.426(14)
1.339(14)
N2-C25
N2-C26
C16-C19-C20
C19-C20-C21
C20-C21-C22
C21-C22-C23
128.9(13)
122.4(12)
128.3(12)
124.4(11)
1.351(14)
N1-C1
C26-C27
C23-C27
C25-C28
C28-C29
C29-C30
C30-C31
C31-C32
N3-C32
N3-C28
N3-C36
C35-C36
C34-C35
N2-C34
1.364(15)
1.375(14)
1.493(15)
1.369(15)
1.397(15)
1.389(16)
1.369(16)
1.332(13)
1.381(13)
1.463(13)
1.529(13)
1.499(14)
1.482(13)
C13-C14
C14-C15
C15-C16
C16-C17
C17-C18
C13-C18
C16-C19
C19-C20
C20-C21
C21-C22
C22-C23
C23-C24
C24-C25

12036 J. Phys. Chem. A, Vol. 114, No. 45, 2010
Coe et al.
TABLE 4: HRS Data and Depolarization Ratios for Salts
5-12 in Acetonitrile
so only ꢀzzz and ꢀzyy are significant. To determine “off-diagonal”
tensor components, we have measured HRS depolarization ratios
F for 5-12 and these are included in Table 4. The parameter F
is the ratio of the intensities of the scattered SH light polarized
parallel and perpendicular to the polarization direction of the
fundamental beam.29 A F value of 5 is the upper limit for purely
dipolar symmetry, corresponding with a single dipolar ꢀzzz tensor
component, and under ideal experimental conditions. Realistic
conditions, such as using a condenser system to collect the SH
light, will decrease the observed F value. The octupolar reference
compound crystal violet gives a relatively low F value of 1.5.
(10-30 esu)
(10-30 esu)
a
b
d
d
salt ꢀ〈ꢀ2HRS
〉
ꢀ0
Fc
k
ꢀzzz
ꢀzyy
5
6
7
8
9
330 ( 35 155 ( 15 3.7 ( 0.6
320 ( 32 160 ( 16 4.4 ( 0.7
310 ( 31 130 ( 13 2.5 ( 0.4
375 ( 37 170 ( 17 3.0 ( 0.5
325 ( 40 150 ( 18 3.5 ( 0.5
800 ( 80
770 ( 77
750 ( 75
900 ( 90
235 ( 30 470 ( 56
120 ( 12 710 ( 71
100 ( 10 615 ( 61
100 ( 10 810 ( 81
2
6
6
8
10 455 ( 45 230 ( 23 2.8 ( 0.4
11 385 ( 38 160 ( 16 2.8 ( 0.4
12 500 ( 50 225 ( 22 2.7 ( 0.4
The values of ꢀzzz and ꢀzyy can be determined from 〈ꢀH2 RS
and F as follows:
〉
a Orientationally averaged first hyperpolarizability without any
assumption of symmetry or contributing tensor elements, measured
by using an 800 nm Ti3+:sapphire laser. The quoted cgs units (esu)
can be converted into SI units (C3 m3 J-2) by dividing by a factor
of 2.693 × 1020. b Static first hyperpolarizability estimated from
〈ꢀ2ZZZ
〈ꢀ2YZZ
〉
〉
〈ꢀH2 RS〉 ) 〈ꢀZ2ZZ〉 + 〈ꢀ2YZZ〉F )
(7)
{
ꢀ〈ꢀ2HRS
〉
via the two-state model.46 c Depolarization ratio.
d Hyperpolarizability tensor components derived from the HRS
intensity and F measurements by using eqs 7-9.
The HRS intensities with parallel polarization for fundamental
and SH wavelengths, 〈ꢀ2ZZZ〉, and for perpendicular polarization,
〈ꢀ2YZZ〉, are given in terms of the molecular tensor components
ꢀzzz and ꢀzyy according to
The precursor Dpaca also crystallizes centrosymmetrically
and shows geometric parameters similar to those of 4-(diphe-
nylamino)benzaldehyde,45 both having a trigonal planar N atom
and effectively identical N-C(phenylene) distances (1.407(3)
Å in 4-(diphenylamino)benzaldehyde). In both aldehydes, the
shortening by up to ca. 0.3 Å of the latter distance with respect
to the two N-C(phenyl) distances is consistent with stronger
π-conjugation with the phenylene ring.
1
〈ꢀ2ZZZ〉 ) ꢀ2zzz
7
6
35
9
35
2
1
ꢀ2zyy〈ꢀ2YZZ〉 ) ꢀz2zz
35
+
ꢀzzzꢀzyy
+
-
{
11
ꢀzzzꢀzyy
+
ꢀ2zyy (8)
105
105
3.5. Hyper-Rayleigh Scattering Studies. We have measured
the ꢀ values of salts 5-12 in acetonitrile solutions by using the
HRS technique27,28 with a 800 nm Ti3+:sapphire laser and the
results are shown in Table 4. This laser wavelength was chosen
because the ICT-1 bands lie between 800 nm and the second
harmonic (SH) at 400 nm (Figure 2). However, most of the
chromophores do absorb relatively strongly at 400 nm, so the
results obtained are strongly enhanced by resonance. Other
available laser fundamentals, for example, 1064 or 1300 nm,
would be unsuitable for most of the samples due to very strong
absorption at their SH wavelengths. The hyperpolarizability data
shown are orientationally averaged ꢀ〈ꢀ2HRS〉 values derived from
the total HRS intensity, regardless of molecular symmetry.
Although most of the compounds show an additional ICT-2
maximum to high energy of the main ICT-1 band and significant
resonance enhancement also occurs, we have used the simple
two-state model46 and the major λmax values (Table 2) to
determine estimated ꢀ0 values; the results are included in Table
4. It should be noted that this analysis is a major oversimplifica-
tion because the DQ2+ derivatives, especially the DS ones, can
not be accurately described as two-state systems. As observed
previously with Dmap/Jd-substituted diquats,15b ꢀ0 increases
when moving from n ) 1 to 2, although the difference for the
pair 5/6 is not significant. The DS compounds show larger ꢀ0
responses when compared with their MS counterparts, except
for the pair 5/9, and the related EDQ2+/PDQ2+ pairs have
indistinguishable ꢀ0 values. However, these observations should
be treated cautiously, given the number of approximations
involved in deriving the data.
and F can be expressed in terms of the parameter k ) ꢀzyy/ꢀzzz
by
15 + 18k + 27k2
F )
(9)
3 - 2k + 11k2
Application of eqs 7-9 to our experimental data gives the
zzz and ꢀzyy values shown in Table 4. Although eq 9 gives two
ꢀ
solutions for k, the positive value has been used based on
theoretical predictions; both finite field-DFT and coupled
perturbed Hartree-Fock (CPHF) calculations predict that ꢀzyy
dominates in DS DQ2+ derivatives.15 Due to the uncertainties
in the depolarization measurements, all k values are rounded to
one significant figure.
Although the individual F values vary considerably, the
average of 3.4 for the MS compounds 5-8, together with their
molecular structures, is consistent with the dominance of a single
dipolar ꢀzzz tensor component. The DS chromophores in 9-12
give an average F value of 3.0, slightly lower than for their MS
counterparts. Because the MS compounds are predominantly
1D dipoles, we have derived only their ꢀzzz responses. However,
the average F value for the DS compounds, taken together with
their molecular shapes, justify the derivation of ꢀzyy values. In
each case, ꢀzyy dominates markedly over ꢀzzz (Table 4). In
addition, ꢀzyy (but not ꢀzzz) increases on moving from n ) 1 to
2. Given that the two types of chromophore are treated
differently, assuming purely 1D dipolar symmetry (C∞V) for the
MS species but essentially C2V symmetry for the DS derivatives,
comparing the ꢀzzz values for MS and DS pairs is inappropriate.
3.6. Stark Spectroscopic Studies. We have studied the salts
5-12 by using Stark spectroscopy32 in butyronitrile glasses at
77 K, and the results are presented in Table 5. For the MS
compounds, the single bands in the visible region were fitted
satisfactorily. However, due to the presence of overlapped minor
The electronic structures and thus hyperpolarizabilities of
these chromophores are actually substantially 2D, and the main
value of our HRS measurements is to assess the relative
importance of different ꢀ tensor components. A C2v symmetric
chromophore has five nonzero ꢀ components, ꢀzzz, ꢀzyy, ꢀzxx, ꢀyyz,
and ꢀxxz. Assuming Kleinman symmetry, ꢀzyy ) ꢀyyz and ꢀzxx
)
ꢀxxz, and if the structure is essentially 2D, then ꢀzxx ) ꢀxxz ) 0,

Nonlinear Optical Properties of Chromophoric Salts
J. Phys. Chem. A, Vol. 114, No. 45, 2010 12037
TABLE 5: ICT Absorption and Stark Spectroscopic Data for Salts 5-12
c
d
e
f
g
i
j
µ12
(D)
∆µ12
(D)
∆µab
(D)
r12
rab
Hab
ꢀ0
Σꢀ0
a
a
a
b
2h
salt
νmax (cm-1
)
λmax (nm)
Emax (eV)
fos
(Å) (Å) cb
(103 cm-1
)
(10-30 esu) (10-30 esu)
5
6
7
8
9
18148
17341
18873
18067
(17502)
551
577
530
554
(571)
2.25
2.15
2.34
2.24
(2.17)
0.49 7.6
0.65 9.0
0.66 8.6
0.54 8.0
0.25 5.5
23.4
28.3
20.9
26.4
28.6
23.2
27.0
33.0
30.0
32.2
23.5
24.3
28.8
25.9
26.2
24.3
28.1
33.7
27.2
31.0
30.6
28.6
29.4
34.3
34.9
33.4
26.1
30.1
31.0
28.1
31.8
26.2
4.9 5.8 0.08
5.9 7.0 0.08
4.4 5.7 0.12
5.5 6.5 0.08
5.9 6.4 0.03
4.8 6.0 0.09
5.6 6.1 0.04
6.9 7.2 0.02
6.3 7.3 0.07
6.7 7.0 0.02
4.9 5.4 0.05
5.1 6.3 0.10
6.0 6.5 0.03
5.4 5.9 0.04
5.5 6.6 0.08
5.1 5.5 0.03
4.9
4.6
6.0
4.7
3.1
5.4
4.4
2.2
4.5
2.9
4.0
5.8
4.2
3.3
5.2
4.1
314
578
334
393
213
350
127
204
572
100
173
385
133
192
452
92
689
876
691
735
18228 (18712) 549 (534)
22987 (22987) 435 (435)
2.26 (2.32) 0.61 8.3
2.85 (2.85) 0.35 5.7
(2.03)
2.15 (2.20) 0.66 8.9
2.73 (2.73) 0.20 4.4
(2.27)
2.35 (2.42) 0.73 8.9
2.89 (2.86) 0.35 5.7
(2.15)
2.24 (2.32) 0.70 8.9
2.77 (2.73) 0.25 4.9
10 (16373)
(611)
0.17 4.7
17341 (17744) 577 (564)
21979 (21979) 455 (455)
11 (18309)
(546)
0.28 5.7
18954 (19519) 528 (512)
23309 (23027) 429 (434)
12 (17341)
(577)
0.24 5.4
18067 (18712) 554 (534)
22342 (22019) 448 (454)
a In butyronitrile at 77 K; observed absorption maxima, maxima for Gaussian fitting functions for 9-12 in brackets. The data in all
subsequent columns relate to fitted curves if used. b For 5-8, obtained from (4.32 × 10-9 M cm2)A where A is the numerically integrated area
under the absorption peak; for 9-12, obtained from (4.60 × 10-9 M cm2)εmax × fw1/2 where εmax is the maximal molar extinction coefficient
and fw1/2 is the full width at half height (in wavenumbers). c Calculated using eq 2. d Calculated from fint∆µ12 using fint ) 1.33. e Calculated
from eq 1. f Delocalized electron-transfer distance calculated from ∆µ12/e. g Effective (localized) electron-transfer distance calculated from
∆µab/e. h Calculated from eq 3. i Calculated from eq 4. j Calculated from eq 5.
Figure 6. Stark spectra and calculated fits for 7, 8, and 10 in external electric fields of 3.57, 3.57, and 3.51 × 107 V m-1, respectively. Top panel:
absorption spectrum illustrating Gaussian curves used in data fitting for 10; bottom panel: electroabsorption spectrum, experimental (blue) and fits
(green) according to the Liptay equation.32a
bands to high energy, Gaussian-fitting of the absorption spectra
with three curves was necessary to successfully model the Stark
data for the DS compounds. Representative absorption and
electroabsorption spectra for 7, 8, and 10 are shown in Figure
6, and the corresponding spectra for the other compounds are
included in the Supporting Information (Figures S1 and S2).
The ICT bands of 5-12 show either no change or slight red
shifts on moving from acetonitrile solutions to butyronitrile
glasses (Tables 2 and 5). The ICT-1 bands do not shift
significantly when n ) 1, but decreases in their Emax values of
0.02-0.05 eV are observed for the compounds with n ) 2. This
behavior contrasts with that of the related Dmap/Jd-substituted
species, which generally display small blue shifts on changing
from solution to frozen glass (up to 0.07 eV).15b All of the ICT-1
Emax trends observed at room temperature are also found at 77
K, that is, decreases on moving from n ) 1 to 2, increases on
replacing EDQ2+ with PDQ2+, and no difference between the
related MS and DS species. The trends evident in the band
intensities at 77 K (i.e., fos and µ12) generally agree with those
already noted for ε at room temperature (see above). When
analyzing these data for cases involving spectral deconvolution,
the total values are used. Moving from n ) 1 to 2 causes fos
and µ12 to decrease, except for the pair 5/6. In every case,
switching from a MS to the corresponding DS dication leads
to marked increases in fos and µ12. Replacing EDQ2+ with PDQ2+
gives small increases in intensity, except for the pair 6/8. It is
worth noting that both of these exceptions (and the differences
between 6 and 10) imply that the intensities measured for 6 are
higher than expected.
Because they are directly related to the chromophore length,
the quantities ∆µ12, ∆µab, r12, and rab should increase on moving
from n ) 1 to 2; such behavior is indeed observed (using
averaged values for the DS compounds), except for the pair
11/12. In similar manner, changing from a MS dication to its
DS counterpart leads to modest increases in these parameters
for all pairs except for 8/12. Moving from an EDQ2+ chro-
mophore to its PDQ2+ analogue is associated with small
decreases in ∆µ12, ∆µab, r12, and rab, but the relative change is
much larger for 10 f 12. Taken together, these observations

12038 J. Phys. Chem. A, Vol. 114, No. 45, 2010
Coe et al.
TABLE 6: Stark-Derived Total Static First
Hyperpolarizabilities for Salts 5-12 and Their Dmap/
Jd-Containing Analoguesa
to, those of their Dmap analogues. These observations are
consistent with the established weaker π-ED ability of the Dpap
group (see above). Nevertheless, the quadratic NLO responses
of the new compounds are relatively impressive, with the largest
(for 10) approaching 10-27 esu. In all cases, the total ꢀ0 response
is considerably larger than that which we obtained for [DAS]PF6
by using the same approach previously (236 × 10-30 esu).48
Indeed, the total NLO responses estimated for the DS cations
in the salts 9-12 are at least ca. 3-fold increased when compared
with [DAS]PF6. Therefore, polar materials containing these new
symmetrical DQ2+ cations can be expected to show very large
and 2D bulk quadratic NLO effects.
3.7. Two-Photon Absorption Studies. The salts 5-12 and
their precursors 1-3 have been investigated in DMF solutions
by using the Z-scan technique33,34 at a number of laser
wavelengths, to afford the dispersion of the real and imaginary
parts of γ, the latter being also converted to the 2PA cross-
sections σ2. The results are collected in Table 7, and Figure 6
shows a representative set of data for 6. Dispersion curves for
all the other compounds are shown in Figures S3-S12 of the
Supporting Information.
Examination of the data in Table 7 reveals several trends.
First, since all of the investigated chromophores except for 3
are noncentrosymmetric, there is no condition disallowing one-
photon absorption (1PA) reachable states to be populated also
by 2PA. Therefore, it is to be expected that 2PA peaks will
appear at wavelengths close to twice those of 1PA peaks, and
this seems to be the case in general. The longest wavelength
2PA maximum for 1 and 2 is close to 700 nm, a little less than
twice the wavelength of the first 1PA peak at ca. 400 nm. On
the other hand, all of the salts 5-12 show 1PA at ca. 520-550
nm and their 2PA spectra show maxima at ca. 1100 nm for
5-10, and at ca. 900 nm for 11 and 12 with a shoulder at ca.
1100 nm for the former. It is also worth noting that while the
1PA spectra for 1-3 show little variation between DMF and
acetonitrile (Tables 2 and 7), the DQ2+ derivatives do display
some solvatochromism. The ICT bands of the n ) 2 species
show blue shifts of ca. 0.1-0.2 eV on moving from acetonitrile
to DMF solutions, while only negligible or very small changes
are observed for the n ) 1 compounds, except for 5. However,
it should be noted that higher concentrations were used for the
DMF solutions, with 1 mm path length cells as opposed to 10
mm cells for the acetonitrile studies.
salt
ꢀ0 (10-30 esu)
ꢀ0 (10-30 esu)
a
ꢀ0 (10-30 esu)
b
5
6
7
8
314
578
334
393
689
876
691
735
297
456
305
525
876
1173
738
1126
439
778
372
705
905
1500
943
1390
9
10
11
12
a All derived by using eq 5 with data measured in butyronitrile at
77 K. b Data for analogous Dmap-containing compound; taken from
ref 15b. c Data for analogous Jd-containing compound; taken from
ref 15b.
indicate that the dipole-moment changes determined for 12 are
unexpectedly low.
D-A electronic coupling and orbital mixing are quantified by
2
the parameters cb and Hab, and these show the same trends as
noted previously with Dmap/Jd-substituted diquats (again using
averaged values for the DS compounds).15b Thus, moving from
2
n ) 1 to 2 causes decreases (or no significant change) in cb
and Hab; this is logical because π-orbital overlap diminishes
2
over longer distances. The values of cb and Hab also tend to
decrease on moving from a MS chromophore to its DS analogue,
and the EDQ2+ species have very similar or slightly smaller
values when compared with their PDQ2+ counterparts.
Given that all of the observed visible absorption arises from
ICT transitions, then even if deconvolution is used, each
individual component can be adequately described as involving
a ground and single excited state. It is hence reasonable to use
the standard two-state model46 (i.e., eq 5) to estimate ꢀ0 values
from the Stark data; the results are included in Table 5, and the
sum totals of the ꢀ0 responses associated with the three fitted
Gaussian components are also quoted for 9-12. In other related
studies, we have demonstrated that using this approach generally
affords total ꢀ0 values similar to those obtained without spectral
deconvolution.39,47 The validity of, and assumptions involved
in, applying this additive two-state approach to relatively
complex systems have been discussed at length previously,15b
but it is worth emphasizing that no other experimental method
besides the clearly limited HRS exists for deriving ꢀ0 values
for chromophores of the type we report here.
The trends observed in the estimated ꢀ0 values are in certain
respects reminiscent of our previous studies with Dmap/Jd-
substituted diquats.15b The only clear pattern for 5-12 is that
moving from a MS chromophore to its DS analogue is associated
with large increases in the quadratic NLO response, in the range
ca. 50-120%. This effect is due largely to the strongly enhanced
µ12 values for the DS chromophores (see above). Although
extending the conjugated systems substantially increases ꢀ0 in
the Dmap/Jd-substituted species,15b the corresponding difference
for the new compounds is only truly significant for the pair
5/6. In keeping with our previous studies,15b the ꢀ0 values for
the EDQ2+ chromophores are similar to those of their PDQ2+
analogues.
Trends are also evident in the magnitudes of the 2PA maxima.
It is clear that the DQ2+ derivatives are much stronger two-
photon absorbers (σ2 values in the range of several hundred
GM units) than their neutral precursors. Such behavior is not
unexpected and may be understood in terms of the sum-over-
states expressions that involve lower lying states in the case of
the ionic species. It is worth noting that Zhou et al. used INDO/
SOS calculations to derive σ2 values of 454 GM at 918 nm and
182 GM at 729 nm for a chromophore analogous to 92+, but
with -NEt(C2H4OH) instead of -NPh2 π-ED groups.15 These
values are of the same order of magnitude as those we have
now measured. It is also to be expected that the magnitude of
σ2 may be affected by the extent of π-conjugation, that is,
increased on moving from n ) 1 to 2. However, this expectation
is not generally confirmed by the data in Table 7. There is a
strong enhancement of σ2 when moving from 5 to 6, and from
9 to 10, but no difference between 7 and 8, and even opposite
trends for the pairs 1/2 and 11/12. Even when taking into
account possible experimental inaccuracies (ca. ( 20% for σ2),
these differences are significant. Therefore, it appears that the
σ2 values are also sensitive to other factors and that the degree
Some comments regarding the performance of the new
chromophores when compared with their Dmap/Jd-substituted
counterparts are instructive. The collected data for these series
of closely related compounds are shown in Table 6. The ꢀ0
values of 5-12 are always smaller than those of their Jd
analogues (but by highly variable proportions, ca. 10-90%),
and in most cases smaller than (by up to ca. 50%), or similar

Nonlinear Optical Properties of Chromophoric Salts
J. Phys. Chem. A, Vol. 114, No. 45, 2010 12039
TABLE 7: Electronic Absorption and Cubic NLO Parameters for Compounds 1-3 and Salts 5-12 in DMFa
Im(γ)maxd(10-33 esu)
Re(γ)exte(10-33 esu)
c
compound
2λmax (nm)
λ2phb(nm)
σ2 (GM)
1
778
580
740
63
0.21
0.29 (950 nm)
2
804
588
790
592
1064
604
1069
750
1060
670
1040
720
700
740
21
0.065
0.63
-0.35 (1000 nm)
1.4 (850 nm)
-6.8 (850 nm)
2.9 (650 nm)
14 (1370 nm)
13 (850 nm)
24 (1190 nm)
14 (900 nm)
12 (1370 nm)
15 (810 nm)
3
180
5
1190
950
1190
950
1130
950
1130
260
280
670
730
310
700
340
2.3
1.6
6.1
3.9
2.5
4.0
2.8
6
7
8
21 (1190 nm)
10 (900 nm)
9
1100
850
1091
818
1060
670
1130
900
1130
280
215
550
2.3
1.1
4.5
5.6 (1280 nm)
-7.5 (1060 nm)
3.2 (1370 nm)
-8.4 (1000 nm)
2.9 (1370 nm)
-7.0 (1060 nm)
10
11
1130 (shoulder)
165
240
1.4
1.4
-4.6
0.72
1.3
950
620
900
770
-1840
12
1040
744
135
3.6 (1500 nm)
-5.0 (850 nm)
350
a Unfortunately, compound 4 is insufficiently soluble to allow reliable determination of the NLO parameters. b 2PA maximum. c Maximum
2PA cross-section; note that negative values correspond with absorption bleaching rather than the usual 2PA. d Maximum of the imaginary part
of γ. e Extrema of the real part of γ.
of π-conjugation is not always decisive. As noted above, moving
from n ) 1 to 2 leads to either no significant change or small
decreases in the 1PA coefficient ε, so somewhat unintuitive
behavior of σ2 is hence not very surprising.
We note that the discussion of σ2 values is equivalent to
considering the imaginary part of γ, Im(γ). Comments on the
dispersion of the real part of γ, Re(γ), can also be made, but
only with caution due to inherently higher errors of determi-
nation of these parameters. In any case, the wavelength
dependences of Re(γ) for all of the compounds appear rather
complicated, with the γ values usually being positive at the long
wavelength end of the measurement range and showing numer-
ous features like extrema, changes of sign, etc., which typically
coincide with the ranges of strong 2PA. It should be emphasized
that the dispersion of Re(γ) may be quite complicated due to
the presence of numerous terms in the perturbative expansion
of the hyperpolarizability involving both one- and two-photon
resonances,49 and the understanding of experimental dispersion
curves may therefore prove difficult.50 Table 7 includes some
of the extremal values of Re(γ) measured for 1-3 and 5-12;
as expected, the scaling of these follows roughly the scaling of
the Im(γ) values.
Although our DQ2+ derivatives show some relatively high
σ2 values, especially considering that they are not very large
molecules, it should be noted that various other chromophores,
including transition metal complexes, possess much larger
activities exceeding even 104 GM.51 However, it is established
that centrosymmetric systems do tend to give stronger 2PA when
compared with similarly sized, related dipolar structures.12k The
polar nature of DQ2+ derivatives that is essential to give nonzero
values of ꢀ can therefore be expected to limit their potential
for 2PA. Although we are not aware of any other experimental
cubic NLO studies with DQ2+ derivatives, Xu et al. have
recently reported Z-scan studies on N-methylpyridinium deriva-
tives having two or three Dmap groups connected via ethylene
bridges;52 these compounds exhibit very strong saturable absorp-
tion when using picosecond pulses, and nanosecond pulses gave
Figure 7. Dispersion of (a) the 2PA cross-section and (b) the complex
cubic hyperpolarizability of salt 6.
reverse saturable absorption in one case but saturable absorption
for the other.
4. Conclusion
We have synthesized a family of new DQ2+-based NLO
chromophores containing one or two Dpap π-ED groups. Their
visible absorption spectra are dominated by intense ICT bands
that red shift on moving from n ) 1 to 2 and blue shift on

12040 J. Phys. Chem. A, Vol. 114, No. 45, 2010
Coe et al.
extending the diquaternizing bridge (EDQ2+ f PDQ2+). Cyclic
voltammograms show two reversible or quasireversible DQ2+-based
reductions and partially reversible Dpap oxidations. Single crystal
X-ray structures have been determined for one PF6- salt and for
the aldehyde precursor Dpaca, with both packing centrosymmetri-
cally. HRS studies with a 800 nm laser show relatively large ꢀ
responses, with depolarization measurements revealing dominant
“off-diagonal” ꢀzyy tensor components for the DS compounds. Stark
spectroscopy allows the estimation of ꢀ0 values and shows that
these do not differ significantly for the EDQ2+ and PDQ2+ pairs,
but large increases occur on moving from a MS species to its DS
counterpart. Z-scan measurements over a range of wavelengths
reveal relatively high σ2 values of up to 730 GM at 620 nm for
one of the DS species. The new chromophores reported are
therefore relatively rare in that they combine high quadratic and
cubic NLO responses with potential redox-switchability in the
absence of transition metal centers.
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Acknowledgment. We thank the EPSRC for support (grants
EP/E000738 and EP/D070732) 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, “Power-
ing 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 and J. O. is a Ph.D.
scholar of the FNP Welcome programme.
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