On the second-order nonlinear optical structure-property relationships of metal chromophores

Article abstract of DOI:10.1021/ic980872q

The synthesis and solvent-dependent second-order polarizability, β, responses of three new platinum(II) chromophores are reported. A strong correlation between the metal chromophore electronic structure and the second-order polarizability is observed that substantiates the use of the two-state model to design, optimize, and study new NLO chromophores.

Full text of DOI:10.1021/ic980872q

Inorg. Chem. 1999, 38, 287-289
287
On the Second-Order Nonlinear Optical Structure-Property Relationships of Metal
Chromophores
Karel Base,† Mark T. Tierney,† Alain Fort,‡ Jacques Muller,‡ and Mark W. Grinstaff*,†,§
Department of Chemistry, P. M. Gross Chemical Laboratory, Duke University,
Durham, North Carolina 27708, and Institut de Physique et de Chimie des Materiaux de Strasbourg,
CNRS 67037 Strasbourg, France
ReceiVed July 24, 1998
The synthesis and solvent-dependent second-order polarizability, â, responses of three new platinum(II)
chromophores are reported. A strong correlation between the metal chromophore electronic structure and the
second-order polarizability is observed that substantiates the use of the two-state model to design, optimize, and
study new NLO chromophores.
Introduction
synthesizing and characterizing highly solvatochromic metal-
lochromophores and measuring the second-order polarizability,
A current challenge in materials chemistry and engineering
is to afford a specific macromolecular property by tuning a
molecular structure through deliberate chemical changes. Ma-
terials that display nonlinear optical (NLO) activity are of
fundamental interest and technological use for a number of
photonic applications, including optical switching, communica-
tions, and data storage and retrieval.1-8 The rational design of
organic chromophores with specific NLO activity is possible
since the development of structure-property relationships
(SPRs).9-13 In comparison, the relationship between structure
and nonlinear optical activity for inorganic chromophores is not
as well defined or understood.14,15 To date, few metal chro-
mophores approach the NLO responses of current organic
chromophores, and we attribute this to the lack of NLO SPRs
to guide the research and to the limited number of studies
conducted.16 Our approach for developing NLO SPRs relies on
â, in solvents of varying polarity.17 Herein, we report the
solvent-dependent â responses of three new platinum(II) chro-
mophores and insights into the relationship between electronic
structure and second-order polarizability.
NLO SPRs for organic compounds are developed by syn-
thesizing donor-acceptor chromophores with a particular
electronic structure and measuring the molecular polarizabilities.
These data combined with computational studies provide a
clearer picture of the factors that affect NLO responses and the
interrelationships between the molecular structure and first-,
second-, and third-order polarizabilities.9-13,18-21 The relation-
ship between the second-order polarizability, â, and the degree
of charge-separated character (i.e., polarization, or bond-length
alternation, BLA, MIX in polymethines) in the ground-state
structure of an organic chromophore can be understood within
the confines of a two-state model,
† Duke University.
µge2∆µ
‡ Institut de Physique et de Chimie des Materiaux de Strasbourg.
§ http://www.chem.duke.edu/∼mwg.
â
2
Ege
(1) For a recent review on optical nonlinearities, see the special issue of
Chem. ReV. 1994, 94 (January).
(2) Marder, S. R.; Perry, J. W. AdV. Mater. 1993, 5, 804-815.
(3) Materials for Nonlinear Optics: Chemical PerspectiVes; Marder, S.
R., Sohn, J. E., Stucky, G. D., Eds.; ACS Symposium Series 455;
American Chemical Society: Washington, DC, 1991.
(4) Nonlinear Optical Properties of Organic Molecules and Crystals;
Chemla, D. S., Zyss, J., Eds.; Academic: New York, 1987.
(5) Marder, S. R.; Perry, J. W. Science 1993, 263, 1706-1707.
(6) Prasad, P. N.; Williams, D. J. Introduction to Nonlinear Optical Effects
in Molecules and Polymers; Wiley-Interscience: New York, 1991.
(7) Kaino, T.; Tomaru, S. AdV. Mater. 1993, 5, 172-178.
(8) Wagmiere, G. H. Linear and Nonlinear Optical Properties of
Molecules; VCH: Weinheim, 1993.
where â is proportional to the square of the transition dipole
moment (µge) and to the dipole difference between the first
excited and ground states (∆µ) and inversely proportional to
the square of the transition energy (Ege) between the ground
(16) For recent reviews on inorganic chromophores for second-order NLO,
see the following three review articles and the references cited therein.
(a) Long, N. J. Angew. Chem., Int. Ed. Engl. 1995, 34, 21-38. (b)
Marder, S. R. In Inorganic Materials; Bruce, D. W., O’Hare, D., Eds.;
John Wiley & Sons Ltd: 1992; pp 115-164. (c) Nalwa, H. S. Appl.
Organomet. Chem. 1991, 5, 349-377.
(9) Marder, S. R.; Beratan, D. N.; Cheng, L. T. Science 1991, 252, 103-
106.
(17) A preliminary account of this NLO study was reported at the 215th
National Meeting of the American Chemical Society, Dallas, TX,
March-April 1998.
(10) Marder, S. R.; Gorman, C. B.; Meyers, F.; Perry, J. W.; Bourhill, G.;
Bredas, J. L.; Pierce, B. M. Science 1994, 265, 632-635.
(11) Meyers, F.; Marder, S. R.; Pierce, B. M.; Bredas, J. L. J. Am. Chem.
Soc. 1994, 116, 10703-10714.
(18) Albert, I. D. L.; Marks, T. J.; Ratner, M. A. J. Phys. Chem. 1996,
100, 9714-9725.
(12) Marks, T. J.; Ratner, M. A. Angew. Chem. 1995, 34, 155-173.
(13) Gorman, C. B.; Marder, S. R. Proc. Natl. Acad. Sci. U.S.A. 1993, 90,
11297-11301.
(19) Meyers, F.; Marder, S. R.; Pierce, B. M.; Bredas, J. L. Chem. Phys.
Lett. 1994, 228, 171-176.
(20) Bourhill, G.; Bredas, J. L.; Cheng, L. T.; Marder, S. R.; Meyers, F.;
Perry, J. W.; Tiemann, B. G. J. Am. Chem. Soc. 1994, 116, 2619-
2620.
(14) Bella, S. D.; Fragala, I.; Ledoux, I.; Marks, T. J. J. Am. Chem. Soc.
1995, 117, 9481-9485.
(15) Cummings, S. D.; Cheng, L.-T.; Eisenberg, R. Chem. Mater. 1997, 9,
440-450.
(21) Barzoukas, M.; Runser, C.; Fort, A.; Barrzoukas, M. M. Chem. Phys.
Lett. 1996, 257, 531.
10.1021/ic980872q CCC: $18.00 © 1999 American Chemical Society
Published on Web 01/05/1999

288 Inorganic Chemistry, Vol. 38, No. 2, 1999
Base et al.
Chart 1
Figure 1. Structure-property relationship for linear polymethines
where D ) donor and A ) acceptor.
Table 1. Values of µ*â(0) and λmax for I-III in Various Solventsa
µ*â (0) (10-48 esu)/λmax (nm)
and first excited states.22-24 As shown in Figure 1, the second-
order polarizability, â, for linear polymethines varies dramati-
cally as the ground-state structure becomes more charge-
separated, going from left to right along the x-axis. The second-
order polarizability first increases going from the neutral polyene
limit to the cyanine limit, and then reaches a maximum for an
intermediate structure. Next, â decreases and passes through 0
at the cyanine limit. Moving from the cyanine limit to the
charge-separated limit, â continues to decrease, becoming
negative, and then reaches a minimum for an intermediate
charge-separated structure. Finally, â increases and returns to
0 at the charge-separated limit. The regions of positive â values
(∆µ > 0) and negative â values (∆µ < 0) are associated with
positive and negative solvatochromism, respectively.
C7H8 (0.099)b CHCl3 (0.259) CH2Cl2 (0.309) DMSO (0.444)
I
II
III
+118/483
-212/689
-523/717
-166/460
-412/634
-635/657
-433/612
-669/621
-480/590
-597/601
a Top entries are µ*â(0) values (in units of 10-48 esu) with an
estimated error of (10%. Bottom entries are the corresponding λmax
(nm) of the chromophore in a particular solvent. b Solvent polarities
increase from left to right, and the normalized ENT values of the solvent
are denoted within parentheses.29
catecholate yielded I, II, and III. The molecular structures are
shown in Chart 1.
Solutions of I, II, or III exhibit an intense absorption at 290
nm for the π-π* transition of diphenylphenanthroline, and a
second charge-transfer absorption at lower energy that is strongly
solvatochromic (Table 1). All three chromophores display a
hypsochromic (or blue) shift with increasing solvent polarity
(i.e., negative solvatochromism),29 and this effect is more
pronounced in complexes II and III. For example, III is soluble
in solvents ranging from toluene to methanol (∆E ≈ 4000 cm-1).
Cyclic voltammograms of I, II, and III in CH2Cl2 (0.1 M
Results and Discussion
Square-planar d8 platinum(II) compounds possess several
favorable structural and physical properties25,26 amenable to
NLO study15 and SPR development, including (i) rigid planar
structures, (ii) high thermal stability, (iii) polarizable metal, (iv)
solvatochromic transitions, (v) significant metal-ligand orbital
overlap, and (vi) charge-transfer transitions. The ground-state
dipole moment (≈10 D) of these donor (thiolate)-platinum-
acceptor (diimine) complexes is aligned in the direction of the
donor.15 Importantly, the electronic structure (e.g., sulfur to
oxygen) of these complexes can be varied dramatically while
not substantially altering the metal coordination and geometry.
To ensure sufficient solubility for physical characterization
and solvent-dependent â measurements, three new Pt(II) diimine
dithiolate complexes containing sterically demanding groups
were synthesized.27,28 Reaction of 4,7-diphenylphenanthroline
platinum(II) dichloride with the corresponding dithiolates or
-
TBA+PF6 vs Ag/AgCl) reveal a quasireversible reduction at
approximately -1.2 V for the diimine reduction. Complexes I
and II possess a quasireversible oxidation at 0.6 V, whereas
III possesses a reversible oxidation wave at 0.4 V. Photolumi-
nescence from I and II is observed at 298 and 77 K. I and II
lie upon the line defined by the energy of emission vs the
difference between the ground-state oxidation and reduction
potentials of other previously characterized Pt(diimine)(1,2-
dithiolate) complexes.25,26 These data support a charge-transfer
transition assignment from a mixed platinum-thiolate HOMO
to a π* diimine LUMO.
With regard to the SPR as presented in Figure 1, the negative
solvatochromism (i.e., the ground state is more charge-separated
than the excited state) of I, II, and III indicates that these Pt
chromophores lie somewhere between the cyanine and charge-
separated limits. Chromophore I possesses the largest oscillator
strength of these three similar metal complexes, suggesting that
it may lie nearest to the cyanine limit. The large solvatochromic
(22) Oudar, J. L.; Chemla, D. S. J. Chem. Phys. 1977, 66, 2664.
(23) Lalama, S. J.; Garito, A. F. Phys. ReV. A 1979, 20, 1179.
(24) Levine, B. F.; Bethea, C. G. J. Chem. Phys. 1977, 66, 1070.
(25) Cummings, S. D.; Eisenberg, R. J. Am. Chem. Soc. 1996, 118, 1949-
1960.
(26) Connick, W. B.; Gray, H. B. J. Am. Chem. Soc. 1997, 119, 11620-
11627.
(27) Base, K.; Grinstaff, M. W. Inorg. Chem. 1998, 37, 1432-1433.
(28) A preliminary account of the synthesis and photophysical properties
of II and III were reported at the 215th National Meeting of the
American Chemical Society, Dallas, TX, March-April 1998.
(29) Reichardt, C. SolVents and SolVent Effects in Organic Chemistry, 2nd
ed.; VCH: Weinheim, Germany, 1988, p 534.

Structure-Property Relationships of Metal Chromophores
Inorganic Chemistry, Vol. 38, No. 2, 1999 289
largest negative second-order polarizabilities measured for a
metal complex. Third, these data for inorganic chromophores
conform to the two-state model which relates the molecular
structure and the second-order polarizability. Fourth, as sug-
gested previously for organic chromophores, this present study
also proposes that NLO spectroscopy may be a new analytical
tool for characterizing the electronic structure of inorganic
complexes. Finally, these results further substantiate the use of
the two-state model and its concepts for the design, optimization,
and study of new NLO chromophores.
Experimental Section
All solvents were dried and distilled prior to use. 1H and 13C NMR
spectra were recorded on a Varian 400 MHz NMR. Elemental analyses
were performed by E+R Microanalytical Labs. Absorption spectra were
measured on a Hewlett-Packard 8452 diode array spectrometer.
Emission spectra were recorded on a Perkin Elmer LS50B.
Pt(dpphen)(dtoc) (I). Pt(dpphen)(dtoc) was synthesized by reacting
1,2-dithiolato-closo-1,2-dicarbadodecaborane(12) with Pt(dpphen)Cl2
and Proton Sponge in CH2Cl2 for 3.5 h (yield 82%). Anal. Calcd: C,
45.07; H, 3.78; N, 3.62. Found: C, 44.94; H, 3.93; N, 3.84.
Pt(dpphen)(dtbdt) (II). Pt(dpphen)(dtbdt) was synthesized by
reacting 3,5-di-t-butylbenzenedithiol with Proton Sponge and
Pt(dpphen)Cl2 in CH2Cl2 for 5 h (yield 25%). Anal. Calcd: C, 58.52;
H, 4.65; N, 3.59. Found: C, 58.65; H, 4.70; N, 3.51.
Figure 2. Second-order polarizability curve for I-III.
effect observed for these Pt(II) complexes also indicates that
the magnitude of ∆µ (the dipole difference between the first
excited and ground states) is significant, and moreover ∆µ
increases from I to III. Chromophore III, the oxygen analogue,
possesses the greatest charge-separated character and should be
at the far right on the charge-separated axis. Qualitatively, the
ground-state structures of I, II, and III display a gradual increase
in charge-separated character and should represent specific
regions between the cyanine and charge-separated limits in a
SPR graph.
Electric-field-induced second-harmonic generation (EFISH)
measurements of µ*â(0) for I, II, and III were performed in
solvents of increasing polarity using a fundamental wavelength
of 1.907 µm.30 A systematic tuning of the ground-state charge-
separated character in these Pt chromophores was achieved by
varying the solvent. The µ*â(0) responses and absorption
maxima as a function of solvent are presented in Table 1.
Importantly, the second-order NLO responses vary with solvent
polarity and span a wide range. As shown in Table 1, I possesses
a positive signal in toluene and a negative one in chloroform.
This change in µ*â(0) sign on going to more polar solutions
indicates that I lies near the cyanine limit. The µ*â(0) responses
of II are negative and decrease from toluene to DMSO.
However, the negative µ*â(0) responses for III first decrease
and then increase as the solvent polarity increases, identifying
the negative peak of the â curve. These results lead to the
interpretation sketched in Figure 2 and allow us to assess the
shape of the â curve for an inorganic chromophore.
[Pt(dpphen)(dtbc) III. Pt(dpphen)(dtbc) was synthesized by reacting
di-tert-butyl catechol with KOH and Pt(dpphen)Cl2 in refluxing CH2-
Cl2 for 24 h (yield 74%). Anal. Calcd: C, 61.03; H, 4.85; N, 3.75.
Found: C, 60.78; H, 4.91; N, 3.68.
EFISH Measurements. The conventional EFISH (electric-field-
induced second harmonic) technique was used to measure the quadratic
hyperpolarizability of the compounds.30,31 The initial wavelength 1.064
µm of a Nd:YAG laser was shifted to 1.907 µm by stimulated Raman
scattering in a high-pressure hydrogen cell. DC electric field pulses
with amplitude up to 30 kV/cm were applied to the solutions placed in
a wedge cell. The second-harmonic signals were calibrated to a quartz
wedge (quadratic hyperpolarizability taken equal to 1.1 10-19 esu at
1.907 µm). Experimental details and analytical expressions for the mean
microscopic hyperpolarizability γ are given in ref 31. Neglecting the
purely electronic contribution (appropriate for medium-sized conjugated
molecules) leads to the expression
µâ(2ω)
γ )
5kT
where kT is the Boltzmann factor, µg is the dipole moment of the
molecular ground state, and â stands for the vector part of tensor âijk-
(2ω). In the two-level approximation the dispersion dependence of the
quadratic hyperpolarizability â(ω) is described by the simple dispersion
factor
ω4max
â(ω) )
â(0)
Conclusions
(ωm2 ax - ω2)(ω2max - 4ω2)
A number of important conclusions can be drawn from these
data, given the strong correlation between the metal chro-
mophore electronic structure and the second-order polarizability.
First, these platinum(II) chromophores possess solvent-depend-
ent second-order polarizability responses, underlying the im-
portance of both molecular structure and the influence of the
solvent medium. Second, â for inorganic chromophores can be
optimized and platinum chromophore III exhibits one of the
where the ω’s are the pulsations corresponding respectively to the
fundamental laser wavelength λ and λmax and â(0) is the intrinsic static
hyperpolarizability.
Acknowledgment. This work was supported by the Petro-
leum Research Fund, administered by the American Chemical
Society (PRF 32875-G3), Lord Foundation, and Duke University.
IC980872Q
(30) Levine, B. F.; Bethea, C. G. Appl. Phys. Lett. 1974, 24, 445.
(31) Oudar, J. L. J. Chem. Phys. 1977, 67, 446.