J . Phys. Chem. 1993,97, 1540-1542
7540
Quadratic Hyperpolarizability of 4- (Dimethy1amino)benzonitrilein Solvents of Differing Polarity Dean W. Robinson. and Charles A. Long Department of Chemistry, Johns Hopkins University, Baltimore, Maryland 21 218 Received: March 9, 1993; In Final Form: May 5. 1993
The hyperpolarizability /3 of 4-(dimethy1amino)benzonitrile has been measured by the method of electric field induced second harmonic generation (EFISH) at 1064 nm, in different solvents. These represented a range of dielectric constant from 2.2 to 36.1. It had been predicted that such measurements would be influenced by, and shed some light on, a widely postulated twisted intramoleculer charge-transfer (TICT) state that has been deemed responsible for the dual frequency fluorescence of the molecule dissolved in polar solvents. Measurements of /3 in these solvents showed no sensitivity to solvent polarity.
Introduction The dual frequency fluorescence of the molecule 4-(dimethy1amino)benzonitrile (DMABN), shown below, and related compounds has been the subject of considerable theoretical and experimental study over the past 30 years. This unusual N
O
C
E
N
H3C
DMABN
phenomenon was first reported by Lippert, Liider, and Boos in 1962.1 In solvents of low polarity, such as hydrocarbons, the fluorescencelies in the vicinity of 29 400 cm-1; in solvents of high polarity, such as acetonitrile, the principal fluorescence is found near 20 000 cm-l, while in solvents of intermediate polarity, e.g. dichloromethane, both emissions are often observed. A number of diverse explanations for this interesting behavior has been provided. They include the original S I , Slevel ~ reversal picture by the Lippert group,l assignment of the long wavelength band to excimer emission by the McGlynn ~ c h o o l , the ~ - ~Kosower team’s attribution of the long wavelength band to a protonated excited state,6v7 the solvent-solute complex formation proposed by the Chandross laboratory,899 and finally the most durable notion of the set, the twisted intramolecular charge-transfer (TICT) tautomer of the emitting state originally suggested by Grabowski and co-workers.lOJ1 The idea presented for the TICT state is that solvents of high dielectric constant will stabilize a low lying excited charge-transfer electronic state, and once a molecule is in this state, any conjugation of the nitrogen’s lone pair electrons into the ring will be lost. This permits the rotation of the amino CNC plane into a plane which approaches perpendicularity to that of the ring (hence the name “twisted”) and the long wavelength, strongly solvatochromic emission originatesfrom this conformation. Some elegant papers from Eisenthal’s laboratory deal with the rates of this process in solvents of varying polarity and viscosity.l2-15 The bulk of the literature on the double fluorescence of DMABN since about 198 1 has described experiments and interpreted them in terms of an exdted TICT electronic state that is stabilized by high dielectric solvents. Effects of chemical derivatization,lOJlsolvent vi~cosity,~J~ polarity, and temperature16 appear to be consistent with this picture. Nonsolvatochromic emission at the higher frequency is thought to originate in a less polar state which is in chemical equilibriumwith the TICT species. So far, there exists no direct structural evidence for this twisted conformation, but the scheme is consistent with an abundance of data that seems to resist alternative explanations. The reports mentioned in the previous paragraphs were concerned largely with experimental studies; theoretical support 0022-3654/93/2097-7540$04.00/0
was provided in some cases. A number of quantum chemical calculationshave also appeared over the years that employ varying degrees of approximation and sophistication. References to these can be found in the most recent publications by Bhattacharyya and co-workers.17 Especially pertinent to the work reported in the present paper are the calculations by that group of polarizabilities and hyperpolarizabilities, and solvent effects thereon.18,19 The quadratic hyperpolarizability 0 2o of a molecule contains terms in its perturbation theory expansion that depend linearly on the difference between the electric dipole moment of each excited stateand that of thegroundstate.*’ Experience has shown that usually only one excited state dominates this effect in ?r-conjugated organic molecules. This dominating state is one that involves intramolecular charge transfer which is responsible for the large Ap. If it is a low lying state, it further contributes to the dominance of one term in the expansion through the presence of a resonance denominator. Essentially all known high j3 molecules possess an electron donating and an electron attracting entity, connected through .rr-conjugation, between which the charge is transferred on excitation. This is the model employed by the Bhattacharyya group in their calculations of &, and 00;here the subscripts refer to the values of 0 at frequency w and frequency zero. It is a two-state molecule in which the charge-transfer state is identified with the TICT transition at about 20 800 cm-I in a solvent of dielectric constant c = 0, which “Stokes shifts” to 12 500 cm-1 in a solvent of c = 35 (that of acetonitrile). It was calculated that, “...@(w) increases smoothly in any solvent as the fundamental frequency (a)is increased and begins to diverge [as] 2w approaches the (T)ICT transition energy of DMABN”.19 The plots of & vs w are hard to read because of their small scale, but for a fundamental wavelength of 1.064 pm in a solvent of dielectric constant c = 35, 8 1 ~ appears 4 to be close to 40 X esu. In a solvent of low dielectric constant it appears to approach zero. A plot of Pw,or 80,vs e of the solvent shows a rapidly increasing function up to dielectric constant of about 20 and then a rather sharp leveling off above that. These results will be compared with experiment below. The quadratic hyperpolarizability /3 of this molecule has appeared before in the literature. Ledoux et al.23 give the values (8.3 f 0.4) X 10-30, (7.5 f 0.4) X 10-30 and (5 f 0.3) X 10-30 esu for the fundamental wavelengths 1064 nm, 1340 nm, and infinity, in the solvent chloroform; Cheng et al.24 quote 5.0 X esu at 1910 nm in dioxane solution.
Experimental Section The determination of 81064 of DMABN was made by the wellknown electric field induced second harmonic generation 0 1993 American Chemical Society
Quadratic Hyperpolarizability of DMABN
The Journal of Physical Chemistry, Vol. 97, No. 29, 1993 7541
TABLE I: Comparison of Experimental and Theoretical HyperpolarizabilitiesB of DMABN in Solvents of Differing Polarities solvent
t
CCL 2.228 HCCl3 4.806 HiCCli 9.08 H3CCN 36.1 H3COH 32.6
Ac) 0.225 0.358 0.422 0.480 0.477
hu./ nm
290 292 292 291 295
&(calc)C 3@0(calc) @o(theory)d
@ l ~ b
6.7 7.6 8.0 7.2 7.1
4.4 4.9 5.2 4.7 4.6
13.2 14.7 15.6 14.1 13.8
8.6 12.8 16.0 17.4 19.4
a From ref 7. b Taking fi = 6.6 D from ref 30. All i3 values are in units of 10-30 esu. Given the difficulties in estimating the accuracy of such measurements, the authors have confidence only to within h0.5 unit. c Calculated with eq 1 from of column 5. From Figure 2 of ref 19, the plot of BO vs f i e ) .
experimental determinations.2233 Although the values in column seven have the magnitude of the average of the theoretical counterparts in column eight, there is no agreement with the predicted dependence of solvent polarity. The lack of dependence on solvent polarity is, however, entirely consistent with the absence of solvatochroism. It is interesting that the absorption maxima are much less susceptible to solvent influence than is the case for the somewhat analogous CT molecule,p-nitroaniline.29Methanol may not be comparable with the non-hydrogen bonding solvents but is included to demonstrate more forcefully the indifference of 80to the molecular environment.
Discussion The determinations reported here fail to shed any light on the nature, or even the existence, of a TICT state. However, this (EFISH) method using apparatus that has been described observation is really quite consistent with the popular picture of elsewhere,25 except that the entire operation was automated, an intramolecular charge-transfer excited state conformationally controlled by a North Star computer. The solvents used were distorted in some fashion from the Franck-Condon, unrelaxed, carbon tetrachloride, chloroform, methylene chloride, methanol, initially-excited singlet (whether or not the distortion involves and acetonitrile, in order to span a range of solvent polarity and the twist of the dimethylamine group). Such a state is only provide one sample with the opportunity for specific, (hydrogen reachable from the ground state by electronic excitationfollowed bonding) solvent-solute interaction. Experience in this laboratory by rearrangement of the nuclei into a conformation that minimizes has shown that these measurements are singularly insensitive to (or a t least decreases) the potential on the excited electronic impurities, so materials were used as received in freshly opened energy surface. containers. The 4-(dimethy1amino)benzonitrile was obtained For frequency doubling to occur, waves at frequency 2w are from Aldrich and the solvents from J. T. Baker or Fisher Chemical. generated from those at w by the induction of an anharmonic Bulk, solution cubic hyperpolarizabilities I’ were obtained a t the distortion of the charge cloud in the molecule, that is, an fundamental frequency of 1064 nm by comparing the intensity anharmonic polarization which cycles with a frequency of w = of the harmonic generated by each solution with that of a quartz 2.8 X 1014s-1 corresponding to the 1064-nm fundamental of the reference crystal as soon as possible before and after each EFISH laser. The experimentsof Eisenthal and co-workers have showni3 run. Intensities were measured as the quartz-referenced amthat the emitter of the longer wavelength band (assigned to the plitudes of the Maker fringes. For quartz thenonlinear coefficient TICT state) reaches equilibrium with its precursor -20 ps after dll = 1.2 X 10-9 esu was used as a reference.26 excitation, or it could “twist” with a frequency of 5 X 1Olo s-l (an Coherence lengths were calculated from the fringe separation eminently believable rotational frequency). Thus, the electronic, and the cell wedge angle. Refractive indices and dielectric anharmonic polarization frequency exceeds the twisting rate by constants were not measured but taken from the literature for the a factor of 5600! How could the ground-state polarizability or pure solvents. This shortcut should not affect the value of the hyperpolarizability, a t the ground-state atomic configuration, be intercept of r D M a B N extrapolated to infinite dilution. As is in any way influenced by the TICT state? It appears from the customary with molecules of the class, donor/conjugated-chain/ above that it is not. acceptor, the molecular analog of the bulk r D M A B N (YDMABN) is The fact that @ is positive simply proves that the excited state taken to be equal to p@/5kT, the orientational contribution, exerting the dominant influence on its magnitude possesses an neglecting a
