7092
J . Phys. Chem. 1989, 93, 7092-7095
Structure-Energy Correlatton of the Intramolecular Charge-Transfer State in a Series of ( N ,N-Methy1anitino)arenes Jack C.-C. Tseng and Lawrence A. Singer* Department of Chemistry, University of Southern California, Los Angeles. California 90089-0744 (Received: February 9, 1989; In Final Form: April 3, 1989)
The energies of the charge-transfer state for a series of (N,N-diethylani1ino)arenes (1-naphthyl, 1-pyrenyl, 9-anthryl, and 3-fluoranthyl) in acetonitrile correlate well with the one-electron redox potentials of the structural moieties, in accord with a radical cation/radical anion description for this state. Solvent-dependent shifts of the fluorescences for these compounds indicate highly polar excited states with dipole moments (pe) ranging from 13.4 to 20.9 D.
Introduction In recent years, there has been growing interest in compounds possessing both donor and acceptor r-electron structural components that are capable of photoinduced intramolecular charge transfer.14 There are many examples where conformational folding brings the donor and acceptor r-electron systems into parallel planes within through-space interaction In another type, where the donor and acceptor groups are directly connected through a single bond, conformational folding is precluded.&I0 Extensive work on 9-(N,h'-dimethylanilino)anthracene and related compounds, as examples of the latter, suggests formation of a twisted intramolecular C T state (TICT)" where the donor and acceptor groups reside in mutually orthogonal planes. The large magnitude of the excited-state dipole moments (typically 14-18 D) estimated with several of these compounds is consistent with a radical cation/radical anion description of the TICT state. Others have pointed to the important role of solvent polarity in stabilizing the resulting highly polar state.'* Additional and complementary evidence for this picture is found in the present study, which correlates the energy of the C T fluorescence from a series of (N,N-diethylani1ino)arenes (1-4) with the one-electron
A
k% \ /
\ / 1
\ /
\ /
N(Et)2
2
4
3 (1) Grabowski, 2.R.; Dobkowski, J. Pure Appl. Chem. 1983,55245, and references cited therein. (2) Heitele, H.; Michel-Beyerle,M. E.; Finckh, P. Chem. Phys. Lett. 1987, -IP4 - . , -771 . -. (3) Imabayashi, S.; Kitamura, N.; Tazuke, S. Chem. Phys. Lett. 1988,153, 23. (4) Eckert, C.; Heisel, F.; Miehe, J. A.; Lapouyade, R.; Ducasse, L. Chem. Phys. Lett. 1988, 153, 357. ( 5 ) Hirayama, F. J . Chem. Phys. 1965, 42, 3163. (6) Mimura, T.; Itoh, M. J . Am. Chem. Sac. 1976, 98, 1095. (7) Davidson, R. S.In Advances in Physical Organic Chemistry; Wiley: New York, 1983; Vol. 19, p 1. (8) Rettig, W. Angew. Chem., Int. Ed. Engl. 1986, 25, 971. (9) Hagopian, S.;Singer, L. A. J . Am. Chem. Sac. 1985, 107, 1874. (10) Detzer, N.; Baumann, W.; Schwager, B.; Frohling, J.-C.; Brittinger, C . Z . Naturfarsch. 1987, 42A, 395. (1 1) Siemiarczuk, A.; Grabowski, Z. R.; Krowczynski, A,; Ascher, M.; Ottolenghi, M. Chem. Phys. Lett. 1977, 51, 315. (1 2) Gilabert, E.; Lapouyade, R.; Rulliere, C. Chem. Phys. Lett. 1988, 145, 262.
0022-3654/89/2093-7092$01.50/0
TABLE I: Observed Fluorescence Maxima (nm) for Compounds 1-4 in Two Solvents compd cyclohexane acetonitrile 1 2 3 4
394
458
422
514
445
550 586
413
redox potentials of the donoracceptor structural components. We believe that the compounds used in this systematic structureenergy correlation provide additional evidence in support of the radical-ion description for the TICT state. The syntheses of compounds 1-4 will be described in detail e1~ewhere.l~
Experimental Section Sample Preparation. Compounds 1 4 were purified by preparative thin-layer chromatography on silica gel prior to use. The purity of each sample was assessed by a combination of analytical scale thin-layer chromatography, gas chromatography/mass spectroscopic (GC/MS) analysis, and ultraviolet-visible spectroscopic analysis. Standard solutions of the purified materials were prepared in acetonitrile (purified by refluxing over and then distilling from phosphorus pentoxide and redistilling from calcium hydride). Other solvents (cyclohexane, benzene, ethyl ether, tetrahydrofuran, and ethanol) were of spectroscopic grade and freshly distilled immediately before use. Spectroscopic Measurements. Ultraviolet-visible absorption spectra were recorded on a Shimadzu Model UV-260 spectrophotometer. Steady-state emission spectra were recorded on nondegassed samples with an American Instrument Co. spectrofluorimeter with a Hamamatsu Model R446 photomultiplier tube. Spectra were not corrected for instrument through-put and response. Time-resolved emission spectra on degassed samples were recorded with a pulsed nitrogen laser (337 nm; IO-ns width; peak power, 100 kW) in conjunction with a Princeton Applied Research boxcar integrator (Model 160) as previously described.14 Results and Discussion Electronic Absorption Spectra. The ultraviolet-visible spectra of 1-@-N,N-diethylani1ino)naphthalene (1) is dominated by the N,N-diethylaniline group. In contrast, the absorption spectra of compounds 2 4 reflect the strong absorptions from the polynuclear aromatic rings that obscure the weaker absorption of the aniline moiety. In these compounds, the long wavelength bands are r , r * transitions localized in the arenes. In all four compounds (1-4), there are little or no C T contributions to the absorption spectra as indicated by only very modest red shifts in going from cyclohexane to acetonitrile as solvent. A typical result is shown in Figure 1 where the electronic absorption spectra of 1-(p-N,Ndiethylani1ino)pyrene (2) in the two solvents (upper) are presented (13) Tseng, J. C.-C. To be published. Observed melting point ("C) for 1, 100-102; 2, 213-214; 3, 195-196; and 4, 179-180.
(14) Brown, R. E.; Legg, K.; Wolf, M. W.; Singer, L. A,; Parks, J. H. Anal. Chem. 1974, 46, 1690.
0 1989 American Chemical Society
Structure-Energy Correlation of (N,N-Diethylani1ino)arenes
The Journal of Physical Chemistry, Vol. 93, No. 20, 1989 7093
I
260001
I
n
200
250
300
350
400
0
I-Naphthyl
A
I-Pyrenyl
450
0 9-Anthryl
nm
0
16OOO 200
250
300
350
400
0.16
0.24
0.32
3-Fluoranthyl
0.40
450
nm
Figure 1 . Upper: UV-vis absorption spectra for compound 2 in cyclohexane (-) and acetonitrile (- -). Lower: UV-vis absorption spectra for 1-phenylpyrene in cyclohexane (-) and acetonitrile (- -). Concentrations used: 4 X 10” M.
and compared with those of 1-phenylpyrene (lower) as a model. Overall, the electronic absorption spectra indicate little electronic interaction between the donor and acceptor groups in the ground state. Fluorescence Studies. The fluorescences from compounds 1-4 are highly solvatochromic. The fluorescence maxima for compounds 1-4 in cyclohexane and acetonitrile are provided in Table I. Wavelength shifts between cyclohexane and acetonitrile correspond to energy differences of 0.45 f 0.05 eV. While the nature of the emitting state in cyclohexane still needs clarification (locally excited A,R* or CT), the strong red shifts in going to acetonitrile indicate a highly polar excited state in this solvent. From the earlier discussion of the electronic absorption spectra, it appears that excitation in these compounds occurs into a locally excited, and relatively nonpolar, A,A* state in either the aniline (compound 1) or the arene moiety (compounds 2-4) that rapidly relaxes with intramolecular electron transfer to the lower lying, ~ but highly polar, TICT state from which emission O C C U ~ S .The fluorescence lifetimes for the CT states are all smaller than 5 ns, which is the limit of our time resolution. Dipole Moments of the Excited (CT) States. The dipole moment of the excited state can be obtained by the following eq~ations’~J~ ~n = ~ n ( 0 )- 2p:Af/hca3
Af = [(e - 1)/(2e
- l)] - [ ( n 2 - 1)/(4n2
(1)
+ 2)]
(2)
where h is Planck’s constant, c is the speed of light, un is the fluorescence maximum in a solvent of dielectric constant t and refractive index n, ~ ~ (is0the ) gas-phase value of the fluorescence maximum, and pc is the static-state dipole moment. The slope of a plot of un vs Afgives the value of &:/a3. The results of such plots for compounds 1-4 in solvents of different polarities are shown in Figure 2. The magnitude of pe may be obtained from the slope by estimating the value of a (the effective radius of the solvent shell around the molecule) for each compound. Structural parameters determined by X-ray analysis” on the single crystal of 1-(p-N,N-diethylani1ino)naphthalene (1) were (15) Onsager, L. J . Am. Chem. SOC.1936, 58, 1483. (16) Mataga, N. In The Exciplex; Gordon,M., Ware, W., Eds.; Academic Press: New York, 1975. (17) Tseng, J.; Huang, S.; Singer, L. A. Chem. Phys. Lett. 1988, 153,401.
Af Figure 2. Fluorescence maxima of compounds 1-4 as a function of solvent parameter AJ 0,1-naphthyl; A, 1-pyrenyl; 0,9-anthryl; 0 , 3-fluoranthyl.
TABLE II: Calculated and Observed p1)s and Parameters Used in Calculations for Compounds 1-4 PeP
comd 1 2
a/A
3
4.8 5.7 5.4
4
6.0
caIca 20.8 24.9 20.1 28.5
obs 13.4 19.2 17.2 20.9
Edb/eV -2.26 -1.87 -1.72 -1.53
Expected dipole moment for full electron transfer. Values were obtained by treating the donor and acceptor as point charges. The distance of the resulting dipole was taken from the center of the aniline ring to the center of the arene ring. bStreitwieser, A., Jr. Molecular Orbital Theoryf o r Organic Chemists;Wiley: New York, 1961.
fed into the three-dimensional molecular modeling program (Chem3D by Cambridge Scientific Computing, Inc.). The program was used to calculate interatomic distances, from which the value of a was approximated as 40% of the long axis of the ellipsoid around the molecule.’* Because of the similarities in structure, key bond angles and bond lengths evaluated by X-ray analysis on compound 1 were used in the molecular modeling to estimate the a values for compounds 2-4. The results are listed in Table 11. The solvatochromic behavior of compounds 1-4 is very similar as evidenced by the slopes, &:/a3, in Figure 2. From these slopes and the values of a determined from molecular modeling were obtained the estimates of pe (observed) in Table 11. A smaller dipole moment for the naphthalene system (1) is expected due to its shorter molecular dimension. The pe value determined for the anthracene system (3) is very close to that elucidated by Brittinger and co-workers for 9-(p-N,N-dimethylanilino)anthracene (- 18.1 D).’O The fluoranthene system (4) shows the largest excited-state dipole moment of the four systems, which may simply reflect the greater molecular dimension in this structure. Potential Energy Surface. Recent quantum mechanical calculations on the equilibrium geometries for l -phenylnaphthalene (a stereoelectronic model for compounds 1,2, and 4) in the ground and excited (S,)states indicate that the phenyl ring is rotated out (18) Beens, H.;Knibbe, H.; Weller, A. J. Chem. Phys. 1967, 47, 1183.
7094
rhe Journal of Physical Chemistry, Vol. 93, No. 20, 1989 Locally Excited
Tseng and Singer
.,
2.8
2.6
I
t
I
-2 -u" 2
-T
Ip
W
W
s
2.4
c)
2.2
solv
2.0
0'
30'
50'
-2.2
-2.0
-1.8
-1.6
.
4
90'
Twist Angle ( 0 ) Figure 3. Generalized potential energy surfaces in polar medium.
of the plane of the naphthalene ring by 50' and 32', respect i ~ e l y . ' ~An * ~X-ray ~ structure analysis of compound 1 indeed shows a twist angle of 58.7' between the two aromatic groups in the ground state." Rotation around the arene-arene bond interconverts equivalent geometries over a barrier at 90' that is estimated to be only 200-300 cal in the ground state.I9 The above observations are incorporated into the potential energy surfaces shown in Figure 3.2' The excited-state surface includes a minimum near 32O for the locally excited x,a* state in the arene and a second minimum at 90' for the highly polar TICT state.22 Excitation occurs from the ground state into the locally excited T,T*state. Relaxation of this state over a barrier leads to the TICT state with an arene-arene twist angle close to 90'. It may be significant that the magnitudes of the dipole moments of the TICT state seem to be only -65-80% of the expected values for full charge separation suggested to occur at the 90' twist angle (where the donor and acceptor moieties are electronically uncoupled). If radiative decay requires some T,?Toverlap (for back electron transfer) between the rings, fluorescence will be occurring from geometries with twist angles less than 90'. Such geometries should have less than full charge separation because of some back electron transfer. Accordingly, they should display solvatochromic behavior characteristic of the smaller dipole moments expected for the nonorthogonal fluorescent state. Grabowski and Dobkowski' have pointed out that the TICT fluorescence occurs from a highly polar excited state to a Franck-Condon (FC) ground state with a much smaller dipole moment (ge = 2-3 D). They estimate the destabilization of this (19) Gamba, A.; Rusconi, E.; Simonetta, M. Tetrahedron 1970, 26, 871. (20) Gustav, K.; Kempka, U.; Suhnel, J. Chem. Phys. Lett. 1980,71,280. (21) An important aspect of Figure 3 is the relationship between the energetics of the charge-transfer fluorescence described by eq 3 and important photophysical processes after solvent relaxation in the excited state. It is important to recognize the progress made by others in describing the solvent/solute dynamics in such systems. For example: Hynes, J. T. Annu. Rev. Phys. Chem. 1985, 36, 573-97, and references cited therein. Hwang, J.-K.; Creighton, S.; King, G.; Whitney, D.; Warshel, A . J . Chem. Phys. 1988, 89, 859, and referenccscited therein. The relevant effects are considered in Figure 3. The excited state has sufficient time for solvent equilibration on the nanosecond time scale of the described chemistry so the excited-state potential energy surface is for a solvent-equilibrated system. The important solvent reorganization associated with chargetransfer fluorescence is depicted by E& Future time-resolved experiments may shed more light on the roles of solvent and solute dynamics. (22) This model would not rigorously apply to 1 where the N,N-diethylaniline chromophore dominates the near-UV absorption spectrum.
Figure 4. Plot of E,,(obs) vs Erd.
FC state over the solvent-equilibrated ground state to be near 0.5 eV as shown in Figure 3. Structure-Energy Correlation. The energetics for the intramolecular charge-transfer fluorescenceu as modified by Grabowski and Dobkowski' where D is the donor and A the acceptor is
where C = e2/rtrepresents the Coulombic energy for two charges at a distance r in a solvent of dielectric t. For r = 5 A, which is the approximate distance from the center of the aniline ring to the middle of the arene moiety, C = 2.9 eV/e. It follows that the Coulombic energy is very significant in cyclohexane (e = 2.0) but much less so in polar media such as acetonitrile (e = 37.5) where C = 0.08 eV. The values of E,, and Erd, which are the one-electron redox potentials of N,N-diethylaniline and the four arenes, are measured in polar media and, therefore, already include the energy of solvation of the ions. E& is the destabilization energy associated with the solvent dipole relaxation in the less polar ground state.24 Ed= was estimated by using the values of a and we (observed) in Table I1 along with pP values (all
