J . Phys. Chem. 1991, 95, 9666-9612
9666
In summary, the results of electron-transfer reactions in a dynamical quenching regime can be qualitatively rationalized. Nevertheless, quantitative aspects relating theory and experiment presently remain unresolved. To this end, a comparison of theory and experiment which focuses on the combined effects of shorttime dynamics and concentration effects on the transient behavior of the fluorescence lifetime would be welcome.
Acknowledgment. We thank Claudia Turro for computational assistance. Financial support from the National Science Foundation (D.G.N., CHE-8705871; R.I.C., CHE-8318101) and the Center for Fundamental Material Research. D.G.N. also gratefully acknowledges a Presidential Young Investigator Award administered by the National Science Foundation and the financial assistance provided b) Dow Chemical Co.
Conformation-Dependent Intramolecular Electronic Energy Transfer in a Molecular Beam Mita Chattoraj, Balakrishna Bal, G. L. Gloss,* and Donald H. Levy* The Department of Chemistry and The James Franck Institute, The University of Chicago, Chicago, Illinois 60637 (Received: May 28, 1991)
The electronic spectrum of a molecule consisting of two chromophores separated by a spacer was observed in a supersonic molecular beam. The chromophores were anisole and dimethylaniline (DMA), the spacer was cyclohexane, and the chromophores were attached to the I - and 4-positions of the spacer. Each chromophore could be attached axial ( a ) or equatorial (e) with respect to the cyclohexane and the conformers ae, ea, and ee were observed. The aa conformer is not energetically allowed and was not observed. Following excitation of the SI state of the anisole moiety, emission was observed from both the anisole and DMA parts of the molecule. Since there was no direct absorption by the DMA, the emission from the DMA was produced by intramolecular electronic energy transfer. Measurement of the relative intensities of the anisole-like and DMA-like emissions provided a measure of the relative electronic energy transfer rates of the various conformers. It was found that energy transfer was considerably slower in the two trans (ee) conformers than in the four cis (ae and ea) conformers. This observation is not consistent with either simple Forster energy transfer theory, which predicts that the electronic energy transfer rate of all six conformers should be nearly the same, or the Dexter formalism, predicting that the trans isomers should be faster.
Introduction In addition to being of intrinsic scientific interest, electronic energy transfer has important implications in various fields such as biological systems, dye lasers, and wavelength shifters.' Recently much work has been done on intramolecular systems of the type A-Sp-D, where A using bichromophoric is an acceptor chromophore, D is a donor chromophore, and S p is a spacer. If the intervening spacer is rigid and the distance between and the relative orientation of the acceptor and donor is known, it is possible to probe the effects of geometrical factors on energy transfer. Several theoretical models have been formulated to account for the energy transfer observed. On the most fundamental level, radiationless transition theory explains the phenomena completely. Often the systems studied are too unwieldy to do a b initio calculations and a simpler formulation for the process becomes essential. ForsterIo modeled the interaction between the donor and acceptor as a simple Coulombic one and came up with one of the more widely accepted and easy to apply theories. When the chromophores are close enough to one another that the overlap of wave functions cannot be ignored, the Forster model becomes inapplicable and the Dexter model," using the ( 1 ) Speiser, S. J . Pholochem. 1983. 22, 195. (2) Getz. D.; Ron, A.; Rubin, M. B.: Speiser. S. J . Phys. Chem. 1980, 84. 768. ( 3 ) Zimmerman, H. E.; Goldman, T. D.; Hirzel. T. H.; Schmidt, S. J . Org. Chem. 1980. ~ 45. .3935. ~ , (4) Hassoon, S.; Lustig (Richter), H.; Rubin, M. B.; Speiser, S Chem. Phys. Lerl. 1983, 98, 345. ( 5 ) Hassoon, S.:Lustig. H.: Rubin. M . B.; Speiser. S . J . Phys. Chem. 1984. ~
88. 6367 ...
(6) Mugnier, J.; Valeur. B.; Gratton. E. Chem. Phys. Lett. 1985, 119, 217. (7) Ernsting, N. P.; Kaschke, M.; Kleinschmidt, J.; Drexhage. K. H.; Huth. V . Chem. Phys. Left. 1988, 122, 431. (8) Gryczynski, 1.; Wiczk, W.; Johnson, M. L.: Lakowicz. J . R . Chem. Phys. Lelf. 1988. 145, 439. (9) Closs. G.L.; Piotrowiak. P.;Maclnnis, J . M., Fleming, G R . J . Am. Chem. Soc. 1988, /IO,2652. (IO) Forster. Th. Discuss. Faraday SOC.1959, 27, 7.
0022-365419 112095-9666$02.50/0
exchange interaction between two charge clouds, has been used to model the phenomenon. Most of the work on energy transfer has been done in condensed phases where the solvent plays an active role and it is not possible to observe solely the intramolecular electronic relaxation process. Only very limited gas-phase studies have been performed.'* In the present work we have observed intramolecular electronic energy transfer in the rigid bichromophoric molecule 1-(4anisyl)-4-(N,N-dimethyl-4-anilinyl)cyclohexane (AC6D) under the isolated conditions of a supersonic jet expansion. The interpretation of the spectra was facilitated by studying the monochromophoric systems 4,N,N-trimethylaniline (TMA) and 4-anisylcyclohexane (AC,) shown in Figure 1. The electronic absorption spectrum of the bichromophore is practically a superposition of the spectra of anisole and N,N-dimethylaniline (DMA), indicating that the electronic interaction between them is weak. As a result, we have been able to exclusively excite the donor and probe the energy transfer process by dispersing the fluorescence emission. In the ultracold environment of a molecular beam it has been possible to isolate individual conformers and examine the dependence of the rate of energy transfer on the conformer excited. We find that the cis isomer in its various conformations transfers energy much more rapidly than the trans isomer. These observations are examined in the light of previous theorq. Experimental Section The model compounds were synthesized by standard methods.I3 The spectra presented here were recorded using resonantly enhanced two-photon ionization, fluorescence excitation, and dispersed fluorescence techniques. Detailed descriptions of the ex( I I ) Dexter. D. L. J . Chem. Phys. 1952, 2 1 , 836. ( 1 2 ) (a) Ebata, T.: Suzuki, Y.; Mikami, N.; Miyashi, T.; Ito, M. Chem. Phys. Left 1984. 110, 597. (b) Felker, P. M.; Syage, J. A.; Lambert. W. R.; Zewail, A. H. Chem. Phys. Lert. 1982, 92, I . i 13) Sigman, M. E.; Closs, G L . J . Chem. Phys., to be published.
0 199 1 American Chemical Society
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The Journal of Physical Chemistry, Vol. 95, No. 24, 1991 9661
\”
b
d
35300
35500
/
Figure 1. Figures of the compounds used. From right to left these are 4,NJ”trimethylaniline (TMA), 4-methylanisole, 4-anisylcyclohexane (AC,), and I-(4-anisyl)-4-(N,N-dimethyl-4-anilinyl)cyclohexane (AC,D).
perimental apparatus may be found e1~ewhere.l~The molecules were heated ( 1 70 “ C for the bichromophore and 80 OC for the monochromophores) to attain sufficient vapor pressure. They were seeded into helium gas of 3-4-atm stagnation pressure and the mixture was expanded into a vacuum chamber through a 0.05mm-diameter orifice. The ionization experiments were performed by skimming the free jet expansion. The resultant molecular beam was then probed by the frequency-doubled output of a Nd-YAG pumped dye laser between the ion extraction grids of a time-of-flight mass spectrometer. The ion signal of the mass of interest was recorded as a function of excitation wavelength. Mass-selected photoionization spectra were recorded for all samples. The appearance of a single mass signal at the expected time indicated that decomposition was not occurring a t the temperature to which the molecules were heated. In the fluorescence experiments the mass spectrometer was replaced with fluorescence collection optics. The excitation spectra were measured by monitoring the total fluorescence as a function of wavelength with a photomultiplier tube. The dispersed fluorescence spectra were measured using a 1 .O-m monochromator and detected by a photomultiplier. Time-resolved studies of the fluorescence emission were obtained by routing the output of the photomultiplier to a C A M A C transient digitizer with a 10 ns per bin digitization rate. The laser pulse width was about 8 4 s full width at half-maximum (fwhm) and our instrument response function, measured by scattering light off the nozzle, was found to be about 2 0 4 s fwhm. The fluorescence decays were fit iteratively to minimize x2.
Results Fluorescence Excitation Studies. Model Compounds. To aid the interpretation of the spectroscopy of the bichromophoric compound AC6D, we have also studied the simpler monochromophoric model compounds AC6 and TMA. The fluorescence excitation spectrum of 4-methylanisole is shown in Figure 2. In agreement with previous workI5 it shows an intense origin peak and several other transitions at higher wavenumbers. Features in the spectra of molecules with conformational degrees of freedom may be due to vibrational activity or multiple conformers.16 In an effort to distinguish between these we have performed power saturation studies. Spectra of methylanisole taken at power densities differing by a factor of 100 (14) Sharfin, W.; Johnson, K.E.;Wharton, L.; Levy,D. H.J . Chem. Phys. 1979, 71, 1292. ( I S ) Breen, P. J.; Bernstein, E.R.; Secor, H. V.;Seecor, J. I. J. Am. Chem. SOC.1989, I l l , 1958. (16) Rizzo, T.: Park, Y.D.: Peteanu, L. A.; Levy, D.H.J . Chem. Phys. 1985, 84, 2534.
35700
35900
WAVENUMBER (CM-’)
Figure 2. Fluorescence excitation spectra of 4-methylanisole. (a) Spectrum recorded at low laser powers. (b) Spectrum recorded at laser power densities increased by a factor of 100.
-
a
i
b
1
35500
35700
35900
36100
WAVENUMBER (CM-’)
Figure 3. Fluorescence excitation spectra of 4-anisylcyclohexane (AC,). (a) Spectrum recorded at low laser powers. (b) Spectrum recorded at laser power densities increased by a factor of 100.
-
1
I
I
I
32 100
32250
32400
32550
W A V ENUMBER (CM-’ )
Figure 4. Fluorescence excitation spectrum of 4,N,N-trimethylaniline (TMA) at its origin.
show an increase in the relative intensities of all the peaks with respect to the origin. Figure 3 shows the fluorescence excitation spectrum of AC6. There are two strong features a t 35603 and 35648 cm-l and weaker transitions a t higher wavenumbers. When the laser intensities are increased to the level a t which transitions are being saturated there is no significant change in the relative intensities of the strong features, although all the weaker ones increase considerably with respect to them. The two-photon ionization spectrum of T M A taken in the wavenumber region 35 000-36 000 cm-’ is weak, broad, and featureless. This molecule was also studied near its origin at 32 155 cm-I (Figure 4) where it shows a sharp spectrum quickly degenerating to a congested spectrum with increasing energy. The Bichromophore. The bichromophore AC6D exists as two isomers, cis and trans, which were chemically separated. The cis (equatorial-axial) isomer shows four strong features in the
9668
The Journal of Physical Chemistry, Vol. 95, No. 24, 1991
Chattoraj et al. a
b b C
C
e
d f
1111 1111 11111 35500 35600 35700
III III IIli l 32000 29000 26000
35400
W A V E N U M B E R (CM-' ) Figure 5. Fluorescence excitation spectra of the bichromophore. (a) The
cis (equatorial-axial) isomer at low laser powers. (b) The cis isomer at laser power densities increased by a factor of -100. (c) The trans (diequatorial) isomer at low laser powers. (d) The trans isomer at laser power densities increased by a factor of 100.
-
a
I
d l 1 111 111 111 1 35000 32000 29000 26000 WAVE NUMB E R (CM-' ) Figure 6. Emission spectra of the model compounds. The intensity scale is different for the three different compounds. The maximum signal a t
35 585 cm-l for the AC, is 30 times stronger than the maximum emission intensity at 29 I 5 3 cm-I for TMA. ( a ) 4-Anisylcyclohexane (AC,) excited at 35 603 cm". 12% of the intensity at the resonant wavenumber is due to scatter. The resolution is 76 cm-I. (b) AC, excited at 35 648 cm-I. 14%of the intensity at the resonant wavenumber is due to scatter. The resolution is 76 cm-l. (c) 4,N,N-Trimethylaniline (TMA) excited at 35 388 cm-l. Within our ability to measure, the signal intensity at the resonant wavenumber is exclusively scattered light. The resolution is 100 cm-l. (d) 1 -(4-Anisyl)-4-(N,N-dimethyl-4-anilinyl)cyclohexane (AC,D) excited at 35 388 cm". Within our ability to measure, the signal intensity at the resonant wavenumber is exclusively scattered light. The resolution is 76 cm-'. fluorescence excitation spectrum (Figure 5 ) at 35 521, 35 607, 35 656, and 35 661 cm-' along with weaker transitions at higher wavenumbers. Again, increasing the power density by a factor of 100 did not cause a significant change in the relative intensities
35000
WAVENUMBER (CM-')
Emission spectra of 1-(4-anisyl)-4-(N,N-dimethyl-4aniliny1)cyclohexane (AC6D). The resolution is 76 cm-'. Within our ability to measure, the signal intensity at the resonant wavenumber is exclusively scattered light, and this spectral region is not shown in the figure. The first letter (a = axial; e = equatorial) describes the anisole chromophore, the second, the dimethylaniline chromophore. (a) Excitation at 35 521 cm-' of the ae conformer of the cis isomer. (b) Excitation at 35661 cm-' of the ae conformer of the cis isomer. (c) Excitation at 35607 cm'l of the ea conformer of the cis isomer. (d) Excitation at 35656 cm-I of the ea conformer of the cis isomer. (e) Excitation at 35 591 cm-' of the ee conformer of the trans isomer. (f) Excitation at 35 637 cm-' of the ee conformer of the trans isomer. Figure 7.
of these peaks, though all the weaker transitions increased with respect to them. The fluorescence excitation spectrum of the trans (diequatorial) isomer (Figure 5) showed two strong features, 35 591 and 35 637 cm-'. Increasing the power density did not cause an experimentally relevant change in their relative peak heights either. Lifetime Studies. Fluorescence lifetime measurements of AC,, TMA, and AC6D were attempted. The lifetime of AC6 was found to be 12.2 f 0.9 ns. The lifetimes of T M A and AC6D were shorter than our experimental resolution. Dispersed Emission Studies. Model Compounds. The fluorescence emission spectra produced by exciting the two strong peaks of AC6 were observed (Figure 6). The two conformers have minor differences in their emission patterns. These spectra are very similar to the emission spectrum of 4-methylanisole" at its origin, indicating that the features seen are ring modes and are not due to low-frequency skeletal modes of the cyclohexyl ring. The emission from each conformer is intense and sharp having no extensive vibrational progression and having virtually all of its intensity within 1700 cm-' of the excited peak. T M A was excited in the wavelength region of the AC6 origin and the emission dispersed. This incident radiation is -3300 cm-I to the blue of the T M A origin. A very broad, weak emission spectrum that was red-shifted from the excitation light by -2500 cm-' was observed, with the excess energy going into the vibrational modes of the molecule. The Bichromophore. These experiments were run on a mixture of the cis and trans isomers since this was available in larger quantity than the separated isomers. In an effort to study the energy transfer process from anisole to DMA, the six intensely emitting transitions of the bichromophore were excited and the resulting emission spectra were observed (Figure 7 ) . The spectra (17) Martinez, S. J.; Alfano, J . C.; Levy, 145, 100.
D.H . J . Mol. Specrrosc. 1991,
Energy Transfer in a Molecular Beam
The Journal of Physical Chemistry, Vol. 95, No. 24, 1991 9669
show two definite components: (a) a sharply emitting region within 1700 cm-l of the exciting wavenumber with the peak positions and contour corresponding to that of AC,; (b) a broad emission which is red-shifted by 3300 cm-’ from the incident wavelength and which strongly resembles that produced by exciting DMA at this energy. The relative intensities of these components differ substantially among the six transitions studied. The resonant emission for these spectra is not shown as the contribution of scatter is too large. The magnitude of the sharp anisole-like emission is much higher for the trans isomers than it is for the cis isomers. Since DMA has a broad, weak absorption in this energy region, the bichromophore was also excited nonresonantly and the emission dispersed (Figure 6). There is no sharp constituent in the emission. A very weak, broad, featureless, red-shifted emission is observed.
Discussion Conformation Assignment. For molecules with conformational
Figure 8. Optimized geometry of 4-anisylcyclohexane (ACd as determined by M M X force field calculation.
flexibility, spectral activity reflects the existence of vibrational progressions as well as multiple conformers. In order to determine the number and likely geometry of the conformers present in the molecules studied, it is necessary to examine the results of power saturation studies. Interconversion between conformers separated by a barrier of sufficient energy will not occur in a molecular beam. The relative intensities of the transitions occurring between two electronic states reflect the initial state population and the Franck-Condon overlap between the two states. Different transitions having a common initial level and different transition strengths, as in vibrational progressions, will power saturate at different rates and their relative intensities will change as a function of power. Transitions having different initial levels but the same transition strengths, as in different conformer origins, will power saturate a t the same rate and will have a constant relative intensity that does not depend on the laser power. Modd Compounds. Microwave studies of anisole’* indicate that the aromatic ring is planar in geometry with the heavy atoms of the methoxy group lying in the plane of the ring. The two in-plane methoxy configurations are symmetrically equivalent, and therefore 4-methylanisole should possess a single conformer. Microwave studies completely validate the fluorescence excitation studies (Figure 2, showing a single intense transition), power saturation studies (showing a change in the relative intensities of all the transitions in the spectrum), and molecular mechanics calculations (which show the structure of the molecule to be as shown in Figure 1). The excitation scan of AC, shows two strong transitions. There are no extensive vibrational progressions and most weaker transitions occur in pairs (Figure 3). Since the chromophore is anisole and the only change is the attachment of the cyclohexyl moiety in place of the methyl group, it may be inferred that the two transitions are due to the existence of two different conformers which are frozen in the beam. The dispersed emission spectra taken a t a resolution of 25 cm-l do not show a vibrational progression of 45 cm-’ for either of the two peaks, supporting the identification of the peaks as conformers, not vibrations built on a single conformer origin. Power density studies (Figure 3) indicate the existence of a maximum of two conformers. The 45-cm-I splitting between the conformers corresponds well with the observed 41-cm-I splitting between the conformers of 4propylani~ole.’~ The anisole chromophore is attached to the cyclohexyl ring which makes it theoretically possible to have two different conformations: axial and equatorial. However, the axial conformation is a t least 2.7 kcal/mol (944 cm-l) higher in energy than the equatorial one,2oand we may expect to find only the equatorial conformer in the environment of the molecular beam. However,
the orientation of the bent methoxy group relative to the spacer skeleton may give rise to the existence of two conformers. The aromatic ring plane is perpendicular to the pseudoplane of the cyclohexane ring.2’ This conclusion is based on an MMX-type molecular mechanics calculation?2 a b initio calculations, and X-ray structure d e t e r m i n a t i ~ n . ~If~ the two peaks of AC6 come from two equatorial conformations of the molecule, the logical structures are those shown in Figure 8, which differ in the orientation of the methoxy group with respect to the ring. DMA is a molecule with greater symmetry, and, when attached to a cyclohexyl ring a t the equatorial position, it can have only one conformer. This is confirmed by M M X calculations. The Bichromophore. The results from the monochromophoric model compounds can be used to infer the number and likely geometry of the conformers of the bichromophore. When DMA and anisole are attached to the cyclohexyl ring a t 1,4-positions, the resultant molecule may be cis or trans. The cis isomer may be equatorial-axial or axial-equatorial with respect to the individual chromophores since the bulk of the phenyl groups makes these conformations energetically of equal probability. However, the trans isomer will be diequatorial since the diaxial conformation is energetically unfavorable. If a chromophore with an aromatic ring is forced into an axial conformation on the cyclohexyl ring, it prefers a conformation where the aromatic ring effectively “eclipses” one of the carbon-carbon bonds of the cyclohexane skeleton (Figure 10). This may be inferred from M M X calculations. Experimentally, from NMR20 experiments it has been determined that the aromatic ring plane does not sit parallel to the symmetry plane of the cyclohexane ring. Each of these three configurations of the bichromophore will possess two conformations determined by the relative orientation of the methoxy residue. The M M X calculations give six possible conformations for the cis-trans mixture of the bichromophore (Figure 9). This is exactly confirmed by what we see spectroscopically, Le., there are six strong transitions in all (Figure 5), four of them from the cis and two of them from the trans isomer. Power density studies also confirm that there exist six separate conformers. The difference in wavenumbers between the transition for the two equatorial conformers is 46 cm-l, which corresponds well with that observed for the AC6 (45 cm-I) and 4-propylanisole (41 cm-I)l9 and may be taken as due to the two possible orientations of the methoxy group. A 50-cm-l splitting is observed between the 35 607- and 35 657-cm-’ transitions in the equatorial-axial isomer. On the basis of this, these two transitions may be tentatively assigned as having the anisole group equatorial and the DMA chromophore axial, Le., ea1 and ea2.
(18) Onda, M.; Toda, A.; Mori, S.;Yamaguchi, 1. J. Mol. Specrrosc. 1986, 144, 41. (19) Martinez, S.J.; Alfano, J. C.; Levy, D. H. J . Mol. Specrrosc. 1989, 137, 420. (20)Squillacote, M. E.;Neth, J. M. J . Am. Chem. Soc. 1987, 109, 198.
J.
(21) Closs, G. L.;Calcaterra, L. T.; Green, N. J.; Penfield, K.W.; Miller, R.J . Phys. Chem. 1986,90, 3613. (22) Serena Software, using methods similar to: Allinger, N. L.; Hindman,
D.;Helmut, H.J. Am. Chem. SOC.1977, 99, 3282. (23) Huffman,J. C. Indiana University Molecular Structure Center, unpublished data, 1986.
9670 The Journal of Physical Chemistry, Vol. 95, No. 24, 1991
AE
EA
Av
AE
\
P
0 EA
c"
I
Chattoraj et a]. excess energy going to the vibrational modes of the molecule. This energy region is -3500 cm-' above the DMA origin transition, so the density of states of these "bath modes" is very large, facilitating energy transfer. The very broad features of the DMA-like region of the bichromophore emission spectrum are due to transitions from the various populated bath modes as is often seen in the intramolecular vibrational energy redistribution of large molecules. The same emission spectrum may be obtained by directly exciting the monochromophoric DMA or by tuning the exciting laser off the anisole resonances in the bichromophore. These spectra are weaker in intensity but in the same wavenumber region (Figure 6). The shifting of the emission wavenumber to lower values relative to the excitation wavenumber results from the predominance of Au = 0 transitions due to the molecular Franck-Condon factors. A kinetic model for the transfer process is
+ DA hu + DA D*A
EE
EE
Figure 9. Optimized geometries of cis- and trans- 1-(4-anisyl)-4-(N,Ndimethyl-4-anilinyl)cyclohexane (AC6D) as determined by MMX calculations. View of ae (axial-equatorial) and ea (equatorial-axial) conformers of the cis isomer and the ee (diequatorial) conformer of the trans
k'l ----,
k,
D*A DA*
DA*
-
DA*
--kDA + hvA
k,
DA*
ae
kl
D*A
D*A
isomer.
-
hu
+ hUD
DA
k,
DA
k,A
DA
A few experimental considerations help in simplifying the kinetic analysis. The cross section for the absorption by the acceptor at this energy region is negligible, so that k',
