Mediated Electronic Energy Transfer: Effect of a Second Acceptor

J. Phys. Chem. 1994, 98, 3361-3368

3361

Mediated Electronic Energy Transfer: Effect of a Second Acceptor State Mita Chattoraj, Dutch D. Chung, Basil Paulson, G. L. CIOSS,~ and Donald H. Levy' Department of Chemistry and The James Franck Institute, The University of Chicago, Chicago, Illinois 60637 Received: December 22. 1993"

Intramolecular electronic energy transfer was observed in bichromophoric molecules with the general structure donor-spacer-acceptor where indole is the donor, naphthalene is the acceptor, and cyclohexane is the inert spacer. Measurements were performed in a supersonic jet where both the absorption and emission spectra of the two chromophores were well-resolved and indole could be selectively excited with essentially no excitation on naphthalene. The emission produced on excitation of indole showed strong naphthalene emission and relatively weak indole emission. Moreover, the fluorescence lifetime was similar to the fluorescence lifetime of naphthalene, indicating that the energy transfer occurred on a time scale faster than the fluorescence lifetime of the donor. Energy transfer was observed in several conformers of the bichromophore, but the rates were similar for each. The relative energy-transfer rates of various vibronic levels was determined by measuring the ratio of the naphthalene and indole emission intensities. The energy-transfer rate was found to be fairly constant as a function of vibrational state until the vibrational levels of indole became isoenergetic with the naphthalene S2 state. From this point, the rate increased and then decreased with greater excitation energy. A modification of the theory of mediated intersystem crossing was used to account for this effect.

Introduction In nonresonant energy transfer, electronic energy transfers nonradiatively from an unlike donor to an acceptor molecule. Any excess electronic energy transforms into vibrational energy of the acceptor. It has been shown that energy transfer between degenerate,nonstationarystates of the overall systemis completely equivalent to other intramolecular electronic relaxation phenomena such as internal conversion and intersystem crossing2 and may be described by radiationless transition t h e ~ r y . By ~ careful choice of the donor and acceptor chromophores, it is possible to manipulate the initial and final nonstationary states to test various aspects of the theory. This work demonstrates the effect of a second, low-lying electronic state of the acceptor. Similar processesin radiationless transition theory have been described in the literature. These include mediated intersystem crossing which involves the effect of a second triplet state4 and mediated electron transfer, which involves the effect of a second singlet state in electronically forbidden photoinduced electron t r a n ~ f e r .Our ~ description of intramolecular energy transfer will be analogous to the treatment of mediated intersystem crossing. By performing the experiment in the isolated environment of a molecular beam, we ensured that the properties measured are intrinsic to the donor and acceptor and are not perturbed by interaction with the medium. Vibrationalenergy does not convert to lattice energy of the medium; as a result, the only modes involved are intramolecular ones. The initial nonstationary state was prepared by exciting the donor in the presence of the covalently connected acceptor, and energy transfer does not occur by collisions between the two chromophores. This ensures that quenching of the excitation energy by conversionto kinetic energy of motion between the donor and acceptor during the collisional process does not occur.6 The bichromophoric system studied is of the type D-SpA, where D is the donor chromophore, A is the acceptor chromophore, and Sp is a rigid spacer, which prevents A and D from forming a sandwich exciplex. The donor is the chromophore with the higher-nergy first excited electronic state. The donor is excited to its SIstate, and the emission spectrum is used to indicate energy transfer to the acceptor. The interaction between the two ~~

Abstract published in Aduunce ACS Abstructs, March 1 , 1994.

0022-3654/94/2098-336 1$04.50/0

chromophoresis very weak, so the spectrum of the bichromophore is the sum of the donor and acceptor spectra. Previ~usly,~ we found that the dipols-dipole interaction of F6rster theory8 could not accurately describe the dependence of the energy transfer rate on the relative orientation of and distance between the chromophores. More sophisticated theories2.9-12 were required to describe the gas-phase intramolecular situation. In the work described in this paper the donor is indole (IND), the acceptor is naphthalene (NPT), and the spacer is cyclohexane (C6), to which the two chromophoresare joined in the 1,4-positions (see Figure 1). The bichromophore may exist in the cis (equatorial-axial) and trans (diequatorial) isomeric forms which can be chemically separated. The first and second singlet excited states of naphthalene are separated by -3300 cm-I. The SI state of indole lies -2600 cm-1 above the naphthalene S1state. The extent of energy transfer was measured as a function of the isomer excited and as a function of the excitation energy in excess of the indole SItransition energy. It was found that the cis and trans isomershave similar energy-transferrates. As the excitation energy increases, the extent of transfer remains constant until the excited indole vibronic level becomes isoenergetic with the Sz state of the naphthalene chromophore. At this point there is a sharp increase in the rate.

Experimental Section The compounds were synthesized by standard methods.l3 Both the bichromophore and the single chromophore forms were studied. The bichromophore, 1-(l-indolyl)-4-(2-naphthyl)cyclohexane (IND-CGNPT), was separated into the cis (also referred to in this paper as equatorial-axial or ea) and trans (equatorial-equatorial or ee) isomers. The trans sample was isomerically pure (- 100%). but the cis isomer was incompletely separated and contained -25% of the trans isomer. The spectra presented here were recorded using resonantly enhanced twophoton ionization (REZPI), fluorescenceexcitation,and dispersed fluorescence techniques. The basic experimental apparatus has been described elsewhere,14 and only a brief description will be given here. The molecules were heated to attain sufficient vapor pressure: 55 OC for 1-indolylcyclohexane (IND-C6), 99 OC for 2-naphthylcyclohexane (CGNPT), and 190 OC for the cis and trans isomers of INbC6-NPT. They were seeded into helium 0 1994 American Chemical Society

Chattoraj, et al.

3362 The Journal of Physical Chemistry, Vol. 98, No. 13, 1994

I I

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.

.

.

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,

.

.

.

35000

I

.

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Figure 2. Fluorescence excitation spectra of CGNPT, IND-C6, and IND-CGNPT. The spectra are taken in the vicinity of the indole SI SOtransition and the naphthalene S2 SOtransition. (a) Spectrum of 2-naphthylcyclohexane. Seetext. (b) Spectrumof I-indolylcyclohexane (INDC6). (c) Spectrum of trans-1-(I-indolyl)-4-(2-naphthyl)cyclohexane (INM6-NPT). (d) Spectrum of cis-l-(l-indolyl)-4-(2-naph-

thy1)cyclohexane (INM6-NPT). ~

"

"

"

"

"

"

"

'

"

I

'

Figure 1. Figures of INDC6, C6-NPT, rrans-IND-CbNPT, and cis-

IND-CGNPT. From top to bottom: 1-indolylcyclohexane(IND-C6), 2-naphthylcyclohexane(CGNPT), rrans- I-( l-indolyl)-4-(2-naphthyl)cyclohexane (INDCGNPT), and cis-l-( l-indolyl)-4-(2-naphthyl)cyclohexane (INDC6-NPT).

gas a t 2-4 atm stagnation pressure, and the mixture was expanded into a vacuum chamber through a 0.100-mm-diameter pinhole. Mass-selected photoionization was recorded for all samples. The ionization experiments were performed by skimming the freejet expansion. The resultant molecular beam was then probed by the frequency-doubled output of an Nd:YAG (Spectra Physics, DCR4G) pumped dye laser (Spectra Physics, PDL-111) between the ion extraction grids of a time-of-flight mass spectrometer. The ion signal was recorded as a function of excitation wavelength. The mass spectrum consisted of a single peak, indicating that decomposition was not occurring under our conditions. 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 incident wavelength with a photomultiplier tube. Dye mixes used for the spectra were R610-R6G dye mix for the 34 300-34 950-cm-1 region, R6G for the 34 950-35 900-cm-1 region, and DCMSR640 for the 31 500-32 500-cm-1 region. The excitation spectra were measured to locate the peaks for the measurements of the characteristics of the emission spectra. The peak positions are more accurate than their relative intensities. Fluorescence from the jet was dispersed with an fl3.8 Acton Research Corporation 0.275-m monochromator and detected with a proximity focused microchannel plate intensified diode array (EGBrG Princeton Applied Research, 1455B-700-HQ). Time-resolved fluorescence emission was obtained by routing the output of the photomultiplier tube (RCA 8575) to the input of a Tetronix 2232 -3db 100-MHz interleaving digital storage oscilloscope interfaced to a computer by means of a GPIB board. The duration of the ultraviolet light pulse was 4 ns. The rise time of the photomultiplier tube was 2.5 ns. The measured fluorescence decay is a convolution of the molecular response and the instrument function. The instrument function was measured by scattering light in the chamber in the absence of a source. A best fit base line was subtracted from the fluorescence decay file. The fluorescence decays were then iteratively fit to a convolution of an assumed trial function and the instrument function.

31700

32000

32300

WAVENUMBER ( C M ' )

Figure 3. RE2PI spectra C6-NPT and trans-IND-C6-NPT. These spectra were taken in the vicinity of the naphthalene SI -SO transition region. (a) Spectrumof 2-naphthylcyclohexane.(b) Spectrumof trans1-(1-indolyl)-4-(2-naphthyl)cyclohexane (IND46-NPT).

Results

Fluorescence Excitation Studies. The single chromophore systems I N N 6 and C6-NPT were initially studied. Figure 2b shows the fluorescenceexcitation spectrum of IND-C6. It shows an intense origin peak a t 34 377.9 cm-1 and several other transitions a t higher wavenumbers. The spectrum qualitatively resembles the spectrum of 1-methylindolel5(origin transition at 34 546 cm-I). The presence of a single strong origin indicates that only one conformation exists in the molecular beam. This was verified separately by power saturation16 of the spectrum which showed a significant increase in the relative intensities of all the peaks with respect to the origin. The RE2PI spectrum of C6-NPT taken near the origin of the SItransition is shown in Figure 3a.17 There are twoconformation origins a t 3 1 784 and 3 1 694cm-I. We confirmed by holeburning and power saturation experiments that these two peaks are separate conformer origins and not vibrational progressions. The spectrum of C6-NPT near the first excited singlet state of INDC6 is shown in Figure 2a. The IND-C6 SI SOtransition is -2600 cm-I higher in energy than the SI SOtransition of

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Mediated Electronic Energy Transfer

The Journal of Physical Chemistry, Vol. 98, No. 13, 1994 3363

a

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n

A 35000

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Figure 4. Emission spectra of IND-C6 and C6-NPT. The intensity scale is different for each of the plots. (a) 1-Indolylcyclohexane (INDC6) excited at 34 378 cm-I. Fifty percent of the intensity at the resonant wavenumber is due to scattered laser light. The resolution is 50 cm-l. (b) I N M 6 excited at 35 822 cm-'. The signal at the resonant wavenumber is exclusively due to scattered light and is not shown in the figure. The resolution is 50 cm-I. (c) 2-Naphthylcyclohexane (C6NPT) excited at 34 178 cm-I. The signal at the resonant wavenumber is exclusively scattered light. The resolution is 160 cm-l. (d) CCNPT excited at 35 749 cm-I. The signal at the resonant wavenumber is exclusively scattered light. The resolution is 50 cm-I.

C6-NPT. The excitation spectrumof C6-NPT is extremely weak and almost unobservable in this region until the onset of the SZ region of naphthalene. The fact that this is a broad, featureless spectrum rather than just a flat base line was confirmed by the observation of a very weak, structuredemission spectrumproduced by excitation in this region. Figure 4c shows that there are characteristic naphthalene emission bands in the spectrum. The S2 region of the fluorescence excitation spectrum (at energies in excess of 35 025 cm-1) is very complex, the features resembling those of the SZ region of naphthalene. In naphthalene, the complexity has been ascribed to the vibronic coupling of the zeroorder SZstate with the many SI bl, symmetry states which are nearby in energy.'* The RE2PI spectrum of trans IND-C6-NPT near the naphthalene SIorigin is shown in Figure 3b. Like C6-NPT, it possesses two ofigins. These transitions at 31 714 and 31 795 cm-l indicate the existence of two ground-state conformations. The fluorescence excitation spectrum of the same molecule taken near the origin of the indole chromophore is shown in Figure 2c. In that figure, the spectrum resembles the spectrum of IND-C6. It possesses only one strong transition which is characteristic of a transition origin at 34 382 cm-1. Power saturation of the peaks in this wavenumber region did not yield any evidence of the presence of the second conformer. The striking similarity in the spectral profiles of IND-C6 and IND-C6-NPT (Figures 2b,c) indicates that the two chromophores are only very weakly interacting in IND-C6-NPT. The onset of the Sz absorption of the naphthalene chromophoreis almost imperceptible,indicating that the absorption cross section and/or the fluorescencequantum yield of the SZstate of naphthalene is much smaller than that of IND-C6, at least in this wavenumber region. The fluorescence excitationspectrum of the cis isomer is shown in Figure 2d. The sampleused contained 25% of the trans isomer. Three distinct origin transitions are observed at 34 360, 34 380, and 34 407 cm-1, and their identification as belonging to different

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WAVENUMBER (CM-')

Figure 5. Emission spectra of the originsof I N M 6 N F T . The intensity scale is different in each plot. The signal at the resonant wavenumber in each of thcse spectra is ezclusivelydue to scattered laser light. (a) The trans (ee) isomer excited at 34 382 an-'. The resolution is 40 cm-l. (b) The cis (ae or ea) isomer excited at 34 407 cm-l. The resolution is 50 cm-'. (c)Thecis(aeorea)isomerexcitedat 34 3 6 0 ~ m - ~Theresolution . is 50 cm-l.

conformationswas confirmed by power saturation experiments. Because the transition at 34 380 cm-l is at the same wavenumber region as the trans isomer, it is possible to positively identify only twoconformationsforthecisisomer. Theonly vibrationalfeatures in this spectrum that may be assigned to the cis conformers exclusively are those at 34 993 and 35 000.5 cm-1. Dispersed Emission Studies. The emission spectra of two transitions of IND-C6, the origin and the vibration at 1444 cm-1, are shown in Figure 4, a and b. The emission spectrum strongly reqmbles that of 1-methylind01e.l~Since the SI transition of indale involves no major change in geometry, the Av = 0 transition at -34 380 cm-l corresponds to the strongest spectral feature for all the vibrations studied in the indole SItransition region. Excitation of the origin and of other features at low vibrational energies leads to well-resolved, structured fluorescence emission. Excitation of the higher-energy vibrationsleads to a more diffuse spectrum although the main features remain qualitatively the same. The diffuseness arises from the fact that the excited-state vibrationalstate accessed cannot be adequatelydescribed in terms of a single SOvibrational mode.20 The emission is intense, and virtually all of the intensity is from emission at wavenumbers 32 000 cm-l or greater, even for the higher-energy vibrations. The emission spectra of C6-NPT in this wavenumber region are shown in Figure 4c,d. These spectra have been greatly magnified to make their features discernible. Both spectra show relatively unstructured broad fluorescence emission very similar to naphthalene.21*22Like naphthalene, C6-NPT shows little change in the general shape of the spectra as excitation energy is varied. Also, the entire spectrum of C6-NPT shifts toward the red as the excitationenergy increases. Sincemost of the intensity of the emission occurs at wavenumbers less than 32 000 cm-1, this shift makes it possible to distinguish whether the emission from I N W 6 - N P T comes from the indole or naphthalene. The emission spectra of the three identifiable origins of conformers of the cis and trans isomers of I N D - C d N P T are shown in Figure 5. The spectra show two distinct components. There is a relatively sharp emission at greater than 32 250 cm-1. The wavenumber and relative intensity of these peaks resemble

3364 The Journal of Physical Chemistry, Vol. 98, No. 13. 1994

Chattoraj, et al.

t

1

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loo0l 500

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0

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Time (ns) 35000

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WAVENUMBER (CM") Figure 6. Emission spectra of transitions of tram-IND-CbNPT, The intensity scale is different in each plot. The signal at the resonant wavenumber in each of these spectra is exclusively due to scattered laser light. (a) Excitation at 453.8 cm-1 above the origin. The resolution is 40 cm-I. (b) Excitation at 743.2 cm-I above the origin. The resolution is5Ocm-I. (c)Excitationat 1446.6cm-1abovetheorigin.Theresolution

Figure 7. Fluorescence decay of I N P C 6 . The excitation wavelength is 584 cm-I above its origin transition. The points are the experimental data, and the line is the best fit to the data.

t

is 50 cm-I. the emission of the origin of I N N 6 (Figure 4). The second (and significantly more intense) component is the broad, diffuse emission at wavenumbers less than 32 000 cm-I. The similarity to the emission of naphthalene excited in this energy region is to be noted. The resonant emission (which presumably contains most of the intensity for the indole transition) is not shown in these figures because the proportion of scattered light is very large. Figure 6 shows the emission spectra of the trans (ee)isomer when excited to vibrations at energies 453,743, and 1446 cm-1 in exceSs of the indole origin. The vibrations accessed in Figure 6a,b possess a greater proportion of indole-like emission, relative to the vibration accessed in Figure 6c, which has comparatively little. Nevertheless, a weak peak characteristic of the Av = 0 transition of IND-C6 excited in this region is still definitely observable. Lifetime Studies. The lifetimes of the total fluorescence emission of trans IND-C6-NPT and I N N 6 were measured. The origin of IND-C6 showed a fluorescence lifetime of 17 f 1 ns, which agrees with the fluorescence lifetime measured for 1-methylindole's(15.9 ns), given our limited time resolution. The fluorescence lifetime of trans IND-C6-NPT exhibited fluorescence lifetimes characteristic of the naphthalene chromophore, e.g. 99.1 f 0.6 ns at 2790.49 A. Figures 7 and 8 show the fluorescencedecay profiles of IND-C6 and trans IND-CGNPT excited at energies 584 and 579 cm-' above their respective origin transitions. The best fits of the data sets are also shown. The lifetimes are 13.6 f 0.5 and 124.3 f 1.1 ns, respectively.

Discussion Conformation Assignment. MMX type molecular mechanics calculations23 give four possible conformations (see Figure 9) for the trans (ee) IND-CGNFT molecule. Each chromophore may exist in two stable orientations with respect to the cyclohexane ring. The twoorientationsareseparatedbya 3.2 kcal/mol barrier for the naphthalene chromophore and a 6.2 kcal/mol barrier for the indole chromophore. Excitation of the naphthalene chro-

500

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Figure 8. Fluorescence decay of tram-IND-CbNPT. The excitation wavelength is 579 cm-' above its origin transition. The points are the experimental data, and the line is the best fit to the data.

mophore in C6-NPT and trans IND-CGNPT reveals two conformational origins. It may be inferred that the barrier for rotation around the carbon-carbon bond joining the chromophores to the cyclohexane ring is sufficient to make the conformations noninterconverting in the environment of a molecular beam. Although four conformation origins are predicted when the naphthalene in trans I N N 6 - N P T is excited, only two conformation origins are observed. Furthermore, only one indole conformation origin is observed when the indole in trans INDC6-NFT is excited even though two naphthalene origins are observed for the same molecule. Apparently, the transition energy for each chromophoreis not sufficiently perturbed by the changing orientation of the other chromophore to be distinguishable at our laser resolution. However, the indole ring itself may exist in two orientations with respect to the cyclohexanering (Figure 9), and only a single conformation origin is observed when exciting the indole chromophore in IND-C6 or trans I N N 6 - N F T . MMX calculations predict the barrier to rotation about the bond joining indole to cyclohexaneshould be almost twice that for naphthalene. Either the SI SO transition energy of indole is only very weakly

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The Journal of Physical Chemistry, Vol. 98, No. 13, I994 3365

Mediated Electronic Energy Transfer ee 1

% ee3

I

ee4

n V

ae 1

ae2

I

isoenergetic with the indole excited states in the bichromophore. Interactionbetween the two chromophoreswill result in themixing of these electronic states, and the excitation energy will be delocalized between them. Since the absorption cross section of indole is much greater than that of naphthalene, the zeroth-order state prepared by optical excitation will be the indole SIstate. The line width of the isoenergeticnaphthalene levels18.21ensures the radiationcoherently excites a linear combination ofvibrational eigenstates which undergo a dephasing pr0cess,2~resulting in energy transfer. Due to our limited time resolution, we were unable to observe the dynamics of this dephasing. The phenomenon may be described by an equivalent kinetic model:

ku

D*A k.D

D*A

+

DA*

+

DA*

DA + huD

b

ea2d

DA

+ huA

Figure 9. MMX optimized geometries of IND-CCNPT.

Both the donor and the acceptor may also undergo nonradiative processes. We make the approximationthat the absorption cross section for naphthalene in this region is negligible compared to that for indole. If nh- is the number of photons emitted over all time by the chromophore X and @A is the fluorescence quantum yield to the acceptor, it may be shown that7

influenced by its orientation or one of the two predicted conformations is not frozen into its potential well during the expansion process. Four conformations are also predicted for the cis isomer (Figure 9), the individual chromophores being in the equatorial-axial and axial+quatorial positions. MMX calculations indicate that in the ground state the configuration with indole in the axial position is energetically more favorable (by 1.1 kcal/mol) than that with the naphthalene in the axial position. Each of these configurationsshould possess two conformationsdetermined by the orientation of the equatorial chromophorewith respect to the cyclohexanering. The axial chromophoreis energeticallyconfined to a single orientation. Figure 2d shows two conformation origins which may be attributed to the cis isomer. These conformations have the indole chromophoreat the equatorial and axial positions, assuming that, similar to the diequatorial isomer, its transition energy is only weakly influenced by the orientation of the equatorial chromophore. Of course, we cannot rule out the possibility that there may exist an additional origin for the cis isomer which completely overlaps the trans contaminant’s transition. Energy-TransferRates. While the dispersed emission spectra obtained on excitation of the indole-like transitions from the bichromophore (Figures 5 and 6) show characteristics of both indole and naphthalene fluorescence emission (Figure 4), naphthalene’s emission greatly predominates. Furthermore, the fluorescence lifetime of the indole-like transitions from the bichromophore is similar to naphthalene’s fluorescence lifetime. Thus, excitation of the indole transitions in the bichromophore results in energy transfer to naphthalene. The predominance of the b = 0 transitions, because of molecular Franck-Condon factors, causes the naphthalene chromophore emission to red shift by 12100 cm-1 from the excitation frequency. This makes it possible to distinguish its spectrum from that of indole in the bichromophore. The origin of the SI SOtransition of the indole chromophore is -2593 cm-1 higher in energy than that of naphthalene. Consequently, naphthalene has a high density of vibronic states which are

The relative values of nhvAand nh, could be measured by integrating the areas corresponding to the emission from the respective chromophores. Since there is some overlap in the emission spectra of indole and naphthalene, the intensity of the indole emission was estimated by determining the intensity of a vibration, whose emission is neither in the resonant energy region (since scattered laser light presents a problem) nor in the region of overlappingemissions, and dividing this quantity by its FranckCondon factor. For theorigin transitions, the 1330-cm-1 vibration was used. The Franck-Condon factor which was used was determined from IND-C6 (0.122 0.006 at 40-cm-1 resolution and 0.133 f 0.002 at 50-cm-1 resolution) because the amount of light scattered could be more accurately estimated than in the bichromophore. In the bichromophore, since most of the emission was naphthalene-likein character and the proportion of scattered laser light in the resonance peak was dominant, it was difficult to estimate the proportion of fluorescence emission which came from indole. To estimatethe ratio of acceptor and donor emissions whenexcitingindolevibrations,theintensityofthe b = Oemission was used to determine the intensity of the indole fluorescence in the bichromophore. The Franck-Condon factor for this transition was again measured from IND-C6. Since the b = 0 transition carries most of the intensity of the emission, it was difficult to distinguish the scattered laser light from the resonant emission in IND-C6 when exciting the vibrations. To estimate this, the power normalized ratio of the intensities of the origin transition (Au = 0) and the vibronic transition (b# 0) was measured in the fluorescence excitation spectrum. The ratio of the intensity of the Au = 0 transition to the intensity of the Au # 0 transition in the dispersed emission spectrum was assumed to be the same as it was in the excitation spectrum. The intensity of the naphthalene emission for all cases was estimated directly by integrating the region to the red of 32 250 cm-1. Since the naphthalene component is so much greater than

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3366 The Journal of Physical Chemistry, Vol. 98, No. 13, 1994

Chattoraj, et al.

TABLE 1: Ratios of Naphthalene to Indole Emission Intensity at Different Excitation Energies of the Diequatorial Isomer of IND-Cs-NPT INPTIIIND re1 energy,“cm-l INPTIIIND re1 energy,”cm-1 re1 energy,“cm-1 INPT I~ND 0.0 101.5 149.6 453.8 467.4

9.5 f 1.3 12.1 & 0.8 10.4 & 0.94 4.8 & 0.3 7.3 f 0.7

587.8 703.8 724.0 738.3 743.2

12.3 f 2.0 11.Of2.2 10.6 f 1.6 11.7f0.3 10.2 f 2.1

826.4 913.0 966.9 1446.6 1688.2

16.3 f 0.1 19.8 f 0.9 16.1 f 2.8 24.1 f 3.8 11.2& 1.5

With respect to 34 381.6 cm-I, the indole chromophore origin of diequatorial INDC6-NPT.

TABLE 2: Ratios of Naphthalene to Indole Emission Intensity at Different Excitation Energies of the Cis Isomer of IND-CCNFT energy, cm-1 re1 energy,“cm-1 INPTIIIND 34 408.4 34 361.1 34 992.8 35 000.5

0.0 0.0 584.5b 63 1.7c 592.2b 639.4c

30

12.1 f 3.5 8.1 f 2.3 6.2 f 0.7 7.7 f 0.8 7.8 f 1.2 9.7 f 1.4

” Withrespecttotheorigintransitionforeachconfomer. b Withrespect tothe transitionat 34408.4cm-I. Franck-Condonfactors used toestimate the indole component of the emission are from IND-C6 at Av = 584.3 cm-I. With respect to the transition at 34 361.1 cm-I. Franck-Condon factors used to estimate the indole component of the emission are from IND-C6 at Au = 696.3 cm-I. that of indole, the error in this measurement due to the small amount of indole emission intensity present is very small. Tables 1 and 2 show this ratio for the transitions of the trans and cis isomers, respectively. The errors were estimated by determining the standard deviation of the ratio for several independent measurements. For vibrations where the component of indole emission is small, and therefore difficult to estimate, the error margins increase. The radiativerate for indole in this energy region may be assumed to be independent of thevibrational state accessed. (The variation in fluorescence lifetime with vibrational energy has been ascribed toan increasein thenonradiative rate.15) The fluorescence quantum yield of n a ~ h t h a l e n and e ~ ~2-methylnaphthalene26is constant in this spectral region. Hence, thevalues of @A and k,, in eq 1 may be assumed to be the same for the different conformations and vibrations, and ratios in Tables 1 and 2 reflect the relative energy-transfer rates. The ratios for the origin transitions of the trans and cis isomers are very similar, indicating that the rates of energy transfer are comparable. This is in contrast to the rates of energy transfer of the cis and trans isomers of 1-(4-anisyl)-4-(N,N-dimethylanilino)cyclohexane, which differ by a factor of 6.7 The ratios for the origin and vibrations of the diequatorial isomer are plotted as a function of excess energy above the indole origin in Figure 10. Although the data are noisy, the plot of discrete points shown in Figure 10 resembles the excitation spectrum of C6-NPT. In Figure 10 the value of the ratio starts to rise at 600 cm-l above the indole origin transition, which corresponds to the beginning of the onset of the S2 transition for the naphthalenechromophore. The ratio then follows theintensity profile of the C6-NPT excitation spectrum, decreasing when the intensity of the naphthalene transition becomes less at 1688 cm-1. The vibrations a t 454 and 467 cm-1, which occur at energies lower than the S2 onset, have a larger component of indole-like emission than any of the other vibrations, including the origin. The two identifiablevibrations of thecis isomer occur at energies below the onset of the naphthalene S2 transition. Since we did not know which conformer origin corresponded to each of them, the amount of indole emission was computed twice, using the the corresponding IND-C6 Franck-Condon factors for each. Using the fluorescence quantum yield for 1-methylindole in cy~lohexane?~ 0.5 f 0.02, the gas-phase fluorescence quantum yield of naphthalene, 0.1 3,25,26our measured lifetime of IND-C6 (17 ns), and the ratios of emission, an approximate value for the

€

0 ‘

0

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800 1000 Wavenumbers

1200

1400

1600

1800

Figure 10. Energy-transfer rates versus excitation energy. Ratio of the naphthalene-liketo the indole-like emission in the trans isomer of 1-(1indolyl)-4-(2-naphthyl)cyclohexane (IND-C6-NFT) as a function of the excitation energy in excess of the indole origin. rate of energy transfer may be estimated from eq 1: 2 X lo9 s-1 at the origin of the trans isomer. From the energy-transfer rate, we may conclude the dephasing time is much shorter than the fluorescence lifetime of the molecule. The emission observed is characteristic of the molecular eigenstates accessedon excitation. The dephasing time, kct-l, will be given by the golden rule-like expre~sion2~

Transitions from the ground state are only allowed to the zerothorder donor states, and these are coupled to a dense manifold of vibronic levels of acceptor “dark states” through the intramolecular coupling matrix element,

(3) The electronic and nuclear parts have been separated under the adiabatic approximation, and it is assumed that the perturbation term, H’,is a function of electronic coordinates only. Forsters approximated the rate of energy transfer in weakly coupled systems by expressing H’ as a multipole expansion and truncating the series after the dipole-dipole interaction term

K

= COS 6 - 3 COS ~

D R COS 6~

(4)

where R is the separation between the chromophores and M.CX represents the electronic transition moment for chromophore X. We tried to see whether the difference in energy-transfer rates among the different conformers could be accounted for by Fijrster theory using eq 4. In order to evaluate the relative rates for energy transfer for the cis and trans isomers, it is necessary to know the direction of the transition moments of the chromophores and the geometry of the molecule. Rotational band contour st~dies28,2~ show that the transition moment from the ground to

The Journal of Physical Chemistry, Vol. 98, No. 13, 1994 3367

Mediated Electronic Energy Transfer

TABLE 3 Estimation of the Relative Energy Transfer Rates Using the Forster Expression and Geometries Obtained from MM2 Calculations B type confomerO e l ee2

R

(AIb

9.714 10.016 9.804 9.696 8.331 8.925 8.247 6.727

A type

K2

K2/R6

K2

1.422 0.770 1.104 0.206 1.166 0.624 0.377 0.63 1

37.030 16.681 27.210 5.429 76.300 27.021 26.227 149.047

0.300 0.304 0.048 1.202 0.015 0.336 0.105 0.123

K21@

7.803 6.598 1.184 3 1.638 1.ooo 14.558 7.275 29.100

ee3 ee4 ael ae2 ea 1 ea2 0 a = axial e = equatorial;the first letter describes the indole chromophore and the second describes the naphthalene chromophore. See Figure 9 for structures, b R = center-to-center distance between the chromophores and K = cos O - 3 cos ODR cos Om,where K ~ / @ has been normalized for comparison between conformers. state, SI,decays into a triplet state, TI,the coupling being provided the first excited state of indole lies in the plane of the ring and by the spin-orbit matrix element, VSO.The presence of a second makes an angle of +4S0 with respect to the A axis. The direction triplet state, T2, enhances the rate of intersystem crossing. The of the transition moment in naphthalene is complicated by the density of T2 levels at the energy of SIis sparse, but the spin-orbit fact that there are two low-lying excited states of naphthalene. matrix element that couples SIand T2 is larger than the matrix The SI SOtransition in naphthalene is symmetry allowed, but element that couples SIand TI because the smaller energy gap due to configuration interaction30 with the S3 state, there is an produces larger Franck-Condon factors. The state T2 is in turn “accidental cancellation”of its transition moment and its spectrum coupled to the dense manifold of states TI by means of vibronic isdominated by vibronically induced bands. Thus, thevibrational coupling. The enhancement of the intersystem crossing rate spectrum of naphthalene has three components: bl, bands which produced by the T2 is due to the fact that the vibronic coupling arevibronically coupled to the& state and are short axis polarized matrix element is larger than the spin-orbit matrix element. (B type), aBbands which borrow intensity from S3 and are long axis polarized (A type), and totally symmetric vibrations (A The existence of a second acceptor state in naphthalene leads type) built on the weakly allowed origin. Substitution on to an analogous situation, and we can use the theory of mediated n a ~ h t h a l e n e makes ~ ~ ~ 3the& ~ +So transition allowed (as indicated intersystem crossing (with a change in language) to analyze our by the relatively intense origin); however, the prominent peaks results. In our case all of the relevant states are singlets and the correspond to the vibrationally induced fundamental transitions coupling is electrostatic rather than spin-orbit, but the dynamics of naphthalene. At energy regions we are investigating for energy are due to an initially excited state decaying into the dense transfer from indole, the actual vibronic states are highly coupled manifold of a second state mediated by a sparse manifold of a by anharmonic terms and second-order terms in the S2-Sl third state. perturbation.l8V2l The initially excited state is the exciteddonor, &,*A. Thedense MMX calculations were used to determine the ground-state manifold of states is built on the first excited singlet state of geometries for theeight conformations (Figure 9) and todetermine naphthalene, C#IDA~*,which has an energy gap of 2600 cm-1 with R and K . Since the donor and the acceptor chromophores do not respect to ~ D * A . The sparse manifold of states is built on the change in the different conformations, the relative rates of energy second excited state singlet of naphthalene, $DA~*, which has an transfer when exciting the origin transition (i.e., keeping the energy gap of -700 cm-1 with respect to &*A. The coupling Franck-Condon factors the same) may be determined from eq between ~ D * Aand either ~ D A and ~ * C$DA~*is primarily due to a 2 and 4 (Table 3). Estimations are made for transfer tovibrations dipole-dipole Forster coupling, while the coupling between $ D A ~ * of naphthalene polarized along the long axis (A type) and the and ~ D A is ~ *primarily due to vibronic coupling. The vibronic short axis (B type). It can be seen that the geometry of the coupling matrix element is larger than the dipole-dipole matrix molecule favors the transfer to B-typevibrations. For both cases, element, and as will be shown below, this leads to an enhancement the increase in the center-to-center distance on going from the of the energy-transfer rate. cis isomers to the trans ones is largely compensated by better Our treatment of the problem follows Amirav et ale’streatment angular overlap between the chromophores. of mediated intersystem c r o ~ s i n g The . ~ mixing of the two sparse Experimentally, when exciting indole in the bichromophore, states, ~ D * Aand &*, is described by a non-Hermitian Hamilwe were able to distinguish only one trans and two cis conformation tonian origins. Consequently, the energy-transfer rates measured at these three origin peaks may be taken to be the sum of the rates for the different conformations excited, weighted by their relative populations in the molecular beam. As indicated by Table 3, the energy-transfer rate predicted by F6rster theory varies substanwhere the imaginary parts of the diagonal matrix elementsdescribe tially from conformer to conformer. Since we know neither the the decay of the sparse states into the dense manifold ~ D A , * These . identity nor the weighting of the conformers we are observing, decay rates are given in the golden rule approximation by we cannot draw any conclusions about whether Forster theory is applicable in this case. Transitions from the ground state are only allowed to the zerothorder donor states, and these are coupled to a dense manifold of where V’Fis a Fdrster-type matrix element coupling ~ D * Ato the vibronic levels of acceptor states through the intramolecular dense manifold while Vvib is a vibronic matrix element coupling coupling matrix element BDA. Were there only one acceptor ~ D Ato ~ thedensemanifold. * Because there is a substantial energy electronic state, we would use FBrster theory to estimate the gap between either of these states and 4 ~ ~these ~ 0coupling , matrix coupling matrix element. In our case, naphthalene was the elementswill be a product of an electronicpart and a small Franckacceptor and the existenceof two electronic states below the energy Condon factor. The off-diagonal matrix element VFthat appears of the donor level complicates the energy-transfer process. in eq 5 is a FBrster-type matrix element which couples the two The situation is very similar to the case of mediated intersystem sparse states. Because the energy gap between the sparse states c r o ~ s i n g . In ~ , ~the intersystem crossing case, an initially excited

-

3368 The Journal of Physical Chemistry, Vol. 98, No. 13, 1994 is small, this matrix element is the product of an electronic part that is similar in magnitude to the electronic part of V’Fand a larger Franck-Condon factor. The eigenvalues of the Hamiltonian are

where the imaginary parts of these eigenvalues give the decay rates of the mixed sparse states into the dense manifold; i.e., the imaginary part of the initially excited state is the energy-transfer rate. This rate can be calculated in two limiting cases, the nearresonance limit and the off-resonance limit. In the near-resonance limit

and the energy-transfer rate is

Chattoraj, et al. A,#] will tend to reduce the amount of enhancement. Only when $JD*A is reasonably close to but not resonant with &* will there be any enhancement at all.

Conclusion Excitation of the indole origin and vibronic levels in the bichromophore IND-C6-NPT results in efficient energy transfer to a dense manifold of isoenergetic naphthalene energy levels. Both the emission spectrum and the fluorescence lifetime of indole change in the presence of the naphthalene chromophore, indicating that the state accessed is dominated by contributions from zeroorder naphthalene vibronic levels. The rate of energy transfer was similar for the different cis and trans conformations. This may be due to the fact that the naphthalene modes in this region are a complicated linear combination of in-plane modes polarized along both the x and y axis of the molecule, making the angular dependence of the rate of energy transfer much less rigid. Also, the eight possible conformations of the molecule could not be separated, and the energy-transfer rates measured were consequently those for a range of conformations. The rate of energy transfer increases at the onset of the naphthalene SZ SO transition, and this may be described using a modification of the theory of mediated intersystem crossing. +

where I’E is the energy-transfer rate enhanced by the second electronic state of the acceptor. In the absence of this second state, the energy-transfer rate would have been

rv= A D = 2

7 ~ ~ ; ; ~

Acknowledgment. This work was supported by theNSF under Grants CHE 8818321 and C H E 8520326. References and Notes

(10)

Thus, in the near-resonance limit the enhancement produced by the second state is

(1) Deceased May 1992. (2) Robinson, G. W.; Frosch, R. P. J. Chem. Phys. 1963. 38, 1187. (3) Bixon, M.; Jortner, J. J . Chem. Phys. 1968. 48, 715. (4) Amirav, A.; Sonnenschein, M.; Joriner, J. Chem. Phys. Lett. 1983, 100,488. (5) Reimers, J. R.; Hush, N. S. Chem. Phys. 1990, 146, 105. (6) Bigman, J.; Karni, Y.;Speiser, S. In Mode Selectiue Chemistry; Jortner, J., Levine, R. D., Pullman, B.,Eds.; Kluwer Academic: Dordrecht, 1991: _ _ - nr 415 (7) Chattoraj, M.; Bal, B.; Closs, G. L.; Levy, D. H. J . Phys. Chem. 1991.95, 9666. (8) FBrster, Th. Discuss. Faraday Soc. 1959, 27, 7. (9) Dexter, D. L. J. Chem. Phys. 1952, 22, 836. (10) Lin, S. H. Mol. Phys. 1971, 21, 853. (1 1) Lin, S. H. Proc. R. SOC.London, A 1973, 335, 5 1. (12) Henry, B. R.; Siebrand, W. J . Chem. Phys. 1971, 54, 1072. (13) Sigman, M. E.; Closs, G. L. J . Phys. Chem. 1991, 95, 5012. (14) Sharfin, W.; Johnson, K. E.; Wharton, L.; Levy, D. H. J . Chem. Phys. 1979, 71, 1292. (15) Hager, J. W.; Demmer, D. R.; Wallace, S.C. J . Phys. Chem. 1987, 91, 1375. (16) Rizzo, T.; Park, Y.D; Peteanu, L. A.; Levy, D. H. J . Chem. Phys. 1985,84, 2534. (17) To assure that the molecular beam and laser were aligned, the molecular beam was seeded with the strongly fluorescing 1-methylindolewhen the spectrum of the region 34 350-34 900 cm-I was measured. The peaks of 1-methylindole that appear in this region have been deleted, producing the short noiseless segments that appear in the figure. (18) Beck, S. M.; Powers, D. E.; Hopkins, J. B.; Smalley, R. E. J . Chem. Phys. 1980, 73, 2019. (19) Bickel, G. A.; Demmer, D. R.; Leach, G. W.; Wallace, S . C. Chem. Phys. Lett. 1988, 145, 423. Bickel, G. A,; Leach, G. W.; Demmer, D. R.; Hager, J. W.; Wallace, S. C. J. Chem. Phys. 1988, 88, 1. (20) Bickel, G. A.; Demmer, D. R.; Outhouse, E. A.; Wallace, S. C. J . Chem. Phys. 1989, 91,6013. (21) Beck, S. M.;Hopkins, J. B.; Powers, D. E.; Smalley, R. E. J. Chem. Phys. 1981, 74, 43. (22) Stockburger, M.; Gattermann, H.; Klusmann, W. J. Chem. Phys. 1975. 63. 4519. (23) Serena Software, using methods similar to: Allinger, N. L.; Hindman, D.; Helmut, H. J . Am. Chem. Soc. 1977, 99, 3282. (24) Freed, K. F.; Nitzan, A. J . Chem. Phys. 1980, 73, 4765. (25) Behlen, F. M.; Rice, S . A. J . Chem. Phys. 1981, 75, 5672. (26) Jacobson, B. A.; Guest, J. A,; Novak, F. A.; Rice, S . A. J. Chem. Phys. 1987, 87, 269. (27) Meech, S. R.; Phillips, D. Chem. Phys. 1983, 80, 317. (28) Phillips, L. A.; Levy, D. H. J . Chem. Phys. 1986, 90, 4921. (29) Phillips, L. A.; Levy, D. H. J . Chem. Phys. 1988, 89, 85. (30) Dewar, M. J. S.; Longuet-Higgins, H. C. Proc. Phys. Soc. London, Sect. A 1954, 67, 795. (31) Warren, J. A.; Hayes, J. M.; Small, G. J. J . Chem. Phys. 1984.80, 1786.

_.

and as long as Vvib is larger than V’F,the effect of the second naphthalene state is to produce an increase in the energy-transfer rate. Note that since both Vvib and V’Fare matrix elements between states with a large energy gap, the Franck-Condon parts of both matrix elements will be small. However, the electronic part of Vvib will be very much larger than the electronic part of V’F, producing a very large enhancement in the energy-transfer rate. Since the energy gap between $JD*A and $JDA~*is small, the more relevant limit is probably the off-resonance limit where

and the enhanced energy-transfer rate is rE = AD

+

v(:AA2 (ED*,

- EDA~*)’

- AD)

+

AD - A A , ) ~

(13)

In the absence of the second state the unenhanced rate is still given by eq 10, and the enhancement due to the second state is

Again, as long as V& . , is larger than V’F,the effect of the second naphthalene state is to produce an increase in the energy-transfer rate. However, in the off-resonance case the increase will be less than the near-resonance case. Even if the ratio Vvib/V’F is much greater than one, the quantity VF’/[(ED*A - E D A ~+ ) ’ AD -