8778
J . Phys. Chem. 1992, 96, 8178-8784
Intramolecular Electron Transfer in a Molecular Beam Mita Chattoraj, Sandra L.Laursen,' Basil Padson, Dutch D. Chug, G. L.Gloss: and Donald H.Levy* The Department of Chemistry and The James Franck Institute, The University of Chicago, Chicago, Illinois 60637 (Received: April 20, 1992; In Final Form: July 20, 1992)
Gas-phase intramolecular electron transfer was studied in molecules of the type D-SgA where D was a positive charge donor, A was a positive charge acceptor, and Sp was a rigid, inert spacer. In our experiments, the donor was always naphthalene, the acceptors were benzene, indole, and biphenyl, and the spacers were cyclohexane and androstane. The naphthalene chromophore was selectively ionized by means of resonantly enhanced two-photon ionization via the naphthalene SIstate. The location of the charge was monitored by observing the resonantly enhanced multiphoton dissociation spectrum of the ion. When the charge was localized on the naphthalene, the naphthalene ion D2 Dospectrum was observed as an action spectrum (ion yield at a given mass versus wavenumber). When the charge had transferred to the acceptor, the naphthalene ion spectrum was not observed. Using this technique we attempted to measure the charge-transfer lifetime by measuring the intensity of the naphthalene ion action s p t r u m as a function of the delay between the ionizing and dissociating lasers. We found that in all cases where charge transfer was thermodynamically possible, the charge-transfer lifetime was less than 2 ns, the time resolution of the experiment.
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systems studied in this paper the charge is prepared on the donor Introduction in the presence of the acceptor, and the interaction between them In the last decade charge transfer in bichromophores consisting is not time dependent. In the collision-free region of the supersonic of a donor and an acceptor separated by a rigid spacer has been actively studied both theoretically3 and e ~ p e r i m e n t a l l y . ~ ~ ~jet the system evolves conserving the sum of its electronic, vibrational, and rotational energies. Charge-transfer rates are not diffusion limited in these systems Another class of chargetransfer reactions studied in supersonic as they are in intermolecular systems. Intramolecular charge jets is the direct excitation of the charge-transfer state of subtransfer also serves as a good model for biological charge transfer. stituted amin~3,5dimethylbe11enzonitriles.~~~~~ In these experiments So far all intramolecular chargetransfer studies have been camed there is only one chromophore, and both the locally excited and out in the condensed phase, which is understandable given the the relaxed states are singlet in character and are very strongly difficulties of designing gas-phase charge-transfer measurements. coupled. Penning ionization within a cluster has also been However, the presence of solvent complicates the analysis since studied.13 The molecules used in the experiments described below the reaction coordinate is dominated by solvent reorganization. consist of two distinct chromophores rigidly separated by distances The important role that the solvent plays in controlling the rate comparable to or exceeding the length of their molecular of electron-transfer reactions is seen in ref 5 , where it is shown framework. There is no possibility of forming e x c i m e r ~ . ~The ~.~~ that changing the polarity of the solvent can result in variation charge is initially prepared on the donor chromophore and there of the electron-transfer rates by several orders of magnitude. is no controversy over whether the state being investigated is Gas-phase measurements of intramolecularelectron transfer would charged or not. characterize the intrinsic nature of the process. There is also current interest in the role of intramolecular vibrations in lowering Experimental Scheme the effective activation energy barrier to electron-transfer proThere are two requirements for an experimental scheme decesses.+* Studies of the transfer process in the gas phase would signed to measure intramolecular charge transfer. First, one must be particularly interesting in this context, since all the acceptor selectively ionize the donor side of the molecule, or at least make modes for the chargetransfer process in an isolated molecule are a nonequilibrium distribution of donor and acceptor ions. In the intramolecular ones. condensed phase experiments? a nonequilibrium distribution was In this work intramolecular positive charge transfer has been produced by pulsed electron bombardment, which produced a observed in several rigid bichromophoric systems of the type nearly statistical distribution rather than the equilibrium distriD-SpA, where D is the positive charge donor, A is the acceptor, bution that strongly favored ionization of the acceptor. In our and Sp is the spacer. Throughout the paper the terms "donor" experiments in the gas phase, selective ionization was achieved and Yacceptornwill be used with respect to the positive charge by one-color two-photon ionization of the neutral molecule in a (hole) transfer. A positive charge was created on the donor by supersonicjet (see Figure 1). The SI Sotransitions of the donor resonantly enhanced two-photon ionization (RE2PI). The dyand acceptor are both in the ultraviolet but are well separated. namics of this initial ionic state was then probed by resonantly Therefore the resonant enhancement achieved by tuning the laser fragmentingthe molecule with a second laser pulse and monitoring to the donor transition produces a clean selective ionization. In the variation of the ion's absorption spectrum as a function of the our studies the donor was always naphthalene. Since the ionization time delay between the two laser pulses. From condensed-phase potential of naphthalene was only slightly less than twice the measurements of similar systemsg it would be expected that the excitation energy of the SIstate, the ions were prepared in a few system would evolve from an initial donor-like state to an ac(or even one) vibronic states. ceptor-like state. However, in all the molecules studied in this The second requirement is a probe for the location of the charge experiment we observe no evolution of the ion state, although there in the molecule. In the condensed-phase experiments, the difis a significant change in the ion absorption spectrum when and ference between the ion and neutral electronic absorption spectra if a second chromophore capable of accepting the charge is present. A distinction should be made between the case studied here was used to follow the location of the charge. In our experiments the absorption spectrum of the donor was used as a probe, but and gas-phase ion-molecule reactions that have been extensively because of the low density in the molecular beam, an indirect form investigated by using ion cyclotron resonance mass spectrometers.'O of spectroscopy was necessary. Since the ions have an open-shell In ion cyclotron experiments the ion is initially prepared in isolation and thermalized, and it is then allowed to collide with the substrate. electronic structure, they absorb in the visible, unlike the neutral During the collision there is strong coupling between the kinetic precursors, which only absorb in the ultraviolet. Sequential absorption of several visible photons by the ion will lead to fragand vibrational energies of the intermediate. In contrast, in the +
0022-3654/92/2096-877~~03.~0/0 0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 22, 1992 8779
Electron Transfer in a Molecular Beam aissociation
D
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SO Naphthalene
SO Acceptor
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Figure 1. Schematic energy level diagram. The naphthalene cation is prepared by ultraviolet RE2PI through the origin of the SI So transition. The ion produced is resonantly excited to the D2 state by the absorption of a second color photon. Further absorption of visible photons leads to dissociation. The acceptor has a first excited singlet state higher in energy than that of naphthalene. The acceptor Do state is at a lower energy than that of naphthalene, and so there will exist higher energy vibronic states of the acceptor isocncrgetic with the ground state of the naphthalene cation.
mentation, which we monitor in a timeof-flight mass spectrometer. If the visible laser is tuned to an absorption transition of the ion, there is a resonant enhancement of the fragmentation, and therefore the absorption spectrum of the ion can be inferred by measuring the action spectrum,the fragment ion yield (or decrease in parent ion yield) as a function of the frequency of the visible In principle, we should be able to measure the charge-transfer rate by following the decay of the donor ion action spectrum as a function of the time delay between the ultraviolet ionizing laser and the visible fragmentation laser. The experiments were conducted by using donor-acceptor pairs that satisfied a large number of criteria. In order to produce vibrationally cool ions with a single laser, the ionization potential of the donor had to be less than, but close to, twice the energy of its first excited singlet state. The acceptor had to have a lower ionization potential than the donor. Moreover, it could not have an excited singlet state lower in energy than the SI state of naphthalene. If the acceptor had a lower SIstate than the donor, energy transfer followed by the direct ionization of the acceptor would have prevented the selective ionization of the donor that was required in this experiment. The ease of synthesis of these compounds was also given consideration. In our experiments the donor was always naphthalene, the acceptor chromophores were biphenyl and indole, and the spacer was either cyclohexane or a steroid.
Experimental Methods The compounds were synthesized by standard methods.I9 The basic experimental apparatus has been described elsewhere20and only a brief description will be given here. The molecules were heated to attain sufficient vapor pressure: 90 OC for 2naphthylcyclohexane (NPT-C6), 160 OC for l-(Znaphthy1)-4phenylcyclohexane (NPT-Ca-BZ), 60 OC for I-indolylcyclohexane (C6-IND), 180 OC for 1-(2-naphthyl)-4-( 1-indoly1)cyclohexane(NPT-C6-IND), 230 OC for 1-(2-naphthyl)-4-(4-biphenyly1)cyclohexane (NPT-C6-BIPH), 209 OC for 342naphthy1)androstane (NPT-ST), and 310 OC for 3-(2naphthyl)-l6-(4-biphenyl)androstane (NPT-ST-BIPH). They were seeded into helium gas at 2 4 atm of stagnation pressure and the mixture was expanded into a vacuum chamber through a pinhole. Pinholes used w m either 0.10 or 0.050mm in diameter. The free jet expansion was skimmed and the resultant molecular beam was probecl between the ion extraction grids of a reflectron2I
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Figure 2. Two-photon ionization spectra of the naphthalene compounds. The SIstates of the bichromophores are only very slightly perturbed from that of 2-naphthylcyclohexane. This ensures the preparation of the naphthalene cation by the RE2PI process. (a) Spectrum of 2naphthylcyclohexane (NPT-C6). There are two origins corresponding to two equatorial conformations of the molecule. (b) Spectrum of 1phenyl-4-(2-naphthyl)cyclohexane (NPT-CCBZ). (c) Spectrum of 1( l-indolyl)-4-(2-naphthyl)cyclohexane (NPT-CCIND). (d) Spectrum of 1-(4-biphenyl)-4-(2-naphthyl)cyclohexane (NPT-C6-BIPH). (e) Spectrum of 3-(2-naphthalene)androstane (NPT-ST). (f) Spectrum of 3-(2-naphthyl)- 16-(4-biphenyI)androstane (NFT-ST-BIPH).
time-of-flight mass spectrometer. Excitation sources consisted of two dye lasers that were pumped by a freqliency-doubled Q-switched Nd:YAG laser. The visible fundamental of one of the dye lasers (Quanta-Ray PDL-3) was doubled with a KDP crystal to form the ionizing beam. An optimized SR640-DCM (Exciton) dye mixture was used. Conventional RE2PI spectra were taken to confirm that supersonic cooling was achieved and to determine the fixed frequency of the first photon. The selectively prepared ions were dissociated with the output of the second dye laser (Quanta-Ray PDL-1) that was scanned across the ion absorption spectrum. A DCM-LDS698 (Exciton) dye mixture was used in this laser. The intensity of this fragmenting light could be varied by a variable attenuator (NRC) and the light could be focused with a 50-cm focal length lens. A prism delay system was used to set the timing between the two laser pulses. The time delay between the two pulses was measured before and after interaction with the molecular beam with a photodiode with a resolution of 500 ps. The duration of the ultraviolet light pulse was 4 ns. The ion signals at both the parent and fragment manses were recorded as a function of fragmenting laser wavelength, a dip in the intensity of the parent ion spectrum and a corresponding rise in the intensity of the fragment ion signal indicating absorption by the ion species. The data collection electronics consisted of a 100-MHz transient recorder (Lecroy TR8818) for collecting mass spectral data and a multichannel gated integrator/digitizer (Lecroy 2249SG) for RE2PI and resonance ion dissociation spectra, both of which were interfaced to the laboratory computer via a CAMAC dataway. All the spectra presented have been taken over a single dye (mix) region. No power normalization was attempted on the spectra for these multiphoton processes. ReSUltS
To aid in the interpretation of the spectroscopy of the bichromophoric systems, monochromophoric model compounds were also studied. The resonance ion dissociation spectra of naphthalene and 2-methylnaphthalene have been studied previously by Syage et a1.I6 The one-color RE2PI spectra of NPT-C6 is shown in Figure 2a. The variations in the intensity of the ion signal are due to resonant transitions between the ground and the first excited singlet state of naphthalene and do not reflect the transitions
Chattoraj et al.
8780 The Journal of Physical Chemistry, Vol. 96, No. 22, 1992 a
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Figure 3. Mass spectra of NFT-C6. (a) With one laser at 31 784 cm-l. (b) With two lasers, the ionizing laser tuned to 31 784 cm-l and the fragmenting laser at 14920 cm-'.
between the first singlet and the ion state. The spectrum shows two features at 31 784 and 31 694 cm-l, both to the red of the naphthalene origin at 32019 cm-1?2which we assign as the origin transitions of two conformers. The identification of these peaks as separate conformer origins and not vibrational progressions has been confirmed separately by using the techniques of hole burning23*24 and power saturation.2s The spectrum closely resembles that of 2-methylna~hthalene,~~ with vibrational progressions built on both origins. Unfortunately, the ionization potential of NPT-C6 is not known. The ionization potential of 2-methylnaphthalene is -64OOO cm-l. The excess energy from the two photons will be distributed between the ejected electron and the vibrations of the ion. If the ionization potential of NPT-C6 is similar to that of 2-methylnaphthalene, excitation of the SIorigin transition of NPT-C6 will produce a maximum vibrational energy in the ion of a few hundred reciprocal centimeters. Mass spectra of the ions are shown in Figure 3. The top mass spectrum is taken with the ultraviolet laser, which is tuned to the SI So origin transition at 31 784 cm-I. This spectrum shows a very strong parent ion signal at 210 amu and very little fragmentation. Figure 3b shows the mass spectrum obtained in the presence of two laser pulses, the ionizing laser remaining at 3 1784 cm-I and the photodissociating laser tuned to 14 920 cm-'. The absolute intensity of the parent ion signal has decreased (this may be difficult to discern in the figure since individual mass spectra are plotted relative to their biggest signal) in the presence of the visible laser and there has been a great increase in the intensity of the daughter ions. In order to avoid the complications of secondary fragmentation, the largest mass fragment ion of reasonable intensity (amu 167) was the fragment inspected for resonance effects. The variation in the intensities of the parent ion and the daughter ion at 167 amu with the wavenumber of the second laser are shown in Figure 4, where the ionizing laser is tuned to each of the two conformer origins. Both parent ion loss and fragment ion formation displayed essentially the same spectral characteristics for each of the conformers. The vibrational frequencies and transition linewidths differ considerably from those previously reported for naphthalene and 2-methylnaphthalene cations.16*26 The vibrational linewidths are considerably smaller, -40 cm-l as opposed to 200 cm-I reported for 2-methylnaphthalene. The characteristic vibrational progressions of -200 cm-I and -40 cm-I that dominate the spectrum have not been observed before in the ion spectra of naphthalenes and may be due to conformational acti~ity.~' To test the proposed scheme for measuring gas-phase electron transfer, the first bichromophoric system studied was NPT-C6BZ. The ionization potentials of benzene and naphthalene are 9.24 and 8.13 eV, respectively.** If a positive charge is created on the naphthalene part of this bichromophore, it cannot transfer to the benzene chromophore. Figure 2b shows the REZPI spectra of
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Figure 4. Action spectrum of D2 Dotransition of NPT-C6. Resonance ion dissociation spectra of the two conformers of 2-naphthyl cyclohexane ("426). (a) Variation in the intensity of the parent mass at 210 amu with the wavenumber of the second laser. The first laser is tuned to 31 784 cm-I. (b) Variation in the intensity of the fragment mass at 167 amu with the wavenumber of the second laser. The first laser is tuned to 31 784 cm-I. (c) The same as (a) except with the first laser tuned to the other conformation origin at 31 694 cm-l. (d) The same as (b) except with the first laser tuned to the other conformation origin at 3 1 694 cm-' .
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Figure 5. Mass spectra of NFT-C6-BZ. (a) With one laser at 31 782 cm-I. (b) With two lasers, the ionizing laser tuned to 31 782 cm-l and the fragmenting laser at 15069 cm-I.
NPT-C6-BZ. It very closely resembles the twephoton ionization spectrum of NPT-C6, indicating that the first excited singlet state for the system is only very weakly perturbed by the presence of the second chromophore. The mass spectra of this molecule in the absence and presence of the fragmenting laser are shown in Figure 5a,b. The fragmentation of the parent ion due to the ionizing laser is minimal, whereas the second laser pulse results in extensive fragmentation. The variation in the intensities of the parent and the 167-amu fragment ion with wavenumber of the photodissociating laser is shown in Figure 6. Once again, they both show the same essential features, a strong absorption at 15 000 cm-l. The vibrational linewidth of the spectrum has
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Electron Transfer in a Molecular Beam 0
The Journal of Physical Chemistry, Vol. 96, No. 22, 1992 8781 a
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Figure 6. Resonance ion dissociation spectra of NPT-C6-BZ. The first laser is tuned to 31 782 cm-’. Spectra have not been power normalized. (a) The variation in the laser power with wavenumber. (b) The variation in the intensity of the parent ion at 286 amu with the wavenumber of the second color photon. (c) The variation in the intensity of the fragment ion at 167 amu with the second color photon. TABLE I: Ionization Potentials of Isolated Donor and Acceptors naphthalene,” acceptor appearancea &,b cm-l chromophore energy, cm-I cm-I toluene 71 100‘ 7000 -1600 64 100d 4-methylbiphenyl 62 5OV 1-methylindole 60200‘ -3900
d
I 111111111)IIII 100
200 MASS
300
Figure 7. Mass spectra of NPT-C6-IND and NPT-C6-BIPH. (a) Mass spectrum of NPT-C6-IND with one laser tuned to 31 795 an-’.(b) Mass spectrum of NPT-C6-IND with two lasers at 31 795 and 15034 cm-l, respectively. (c) Mass spectrum of NPT-C6-BIPH with one laser at 31 776 cm-I. (d) Mass spectrum of NPT-C6-BIPH with two lasers each tuned to 31 776 and 15 147 cm-I, respectively.
From photoelectron spectroscopy. The difference in ionization potential for the donor-acceptor pairs. eFrom ref 37. dFrom ref 38. From refs 30 and 3 1. /From ref 29.
increased considerably and only one vibrational progression of 176 cm-l is observed. The adiabatic ionization potential for 1-methylindole is 7.46 eV.29 Hence in a bichromophoric system consisting of naphthalene and indole, a positive charge initially created on naphthalene can transfer to the indole chromophore (see Table I). The RE2PI spectrum of NFT-C6-IND is shown in Figure 2c. Once again the SIstate closely resembles that of NPT-C6. Mass spectra of the system taken in the presence of one and two dye laser pulses are shown in Figure 7a,b. In both cases there is some delayed fragmentation (i.e,, fragmentation of the parent ion occurring in the field-free region before the reflectron grids in the time-of-flight mass spectrometer), which shows up as a broadened feature in the mass spectrum. The amount of additional fragmentation produced by the second laser is very small. The spatial alignment of the two light pulses and the molecular beam interaction region was verified by bleeding 2-ethylnaphthalene into the helium gas lines and observing the extensive fragmentation of this molecule occurring under these conditions. It may therefore be inferred that the difficulty in fragmentation is intrinsic to the ion, which does not readily absorb photons in this wavenumber region. The lack of variation in the intensity of the parent and one daughter fragment ion (at mass 123) with the wavenumber of the fragmenting laser is shown in Figure 8b,c. The only variation in intensity mirrors the visible dye power curve (Figure 8a), the characteristic absorption spectrum of the naphthalene cation being absent. Changing the time interval between the ionizing laser pulse and the fragmenting laser pulse produced no change in the ion spectrum. At the earliest possible sampling time with the intensity maximum of the visible laser coming well before that of the ultraviolet laser, there was significant fragmentation due to the visible laser, but no change in the ion s p e c ” was observed. The adiabatic ionization potential of 4-methylbiphenyl is 7.75 eV,3093’there being a difference of 1600 cm-I in the adiabatic ionization potential of biphenyl and naphthalene (Table I). Figure 2d shows the RE2PI spectrum of NPT-C6-BIPH. It is, again, very similar to the spectrum of NPT-C6. The mass spectra taken with the ionizing laser at the origin transition of the more intense conformer in the absence and presence of the visible laser are
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Figure 8. Resonance ion dissociation spectra of NPT-C6-IND and NPTC6-BIPH. The spectra have not been power normalized. (a) The variation of the laser power with wavenumber. (b) The variation of the intensity of the NPT-C6-IND parent ion at 325 amu with the wavenumber of the second laser. The first laser is tuned to 31 795 cm-’. (c) Same as (b) except the mass inspected is that of the fragment ion at 123 amu. (d) The variation of the intensity of the NPT-C6-BIPH parent ion at 362 amu with the wavenumber of the second laser. The first laser is tuned to 31 776 cm-I. (e) Same as (d) except that the mass inspected corresponds to the fragment at 167 amu.
shown in Figures 7c,d. There is little fragmentation due to the ionizing laser. However, the visible laser has a definite, if weak, effect, the intensity of the parent ion decreasing and the intensity of the fragment ions increasing. The ion absorption spectra are shown in Figure 8d,e. Like the dissociation spectra of NFTC6-IND, they are featureless, reflecting only the power curve of the dye. Changing the timing between the pulses did not result in any change in the spectrum. In order to decrease the interaction between the two chromophores, the distance between them was increased by changing the spacer from cyclohexane to androstane. Figure 2e,f shows the RE2PI spectra of NPT-ST and NFT-ST-BIPH, respectively. The
8782 The Journal of Physical Chemistry, Vol. 96,No. 22, 1992
Chattoraj et al.
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150 300 450 600 Figure 9. Mass spectra of NPT-ST and NPT-ST-BIPH. (a) Mass spectrum of NPT-ST with one laser at 31 766 cm-I. (b) Mass spectrum of NPT-ST with two lasers at 31 766 and 15 147 cm-I. (c) Mass spectrum of NPT-ST-BIPH with one laser at 31 767 cm-'. (d) Mass spectrum of NPT-ST-BIPH with two lasers at 31 767 and 15 147 cm-'.
spectra are very similar to that of NPT-C6. Figure 9a,b shows the mass spectrum of NPT-ST and Figure 9c,d shows the same for NPT-ST-BIPH, taken in the absence and presence of the fragmenting laser at 15 147 cm-l. Negligible fragmentation or decomposition is detected in the absence of the visible laser. The visible laser results in a significant amount of fragmentation for NPT-ST, but for NPT-ST-BIPH the intensity of the visible laser had to be increased substantially to obtain significant fragmentation. The photodissociation action spectrum of NPT-ST cation is shown in Figure 10. Features of the naphthalene cation spectrum can be identified in both the parent and the daughter ion spectra, although secondary fragmentation processes complicate the latter. Several independent scans of the same spectrum consistently show two broad features at 15 090 and 15 300 cm-I. The photodissociation action spectrum of NPT-ST-BIPH is shown in Figure 1 1. We cannot unequivocally identify any features in this spectrum as belonging to the naphthalene cation. The variation in the intensity of both the parent and the daughter ion signal is closely correlated with the dye laser power changes. The signal-to-noise ratios in these spectra are not high, and the inferences drawn from them are more ambiguous than those drawn from spectra of molecules with cyclohexane spacers.
Discussion To summarize the results, the photodissociation spectra of a series of ions of the general structure [D-SPA]+ have been measured, where D is naphthalene and A is H, benzene, indole, and biphenyl. The spacers (Sp) are cyclohexane and androstane. The experiments can be separated into those in which there is either no acceptor (A = H) or an acceptor with a larger ionization potential than naphthalene (A = phenyl) and those with an acceptor that may be ionized with less energy than naphthalene (indole or biphenyl). All the cations are prepared by REZPI through the naphthalene SIstate. In the first group of cations (NPT-C6 and NPT-CCBZ), the charge remainslocalized on the naphthalene and, likenaphthalene itself, these ions have a characteristic photodissociation spectrum centered at 15OOO cm-l. The second group of ions (NPT-C6BIPH and NIT-C&IND) do not have this characteristicspectrum, presumably because the positive charge has transferred from the naphthalene to the acceptor before the absorption of a visible photon. Increasing the distance between the donor and the acceptor with a steroid spacer in NIT-ST-BIPH did not change the
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Figure 10. Resonance ion dissociation spectra of NPT-ST. The spectra have not been power normalized. The ionizing laser is tuned to 3 1 766 cm-I. (a) The variation of the laser power with wavenumber. (b) The variation of the intensity of the parent ion at 386 amu with wavenumber. (c) The variation of the intensity of the fragment ion at 154 amu with wavenumber of the second laser.
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Figure 11. Resonance ion dissociation spectra of NPT-ST-BIPH. The spectra have not been power normalized. The ionizing laser is tuned to 31 767 cm''. (a) The variation of the laser power with wavenumber. (b) The variation of the intensity of the parent ion at 538 amu with wavenumber. (c) The variation of the intensity of the fragment ion at 177 amu with wavenumber of the second laser.
spectrum. This is true regardless of the delay between the ultraviolet and visible lasers. For each pair of chromophores studied, naphthalene has the lower energy SIexcited state with at least --26OO-~m-~separation between it and the SIstate of the acceptor (Table 11). Therefore, in each of the bichromophom the excitation energy of the SIstate is almost completely localized on the naphthalene chromophore,
Electron Transfer in a Molecular Beam
The Journal of Physical Chemistry, Vol. 96, No. 22, 1992 8783
TABLE 11: Transition Energies to the First Excited Singlet States of ISOhted DOIIOr 8d Acceptors
C6-NPT, cm-’
acceptor chromophore phenylcyclohexane
31 784
4-methylbiphenyl
energy, cm-l
A,??: cm-l
37 635 34 64gb 34 378
585 1 2865 2594
1-indolylcyclohexane The difference in transition energies to the first singlet energy levels for the donor-acceptor pairs. From ref 39. there beiig no energy transfer from the SIlevel. The spacers keep the chromophores rigidly apart at fmed distances and orientations, preventing the formation of strongly interacting exciplexes. Each bichromophore is ionized by two photon absorption, the wavenumber of the incident photons being resonant with the origin excitation to the SIstate. For isolated indole, the doublet ground state of the ion is -4000 cm-l lower in energy than that of naphthalene (Table I). There will exist a manifold of zero-order vibronic levels of indole isoenergetic with the naphthalene ion states. For biphenyl the presence of low-frequency torsional vibrations will yield a high density of isoenergetic states, even though the energy gap between the two zero point levels is smaller. Interaction of the two chromophores will result in a perturbation of the zeroth-order naphthalene ion levels and the positive charge will be delocalized over both chromophores. Because of the high density of acceptor states, the molecular eigenstates would have a small amount of naphthalene character and a large amount of acceptor character. The intensity of the ionizing transition, however, is exclusively due to the component of naphthalene states, which is the only one radiatively coupled to the SIstate accessed with the first photon. Depending on the transform limited frequency spread of the incident light and the natural width of the molecular eigenstates compared to their spacing,32we may be preparing individual vibrational eigenstates or we may be coherently exciting a linear combination of the eigenstates. In the former case no relaxation proctsses will be observed. In the latter, depending on the density of states, a reversible or irreversible relaxation process will occur. In our experiments the transform limited frequency spread of the laser used to prepare the ion is 0.0006 cm-’. In all cases where electron transfer is energetically possible, we see no change in the spectrum down to the time resolution of the experiment, which is approximately 2 ns. Therefore, either we are initially preparing eigenstates of the ion that do not evolve in time or we are preparing a coherent superposition of states that unphases in less than 2 ns. The ion dissociation spectra of NPT-CCBZ and NPT-ST are considerably less structured than that for NPT-C6, although each of the ions is prepared by RE2P1, using the naphthalene SIorigin as an intermediate stage. Increase in the size of the molecule will increase the number of low-frequency vibrational modes associated with it. There may also be a lowering of the ionization potential of the molecule with size. However, the SI Sotransition energies remain almost invariant. In the ionization process energy in excess of the ionization potential will be partitioned between the ejected electron and the ion. An increase in the number of available vibrational modes will result in the production of a larger number of populated ion states and a broadening of the dissociation spectrum. The dissociation spectrum of the isolated naphthalene ion consists of some fairly broad but discrete features and a continuous background that extends to the long-wavelength side of any of the discrete features. This led to speculation about the existence of a DIelectronic state 4000 cm-’ below the D2state. The D2 state was thought to be responsible for the discrete features.16 Transitions from the ground Dostate of the ion to the DI state would be formally forbidden but could produce broadening and a continuum by borrowing intensity from the D2 Dotransition. A complementary experiment to the present one would be to measure appearance of the acceptor ion spectrum as well as the disappearance of the donor ion spectrum. We have been unable to do this because isoenergetic electron transfer to the acceptor states results in the formation of a distribution of vibrationally
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hot acceptor ions. Sequence congestion prevents observation of the acceptor ion spectrum. It is useful to compare electron transfer in solution with what is expected in the gas phase. The major difference between intramolecular electron transfer in the condensed phase and in a molecular beam is the presence of solvent molecules. In solution it is necessary to consider the entire supermolecule, i.e., the molecule and its solvation shell. The change in electrostatic character accompanying electron transfer results in the relaxation of the internal coordinates of the molecule as well as of the solvation shell. The often observed activation energy for electron-transfer reactions occurring in condensed phases includes both the standard free energy of the reaction and the reorganization energy of the ~ o l v e n tbecause ,~ the degree of solvation of the charged species, reactant or product, will substantially influence the direction and propensity of the reaction. In contrast, in the gas phase the energy can be distributed among the internal modes only, and so the ionization potential is the sole determining factor. A comparison with previously reported chargetransfer reactions in solution is of interest, particularly since two of the systems studied here are identical with the ones measured in solution. The rates of positive charge transfer for NPT-ST-BIPH and NPTC6-BIPH are 1.27 X lo6 and 1.0 X lo9 s-l, respectively, in a relatively polar solvent, 1,2-di~hloroethane.~~ However, the direction of charge transfer is the opposite to that predicted by gas-phase ionization potential measurements, the relatively compact naphthalene ion being more stable in solution than the cation of biphenyl. The effect of the solvent is dramatically demonstrated in the rata of charge transfer in the neguriue ion of NPT-ST-BIPH in polar methyltetrahydrofuran (1.5 X lo6 s-l) and nonpolar These measurements cannot be directly isooctane (1.5 X lo9SI).” compared to our gas-phase measurements since the direction of charge transfer and indeed (in the solvent polarity comparison) the very nature of the charged species has been reversed. A significant difference between intramolecularelectron transfer in solution and in the gas phase is the greater geometry change between reactants and products that occurs in solution due to the interaction of the electron with the solvent. The matrix element that produces electron transfer can be separated into the product of an electronic part and three Franck-Condon factors: one determined by the change in the donor geometry, one determined by the change in the acceptor geometry, and one involving the other degrees of freedom, those of the solvent (in solution) and the spacer. If the reactants and products have different electronic energies, then the excess electronic energy must be converted into vibrational energy. This means that at least some of the Franck-Condon factors will be between states where Au # 0. If there was no change in the potential energy curves between the reactants and the products except for a change in their zero-point energies, the set of vibrational eigenfunctions would be identical for the two states. The Au # 0 Franck-Condon factors would then be zero due to orthogonality, and electron transfer would be forbidden. Therefore, electron transfer requires a geometry change along some degree of freedom. The situation is illustrated in Figure 12, which shows the potential energy curves for the reactant and the product for three cases. In the weakly coupled case, there is only a small geometry change and the vibrational overlap is poor, leading to small Franck-Condon factors. This would be the case in the gas phase if neither the donor nor the acceptor nor the spacer vibrational potentials changed much upon electron transfer, i.e., if there is not much difference between the ionic and neutral potential energy curves. In this case, the overlap gets worse as the difference in zero-point energy increases and the system obeys an energy gap law.3s This situation may be fairly common in the gas phase and may well describe the molecules studied in this work. For example, there is experimental evidence to indicate that the equilibrium geometry change for ionization of naphthalene in the gas phase is In the strongly coupled case there is enough of a geometry change that there is good vibrational overlap between initial and
8784 The Journal of Physical Chemistry, Vol. 96, No. 22, 1992 Weakly c o w l e d
S t r o n g l y coupled
Chattoraj et al. fellowship support from the Pew Midstates Science and Mathematics Consortium.
Activated Process
References and Notes
Reactlor Coordinate
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Reaction Coordinate
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(1) Present address: Department of Chemistry, Kalamazoo College, Kalamazoo, MI 49006. (2) Deceased May, 1992. (3) Marcus, R. A.; Sutin, N. Biochim. Biophys. Acra 1985. 811, 265. Newton, M. D. Chem. Rev. 1991, 91, 767. Marcus, R. A.; Almeida, R.J . Phys. Chem. 1990,94,2963. Kuznetsov, A. M.; Ulstrup, J. J . Chem. Phys. Reaction Coordinate
*
Figure 12. Three cases of changes in equilibrium configuration. In the weakly coupled case the relative horizontal displacement is small. In the strongly coupled case the change in the equilibrium configuration between the two states is large enough to result in the crossing of the surfaces near the zero-point energy of the initial surface. In the third case the displacement between the two surfaces is so large that the process becomes an activated one.
final states and a relatively large Franck-Condon factor. This could occur in the gas phase if there were a large potential shift produced by the charge transfer. It is much more likely in solution where the interaction between the solvent and the chromophore will change greatly when the electron is transferred. Finally, if the change in geometry is too great, the system is described by the right-hand drawing in Figure 12. Here the region of good vibrational overlap is higher than the zero-point level of the reactant, and electron transfer is a thermally activated process. This is the Marcus region where the electron-transferrate increases with an increase in the difference in the zero-point energy. This situation is possible only in solution, especially in the more polar solvents. For all three cases, the increase in the density of states due to the presence of low-frequency solvent vibrational modes would be expected to increase the rate of the transfer in solution. For naphthalene and biphenyl, electron transfer is an activated process in solution, even in nonpolar solvents, making it difficult to compare with the gas phase. However, NPT-CCIND is expected to be in the nonactivated regime in many solvents and it would be interesting to compare the limit measured in the gas phase with condensed phase measurements.
Conclusion Intramolecular electron transfer in the gas phase has been observed. An experimental scheme is developed by using resonantly enhanced two-photon ionization to selectively prepare an ionic state followed by resonant ion dissociation to characterize its dynamics. No time evolution is observed in any of the molecules within our limit of resolution (-2 ns) but definite changes in the spectrum due to charge delocalization are observed. Measurement of the dynamics in the gas phase by probing at shorter time scales or by decreasing the rate of the transfer process would be of great interest. The latter may be achieved by using longer spacers in conjunction with donor chromophores, which possess an intrinsically more intense and welldefined ion absorption spectrum than naphthalene. Acknowledgment. This work was supported by the NSF under Grants CHE 8818321 and CHE 8520326. S.L.L. acknowledges
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