Article pubs.acs.org/JPCB
Structural Dynamics of a Noncovalent Charge Transfer Complex from Femtosecond Stimulated Raman Spectroscopy Tomotsumi Fujisawa, Mark Creelman, and Richard A. Mathies* Department of Chemistry, University of California, Berkeley, California 94720, United States S Supporting Information *
ABSTRACT: Femtosecond stimulated Raman spectroscopy is used to examine the structural dynamics of photoinduced charge transfer within a noncovalent electron acceptor/donor complex of pyromellitic dianhydride (PMDA, electron acceptor) and hexamethylbenzene (HMB, electron donor) in ethylacetate and acetonitrile. The evolution of the vibrational spectrum reveals the ultrafast structural changes that occur during the charge separation (Franck−Condon excited state complex → contact ion pair) and the subsequent charge recombination (contact ion pair → ground state complex). The Franck−Condon excited state is shown to have significant charge-separated character because its vibrational spectrum is similar to that of the ion pair. The charge separation rate (2.5 ps in ethylacetate and ∼0.5 ps in acetonitrile) is comparable to solvation dynamics and is unaffected by the perdeuteration of HMB, supporting the dominant role of solvent rearrangement in charge separation. On the other hand, the charge recombination slows by a factor of ∼1.4 when using perdeuterated HMB, indicating that methyl hydrogen motions of HMB mediate the charge recombination process. Resonance Raman enhancement of the HMB vibrations in the complex reveals that the ring stretches of HMB, and especially the C−CH3 deformations are the primary acceptor modes promoting charge recombination.
1. INTRODUCTION Photoinduced charge transfer (CT) is the universal process underlying the conversion of light into electrochemical energy. The construction of high-performance dye-sensitized solar cells and artificial photosynthetic systems is a global challenge to develop more efficient utilization of solar radiation.1,2 The essential process is photoinduced charge separation, while charge recombination is one of the undesired outcomes. The mechanisms of CT have been investigated using electron transfer theory,3 and experiments have been performed on a huge selection of noncovalently and covalently linked electron acceptor/donor systems including conventional π-complexes,4,5 metal−ligand complexes,6,7 and porphyrin and chlorophyll chromophores connected to various acceptors.8−10 As electron transfer rates in the nonadiabatic limit are expressed by the product of an electronic matrix element and a Franck−Condon weighted density of states,3,11,12 nuclear motions in a wide frequency range are generally involved in the CT process of an electron acceptor/donor system. To observe and understand this dynamical molecular aspect of the CT process, time-resolved vibrational spectroscopy that directly probes the vibrational structural changes during the CT process is a useful technique. This information on the specific inter- and intramolecular vibrations that can modulate the electronic coupling and the associated Franck−Condon factors should lead to a more detailed picture of the CT mechanism in electron acceptor/donor systems. Pyromellitic dianhydride (PMDA) is a typical organic electron acceptor that has been used for electron transfer © 2012 American Chemical Society
studies because it is easily linked to a variety of donors including Zn-porphyrins, functionalized aromatic imides, and CdSe/ZnS quantum dots.13−15 When PMDA is simply mixed with an electron donor, such as alkylbenzenes, their association exhibits a new absorption band ascribed to the CT complex formation whose peak energy strongly correlates with the donor strength (Figure 1).16 The CT absorption band is an optical transition between the ground state (neutral character) and the excited state (charge-separated character) which arise from the mixing between the HOMO of the electron donor and the LUMO of the acceptor.5 Electronic excitation in the CT band initiates the formation of a contact ion pair (CIP) from the excited state complex.17 Upon excitation, the system is prepared in an already highly dipolar Franck−Condon state.18 As the system evolves, the degree of charge separation increases until the CIP is formed. Here this process will be referred to simply as “charge separation”. The charge separation is followed by either charge recombination or dissociation of the ions.17 Charge recombination of the PMDA/donor complexes has been extensively studied17,19−24 since Mataga and co-workers17,23,24 found that the log of the charge recombination rate of the CIP linearly decreases as a function of the free energy gap, instead of following the classical Marcus bell shape observed in Special Issue: Richard A. Mathies Festschrift Received: January 4, 2012 Revised: March 20, 2012 Published: March 22, 2012 10453
dx.doi.org/10.1021/jp3001306 | J. Phys. Chem. B 2012, 116, 10453−10460
The Journal of Physical Chemistry B
Article
acetonitrile, and all other chemicals were used as received. The concentration dependence of PMDA/HMB charge transfer (CT) complex formation was checked using UV−vis absorption to confirm the formation of only the 1:1 complex under the experimental conditions. Spontaneous Raman spectra of HMB (32 mM), PMDA (68 mM), and their complex (68 mM of PMDA and 57 mM of HMB in acetonirile, 68 mM of PMDA and 75 mM of HMB in ethylacetate) were collected using the 413.1 nm line from a Kr+ laser (Spectra-Physics, model 2025). The excitation beam was focused to a diameter of ∼60 μm with a 50 mm focal length excitation lens into the sample capillary (1.5 mm ID). Raman scattering was collected at 90° from the excitation beam, focused into a subtractive double spectrograph (Spex 1400), and dispersed onto a cooled, back-illuminated CCD (LN/ CCD-1100/PB; Roper Scientific) for detection. The Raman spectrum was taken with 60 s exposures and averaged 30 times. Since the spectrum of the PMDA/HMB mixed solution containing the CT complex is dominated by the vibrational bands of the free acceptor and donor, the resonance Raman spectrum of the PMDA/HMB complex was obtained by subtracting the absorption-corrected off-resonance Raman bands of the solvent, free HMB, and PMDA using the intensity of the solvent bands as a reference. No concentration dependence was observed in the measurement. The photoinduced charge separation/recombination dynamics of the PMDA/HMB complex after excitation of the CT absorption band were measured using time-resolved FSRS. The FSRS instrument has been described in detail elsewhere;27−29 however, some modifications have since been made to the detection system. Briefly, a regenerative Ti:sapphire amplifier (Spitfire, Spectra-Physics) produces a 1 kHz train of ∼800 μJ pulses at 800 nm with a duration of ∼150 fs. This fundamental output was split and used to generate the three pulses necessary for FSRS. The actinic pulse (400 nm, 350 nJ/pulse) used to excite the CT complex was generated by frequency doubling of the fundamental in BBO. The Raman pulse (625 nm, 300−400 nJ/pulse) was produced using a home-built narrow band OPA. The broadband continuum necessary for the probe pulse was generated by focusing a small portion of the fundamental into a sapphire plate. This continuum was then compressed in a SF10 prism pair and used to simultaneously stimulate the Stokes Raman transitions when the Raman pulse was coincident on the sample. These three pulses are focused onto the sample using a 100 mm focal lens. After passing through the sample, the actinic and Raman pulses are blocked by an aperture, while the probe pulse is recollimated with a 100 mm lens and focused into a spectrograph (Spex 500, 600 gr/mm, 500 nm blaze) and dispersed onto a front-illuminated CCD (Pixis 100, Princeton Instruments). The CCD, which is synced to the 1 kHz amplifier pulse train, reads out each pulse. By using phase-locked mechanical choppers (MC2000, Thorlabs) to modulate the Raman pulse at 500 Hz, and the actinic pulse at 250 Hz, the ground-state and excited-state FSRS spectra for a given delay time were collected in four laser shots (4 ms) (see Supporting Information). Transient absorption data within the probe window were also captured for each time point within these four pulses. Time-resolved FSRS spectra were collected by varying the time delay between the actinic pulse and the Raman/probe pulse pair. The time-resolved FSRS spectra of the photoinduced reaction were produced by subtracting the ground state FSRS spectrum from each of the electronically excited
Figure 1. Absorption spectrum of the pyromellitic dianhydride (electron acceptor, PMDA)/hexamethylbenzene (electron donor, HMB) charge transfer complex in ethylacetate. The calculated structure of the complex (at the ωB97xd/6-311+G(d,p) level) and the photoreaction scheme are illustrated. Abbreviations: A, acceptor; D, donor.
solvent-separated ion pairs. This non-Marcus behavior, along with the small activation energy and the similarity to the “energy gap law” for nonradiative transitions,25,26 was interpreted as arising from Franck−Condon vibrational overlap between high-frequency modes of the CIP and the ground state.6,7 A recent study using both femtosecond transient absorption and transient grating methods measured the charge recombination rate in the CT complexes composed of PMDA and methoxy-substituted benzenes which undergo charge recombination faster than solvent diffusive motion.21 The non-Marcus trend was reproduced by accounting for the intramolecular high-frequency modes and dynamic solvent effects on the charge recombination.20 However, due to the lack of structural information, the structural change of the complex during the charge recombination and the vibrational modes responsible remain undetermined. Furthermore, it has not been possible to characterize the charge separation process of the PMDA/donor complex because clear spectral dynamics of the charge separation have not been observed in transient electronic spectra. In this article, femtosecond stimulated Raman spectroscopy (FSRS),27,28 an especially powerful method for probing the femtosecond structural dynamics in photochemical reactions, is applied to the photoinduced CT processes of the electron acceptor/donor complex formed between PMDA and hexamethylbenzene (HMB). FSRS reveals the unambiguous structural evolution in the ultrafast charge separation of the PMDA/HMB complex, which is then followed by charge recombination. The vibrational spectrum of the Franck− Condon excited state indicates that the excited state complex has significant charge-separated character. The charge separation (excited state complex → CIP) is shown to be facilitated by solvation dynamics. The charge recombination rate is found to slow significantly when HMB is replaced by perdeuterated HMB. The steady state resonance Raman spectrum of the CT complex reveals that the C−CH3 deformations of HMB are the primary acceptor modes that promote charge recombination.
2. MATERIALS AND METHODS Pyromellitic dianhydride (PMDA), hexamethylbenzene (HMBh18), perdeuterated hexamethylbenzene (HMB-d18), and the solvents (acetonitrile and ethylacetate) were purchased from Sigma-Aldrich. PMDA was purified by recrystallization in 10454
dx.doi.org/10.1021/jp3001306 | J. Phys. Chem. B 2012, 116, 10453−10460
The Journal of Physical Chemistry B
Article
time points and then subtracting a featureless baseline to highlight the Raman bands. The 200−250 fs temporal instrument response was measured using the optical Kerr effect between the actinic and probe pulses. The solution of the CT complex (∼68 mM of PMDA and ∼32 mM of HMB)21,23,24 was continuously circulated through a 1 mm path-length optical flow cell (Starna, 48-Q-1) during the measurement. UV−vis absorption performed after FSRS experiments shows no signs of sample degradation.
3. RESULTS AND DISCUSSION 3.1. PMDA/HMB Complex. PMDA and HMB readily form a 1:1 π-complex at concentration ranges of 23−68 mM (PMDA) and 6−57 mM (HMB). The extinction coefficients (ε) of the PMDA/HMB complex measured by the Benesi− Hildebrand procedure30 (see Supporting Information) are 1408 ± 195 and 2834 ± 221 M−1 cm−1 in ethylacetate and acetonitrile, respectively. These values are similar to those reported for PMDA/pyrene (1048 ± 25 M−1 cm−1) and PMDA/triphenylene (1379 ± 24 M−1 cm−1) systems in methylene chloride.31 The association constant (K) is larger in ethylacetate (K = 1.40 ± 0.18 M−1) than in acetonitrile (K = 0.57 ± 0.04 M−1), reflecting the dependence of the complex’s stability on solvent polarity. The calculated structure of the CT complex at the ωB97xd/6-311+(d,p) level (Figure 1, inset) shows a displacement of HMB from the center of PMDA such that the HOMO of HMB and LUMO of PMDA are well overlapped in space and phase to facilitate electron sharing. The calculated intermolecular distance between PMDA and HMB is 3.2 Å. 3.2. Spontaneous Raman Spectrum of the PMDA/ HMB Complex. The spontaneous Raman spectra of PMDA and HMB-(h18 and d18) and of their complexes in ethylacetate are shown in Figure 2. The Raman bands of HMB and PMDA were assigned with the help of density functional theory (DFT) calculations (B3LYP/6-311+G(d,p)) along with assignments from earlier studies.32 In the resonance Raman spectrum of the PMDA/HMB-h18 CT complex, the observed peaks at 455, 1301, and 1575 cm−1 are assigned to the C−CH3 deformation, ag ring breathing stretch, and eg ring stretch of the donor HMBh18 whose frequencies are almost identical to free HMB. The same trend is observed for the PMDA/HMB-d18 complex with the C−CD3 deformation at 422 cm−1 and eg ring stretch at 1565 cm−1 downshifted. However, the resonance enhancement of the PMDA vibrations in the complex is negligible. This is also the case when the experiments are performed in acetonitrile. The resonance Raman intensities of the PMDA/HMB complex depend on the displacement between the ground and excited state potentials for the vibrational modes of the complex.33 Therefore, the weak resonance Raman enhancement of PMDA modes in the CT complex indicates a small structural change in PMDA relative to that of HMB upon photoexcitation into the CT absorption band. The ring stretches of HMB (at 1301 and 1575 cm−1 for PMDA/HMBh18, 1297 and 1565 cm−1 for PMDA/HMB-d18) gain relatively strong intensity in the spectrum of the PMDA/HMB complex, while the C−CH3 (or C−CD3) stretching modes (at 453 and 555 cm−1 for PMDA/HMB-h18, 422 and 507 cm−1 for PMDA/ HMB-d18) are weaker. The HMB radical cation has an asymmetric ring where opposite CC bonds stretch by ∼0.06 Å compared to the symmetric neutral HMB structure (see Supporting Information, Figure S3). Thus, the vibrational
Figure 2. Ground state off-resonance spontaneous Raman spectra of PMDA (bottom, 68 mM), HMB-(h18 and d18) (middle, 32 mM), and resonance Raman spectrum of the CT complex (top, HMB-(h18 and d18): 75 mM, PMDA: 68 mM) in ethylacetate (λex = 413.1 nm). The Raman bands from free HMB and PMDA are subtracted from the spectrum of the CT complex solution using the intensity of the solvent as the reference. Assignments are made according to ref 32 and DFT calculation at the B3LYP/6-311+G(d,p) level. Symmetry labels are based on D2h for PMDA and D3d for HMB. Asterisk denotes the positions of the solvent Raman bands.
modes with the asymmetric ring motion have a large displacement and become highly Franck−Condon active as a result of electronic excitation. The resonance Raman features of the PMDA/HMB complex contrast with those of the tetracyanoethylene/HMB complex where vibrational modes of both the donor and the acceptor are enhanced and observed at lower concentrations in CCl4.34 Tetracyanoethylene (Ered = 0.24 V) is a strong acceptor compared to PMDA (Ered = −0.55 V).22 Therefore, the tetracyanoethylene/HMB complex shows a high association constant of ∼150 M−1 in CCl4, and the increased π-orbital overlap results in a large extinction coefficient of ∼5000 M−1 cm−1 relative to the PMDA/HMB complex.35 The negligible resonance enhancement in the case of the PMDA acceptor apparently originates from the relatively small structural change upon photoexcitation as well as its weak acceptor properties that reduce the CT character and the transition dipole moment.36 10455
dx.doi.org/10.1021/jp3001306 | J. Phys. Chem. B 2012, 116, 10453−10460
The Journal of Physical Chemistry B
Article
3.3. Time-Resolved FSRS of the PMDA/HMB Complex. The real-time excited state structural dynamics of the PMDA/ HMB complex were observed by FSRS. Figure 3 presents a
transfer occurs include splitting of the ring breathing mode at 1246/1311 cm−1 and an increase in the intensity of the C−C stretch at 1106 cm−1. The observed evolution of the FSRS spectrum provides kinetic structural data on the charge separation process of the PMDA/HMB complex from the Franck−Condon excited state to the CIP. The Raman bands of the excited state complex at 100 fs have frequencies similar to PMDA•−. This structural information confirms that the Franck−Condon state has significant chargeseparated character before the subsequent enhanced polarization that produces the CIP.37 However, the relative intensity of the C−C stretch at 1109 cm−1 drastically increases from the Franck−Condon state to PMDA•−, indicating a dynamic change of the resonance enhancement factor in the charge separation process. In addition to the relative growth of the C− C stretch, the splitting of the excited-state ring breathing mode (1306 cm−1) is an indicator of charge separation. The splitting is not reproduced by DFT calculations of PMDA•− (B3LYP/6311+G(d,p)). We thus assign the ring breathing doublet to a Fermi resonance arising from anharmonic coupling with the overtone of the COC bending mode at 631 cm−1. A trace of the Fermi resonance is also found as a weak shoulder on the 1306 cm−1 ring breathing mode of the excited state complex at 100 fs. The formation of PMDA•− may modify the COC bending and/or ring breathing frequencies, resulting in the clearly split feature. The kinetics of the Raman intensities were modeled by the rise and decay of PMDA•− together with the decay of the •− excited state complex, i.e., aPMDA•−[exp(−t/τdecay PMDA ) − exp(−t/ rise •− decay τPMDA )] + aExc[exp(−t/τExc )], convoluted with the instrumental response of 225 fs (Figure 4). It was necessary to add the excited state decay component to properly fit the delayed appearance of the radical cation. This kinetic analysis shows that the time constants for the rise (charge separation) and decay (charge recombination) of PMDA•− are 2.5 ± 0.2 ps and 18.9 ± 1.3 ps, respectively (for PMDA/HMB-h18 in ethylacetate). The lifetime of the excited state of PMDA/HMB-h18 is 2.5 ± 0.1 ps. When the complex is formed with the perdeuterated donor (PMDA/HMB-d18), the time constant of the charge recombination is 27.0 ± 0.6 ps, 1.4 times larger than that of PMDA/HMB-h18. On the other hand, the charge separation process does not exhibit any significant change upon isotope substitution of the donor; the PMDA•− rise time and the excited-state lifetime for PMDA/HMB-d18 are 2.7 ± 0.4 and 2.2 ± 1.1 ps, respectively. Since the effect of deuterium substitution is to reduce the Franck−Condon factor and cause the kinetic isotope effect (KIE),38 this indicates that the acceptor modes that facilitate the charge recombination include the vibrations coupled with methyl hydrogen motion such as CH3 bend, C−CH3 stretch, or the C−H stretch of the HMB portion of the complex. The charge separation and recombination in the PMDA/ HMB complex proceed differently in acetonitrile (Figure 5). The ring breathing doublet characteristic of PMDA•− is immediately apparent at 1250/1303 cm−1 upon photoexcitation, and it fully decays in 15 ps (Figure 6A). The ring breathing doublet exhibits different relative intensities in acetonitrile and ethylacetate, indicating sensitivity to the solvent polarity and/or the acceptor−donor interaction. The decay time constant is 5.3 ± 0.4 ps for PMDA/HMB-h18 and 7.3 ± 0.2 ps for PMDA/HMB-d18; the KIE is 1.4. Because charge separation takes place with a time constant similar to the instrument response function (∼225 fs), the estimation of the
Figure 3. FSRS spectra of the PMDA/HMB-h18 complex in ethylacetate after electronic excitation at 400 nm. The Raman pump was placed at 625 nm. The assignments are based on the PMDA/ HMB-d18 result at 100 fs, the ground state spectrum of PMDA, and DFT calculation (B3LYP/6-311+G(d,p)). Asterisk denotes the positions of the solvent Raman bands.
stacked plot of the time-resolved FSRS spectra taken in ethylacetate after photoexcitation of the CT band. Upon electronic excitation, a vibrational spectrum featuring strong bands at 1306 and 1545 cm−1 and weak and moderate bands at 493, 956, and 1109 cm−1 arises. Since these spectra do not exhibit any frequency shift or intensity variation upon perdeuteration of HMB, the Raman bands have been assigned to the vibrational modes of PMDA in the excited state CT complex. The assignments are illustrated on the FSRS spectrum above the 100 fs time point: 1545 cm−1, ring stretch; 1306 cm−1, ring breathing stretch; 1109 and 956 cm−1, C−C stretch; 493 cm−1, CO bend. The excited-state vibrational spectrum develops over several picoseconds into one that has frequencies similar to the 100 fs spectrum but shows the appreciably different relative intensities. The resulting vibrational spectrum, which disappears with the decay of the transient absorption (550−700 nm, D0 → D2) of the PMDA radical anion (PMDA•−), is the resonance Raman spectrum of PMDA•− of the contact ion pair (CIP). The spectral changes as charge 10456
dx.doi.org/10.1021/jp3001306 | J. Phys. Chem. B 2012, 116, 10453−10460
The Journal of Physical Chemistry B
Article
Figure 5. FSRS spectra of the PMDA/HMB-h18 complex in acetonitrile after electronic excitation at 400 nm (Raman pump: 625 nm). Asterisk denotes the positions of the solvent Raman bands. Figure 4. Intensity of the ring breathing doublet (at ∼1246 and ∼1306 cm−1) for PMDA/HMB-h18 (○) and PMDA/HMB-d18 (●) vs time in ethylacetate. The fit is based on the model function expressed as rise •− •− aPMDA•−[exp(−t/τdecay PMDA ) − exp(−t/τPMDA )] + aExc[exp(−t/τExc)] convoluted with instrumental response of 225 fs. Solid gray line: total fit. Dotted line: excited state contribution. Dashed line: PMDA•− kinetics.
at least in HMB. Generally, the charge separation process in CT complexes is also influenced by the strength of the acceptor and donor interaction. A transient absorption study of the complex composed of HMB and tetracyanobenzene43,44 (a weaker acceptor than PMDA) revealed the fast and slow rises of the CIP absorption that are attributed to the solvent-induced partial charge separation (
