Interrogating the Intramolecular Charge-Transfer State of a Julolidine

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Interrogating the Intramolecular Charge-Transfer State of a Julolidine-Anthracene Donor-Acceptor Molecule with Femtosecond Stimulated Raman Spectroscopy Jenny V. Lockard, Annie Butler Ricks, Dick T. Co, and Michael R. Wasielewski* Department of Chemistry and Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern University, Evanston, Illinois 60208-3113

ABSTRACT The nature of the lowest-energy charge-transfer (CT)excited state of the donor-acceptor molecule, 3,5-dimethyl-4-(9-anthracenyl)julolidine (DMJAn) is investigated using femtosecond stimulated Raman spectroscopy. Transient Raman spectra are presented with subpicosecond time resolution, and peaks are assigned based on the published Raman modes of the reference molecule phenyl anthracene. The results indicate that the CT excited state is dominated by a fully charge separated radical ion pair state with minimal contribution from the local anthracene π-π* state. SECTION Kinetics, Spectroscopy

spectrum of DMJ-An (Figure 2 inset) shows that the structured anthracene S1 band is nearly isoenergetic with the CT band that appears as a lower-energy shoulder. For the nearly identical julolidine-anthracene molecule, studies employing a three-state mixing model to accommodate the enhanced absorption intensity of the CT band estimate almost equal contributions of the low-lying anthracene S1 state and the RP state to the CT state.3 However, studies of DMJ-An using the fluorescence solvatochromatic shift method2 lead to an estimated charge separation distance of 5.4 Å. This suggests essentially quantitative charge separation within the CT state of DMJ-An since the charge separation distance for DMJþ•-An-• calculated using the centers of spin density distributions in DMJþ• and An-• is very similar at 5.6 Å.8 The transient absorption spectrum of DMJ-An (Figure 2), obtained using 400 nm excitation from a Ti:sapphire laser system previously described,10,11 contains two features with rise times limited by the instrument response time and peak maxima at 500 and 675 nm in THF. The 500 nm band is assigned to a julolidine-localized transition based on spectra of DMJþ•-An obtained by chemical oxidation. The 675 nm band is associated with an anthracene transition, but its assignment is more complicated since the spectrum of the chemically reduced DMJ-An-• occurs in the same energy range as the transient absorption spectrum reported for the S1 state of anthracene.12,13 In this study, we use the recently developed time-resolved vibrational spectroscopy technique, femtosecond stimulated Raman spectroscopy (FSRS),14 to provide more definitive evidence that the CT state in DMJ-An is in fact best described

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uch work has been directed toward understanding the nature of the lowest-energy charge-transfer (CT) excited state of electron donor-acceptor (D-A) molecules.1-5 The CT state can be represented as a combination of the locally excited (LE) states of the donor or acceptor moiety and the radical ion pair (RP) state, Dþ•-A-•, but the relative contributions remain somewhat elusive for many D-A systems. D-A molecules typically display CTabsorption and emission bands and exhibit large excited-state dipole moments, which imply that significant electron density is transferred from the donor to the acceptor and that LE states play a minimal role in the excited state. At the same time, a CT absorption band lying close in energy to strongly allowed LE bands will often exhibit an unusually high extinction coefficient, which suggests intensity borrowing and thus significant mixing of the LE and RP states. Furthermore, the relative contributions of the LE and RP states to the CT excited state are also controlled by solvent polarity, which influences the respective energy gap.2-7 4-(9-Anthryl)-N,N-dimethylaniline (ADMA) derivatives represent one group of aromatic D-A molecules that has been extensively studied in an effort to elucidate the electronic structure and molecular conformation of the CT state.4,5 3,5-Dimethyl-4-(9-anthracenyl)julolidine (DMJ-An) is a rigid ADMA derivative in which the julolidine structure severely restricts rotation about the nitrogen-phenyl bond. The two methyl groups enforce a geometry in which the DMJ π system is ∼90° to that of anthracene, as determined by MMþ geometry-optimized models created using HyperChem (Figure 1).8 These structural constraints simplify the photophysics of DMJ-An by removing the rotational degrees of freedom that are present in the parent ADMA molecule. DMJ-An was synthesized and purified according to a previously outlined procedure.9 The ground-state absorption

r 2009 American Chemical Society

Received Date: October 13, 2009 Accepted Date: November 10, 2009 Published on Web Date: November 17, 2009

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DOI: 10.1021/jz900136a |J. Phys. Chem. Lett. 2010, 1, 215–218

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Figure 1. Molecular structure and MMþ geometry-optimized model of DMJ-An.

Figure 3. Schematic of the FSRS experimental setup: BS, beam splitter; SD, sapphire disk.

broad-band Raman probe pulse by continuum generation in a 2 mm thick sapphire disk. In this experiment, a sub-100 fs pulse duration is preserved for both the actinic pump and the Raman probe pulse, while the narrow-band Raman pump pulse acquires a ∼3 ps pulse duration after passing through the spectral filter due to the time-bandwidth product. With respect to the three pulses, the Raman spectral resolution is dictated by the Raman pump pulse, which has a bandwidth of ∼5 cm-1. The spectral resolution is also limited by the spectrograph resolution (∼0.5 nm) and at very early times by vibrational dephasing. The time resolution for this experiment is governed by the convolution of the actinic pump and Raman probe pulses (300 fs measured by optical Kerr effect (OKE) cross correlation at a 700 nm probe wavelength). In the simplest sense, once the excited electronic state is populated by the actinic pump pulse, the Raman transition is simultaneously prepared and stimulated after some time delay (0-6 ns) by the Raman pump and Raman probe pulses, respectively. The key to the sub-ps time resolution is that the stimulated Raman transition is gated by the short Raman probe pulse. Florescence backgrounds are rejected because the stimulated Raman spectrum is measured as a gain signal on the intense Raman probe through heterodyne detection using a photodiode array. The ground-state (gs) Raman spectrum of DMJ-An in THF (bottom trace, Figure 4) was obtained using a CW Raman setup consisting of a triple monochromator (Acton Research) equipped with a LN2-cooled deep depletion CCD detector (Princeton Instruments Spec-10:400BR). The 785 nm excitation was provided by a Ti:sapphire laser (Spectra-Physics Tsunami) in CW mode. The experimental setup has been described previously.21 The Raman peaks due to the solvent have been subtracted from the spectrum. No resonance enhancement of the Raman intensity is expected at this excitation wavelength for DMJ-An in its ground state.

Figure 2. Transient absorption spectrum of DMJ-An in THF at 3 ps (400 nm pump). Inset: Steady-state absorption spectra of DMJ-An (black), julolidine (blue), and anthracene (red) along with a Gaussian approximation of the CT state (green).

as the fully charge separated radical ion pair state, DMJþ•-An-•, as suggested by the fluorescence solvatochromatic shift study.2 Time-resolved vibrational spectroscopy methods have been previously employed to help understand the CT excited states of D-A molecules.1,15-18 Time-resolved resonance Raman spectroscopy, for example, was used to study the CT state of 4-dimethylaminobenzonitrile.18 The time resolution of this method, however, is limited by the picosecond Raman probe pulses needed to maintain adequate Raman spectral resolution. Also, Raman spectra obtained using this method can be easily overwhelmed by fluorescence. Other picosecond time-resolved resonance Raman studies of this D-A molecule apply a Kerr gate to reject fluorescence but are still limited to a time resolution of a few picoseconds.15-17 The FSRS technique is employed here as a means to obtain subpicosecond time-resolved vibrational information on the CT state of DMJ-An without the complication of fluorescence overwhelming the signal. The theoretical and experimental facets of FSRS have been described in detail elsewhere,14,19,20 and a detailed description of the experimental setup employed for these specific studies is outlined in the Supporting Information section. Briefly, FSRS is a three-pulse technique based on the output of a 1 kHz Ti:sapphire laser system (Figure 3). In our case, 800 nm, 40 fs fundamental pulses are used to generate (1) a 400 nm actinic pump pulse by second harmonic generation with a 1 mm thick LBO crystal, (2) a narrow-band 800 nm Raman pump pulse by employing a spectral filter equipped with a grating, lens, and slit, and (3) a


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Table 1. Raman Frequencies of Anthracene Localized Vibrational Modes of DMJ-An and PA DMJ-An (gs) (cm-1)

DMJ-An (CT) (cm-1)

PA-•b (cm-1)

PA (S1)c (cm-1)

1*

1258

1264

1248

1246

1395

1391

1357

1364, 1374

1385

1560

1567

1546

1546

1498

a

1254

Reference 22. b Reference 23,24. c References 12 and 25.

Raman enhancement would be expected for the anthracene localized vibrational modes associated with the LE state and RP state components of the CT excited state since both 1*An and An-• absorb at the Raman pump wavelength. Julolidine localized vibrational bands are not expected to be enhanced since the Raman pump is far from resonance with the transient absorption attributed to the DMJ donor. The transient Raman frequencies of DMJ-An are compared to those of 1*PA and PA-• to determine the degree of charge separation within the CT state. The anthracene aromatic vibrational modes frequencies of 1*PA12,25 and PA-•23,24 are both downshifted relative to the PA ground-state values, but the amount of this shift differs significantly between the two. The Raman frequency comparison between DMJ-An and PA is summarized in Table 1. The striking similarity of the DMJ-An transient Raman frequencies to the Raman frequencies of PA-• strongly suggests that the CT excited state is fully charge-separated. This is particularly evident in the comparison of the highest-energy anthracene vibrational mode listed in Table 1, which exhibits a frequency of 1546 cm-1 for DMJ-An and 1546 cm-1 for PA-• but only 1498 cm-1 for 1* PA. Contributions from LE components to the CT state cannot be completely discounted as some of the weaker bands in the FSRS spectra of DMJ-An may be associated with the anthracene S1 state. For example, the less intense DMJ-An band at 1485 cm-1 may correlate to the 1498 cm-1 transient Raman band associated with the S1 state of PA. However, the strong resemblance of the more resonantly enhanced mode frequencies to those resulting from An-• strongly points to the RP state being an excellent description of the CT excited state. In this work, we used femtosecond stimulated Raman spectroscopy to characterize the CT excited state of DMJ-An. By comparison with the resonance Raman spectra of a PA reference molecule, the transient Raman peaks of DMJ-An were assigned to local vibrational modes of the anthracene radical anion. These results strongly point to the RP state as dominating the CT excited-state structure for this D-A molecule. Work is currently underway to use FSRS to study bridge dynamics of D-A molecules incorporating DMJ-An as an electron donor.

Figure 4. Spontaneous Raman spectrum (gs) and transient stimulated Raman spectra of DMJ-An at various time points after the actinic pump in THF. FSRS spectra obtained as RG = (I bkgd)pump-on/(I - bkgd)pump-off. Solvent peaks were subtracted from both gs and transient Raman spectra of DMJ-An.

The Raman peaks at 1258, 1395, and 1560 cm-1 are attributed to anthracene localized vibrational modes. These assignments are based on the reported Raman data of the closely related molecule, 9-phenylanthracene (PA), whose vibrational mode assignments have been well-documented.22 The Raman spectrum of PA shows anthracene localized vibrational modes at 1264, 1391, and 1567 cm-1. The relatively weak conjugation between the phenyl group and the fused ring system of the anthracene unit of PA in the ground state leads to vibrational modes that are localized on either the phenyl or anthracene unit.22 DMJ-An is also expected to have donorand acceptor-localized vibrational modes in the ground state especially due to the methyl groups that enforce a near-90° orientation between the two π systems. For this reason, the remainder of the peaks in the DMJ-An Raman spectrum, namely, those at 1350 and 1413 cm-1, are attributed to vibrational modes localized on the julolidine unit. Upon populating the CT excited state of DMJ-An in THF with the 400 nm actinic pump, the stimulated Raman gain signal induced by the 800 nm Raman pump and broad-band probe is subsequently measured at various time delays. The stimulated Raman spectrum of THF is subtracted at each time point to remove the solvent peaks. The resulting transient Raman spectra of DMJ-An are shown in Figure 4. As expected, the spectrum before t=0 ps strongly resembles the ground-state spectrum with the most intense feature around 1400 cm-1. After significant broadening of the vibrational bands at around t = 0, three prominent stimulated Raman peaks emerge by 500 fs at 1248, 1357, and 1546 cm-1. These Raman peaks are attributed to the same anthracene localized vibrational modes observed in the ground-state spectrum but with downshifted frequencies. The increased intensity of these bands is attributed to resonance (or preresonance) Raman enhancement due to the close proximity in energy of the 800 nm Raman pump to the anthracene transient absorption band of photoexcited DMJ-An. Resonance


PA (gs)a (cm-1)

SUPPORTING INFORMATION AVAILABLE Experimental procedure. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. E-mail: [email protected].

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ACKNOWLEDGMENT This research was supported by the

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Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, DOE under Grant No. DE-FG0299ER14999. We would like to thank Prof. David McCamant for many helpful discussions concerning FSRS and assembling our first apparatus and Dr. Jon Dieringer and Prof. Richard Van Duyne for their assistance in obtaining the steady-state Raman data. J.V.L. would like to acknowledge the ACS-PRF-AEF for funding.

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Interrogating the Intramolecular Charge-Transfer State of a Julolidine

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