Letter pubs.acs.org/JPCL
Cite This: J. Phys. Chem. Lett. 2018, 9, 6289−6295
Mapping Vibronic Couplings in a Solar Cell Dye with PolarizationSelective Two-Dimensional Electronic−Vibrational Spectroscopy James D. Gaynor, Alessio Petrone, Xiaosong Li, and Munira Khalil* Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195, United States
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S Supporting Information *
ABSTRACT: This study uses polarization-selective two-dimensional electronic−vibrational (2D EV) spectroscopy to map intramolecular charge transfer in the well-known solar cell dye, [Ru(dcbpy)2(NCS)2]4− (N34−), dissolved in water. A static snapshot of the vibronic couplings present in aqueous N34− is reported. At least three different initially excited singlet metal-to-ligand charge-transfer (MLCT) states are observed to be coupled to vibrational modes probed in the lowest energy triplet MLCT state, emphasizing the role of vibronic coupling in intersystem crossing. Angles between electronic and vibrational transition dipole moments are extracted from spectrally isolated 2D EV peaks and compared with calculations to develop a microscopic description for how vibrations participate with 1MLCT states in charge transfer and intersystem crossing. These results suggest that 1MLCT states with significant electron density in the electron-donating plane formed by the Ru-(NCS)2 will participate strongly in charge transfer through these vibronically coupled degrees of freedom.
T
dictate ultrafast photoexcited CT dynamics in N34−? We investigate this question using polarization-selective 2D EV spectroscopy. The 2D EV spectrum measures IR-active vibrational changes due to electronic excitation as a frequency−frequency correlation plot of the electronic excitation frequencies (ω1) and vibrational detection frequencies (ω3).16,17,37,38 The 1D analog, transient-IR (tIR) spectroscopy, lacks the excitation spectral resolution afforded by 2D EV, but it provides a starting point for our investigation by identifying structural differences between the ground state and the excited 3MLCT states probed. In the following, we present experimental evidence of vibronic couplings in N34− between several excited 1MLCT states and vibrational modes observed in the lowest energy 3 MLCT state. Distinct ω1-dependent features of 3MLCT vibrational modes in the polarization-selective 2D EV spectra are associated with different initially excited 1MLCT states based on the agreement of measured and ab initio calculated angles between the electronic and vibrational transition dipole moments probed during the experiment. The electron density reorganization upon exciting these associated 1MLCT states lends insight into the microscopic nature of these vibronic couplings. Calculations of N3 and related compounds discuss the complexity of CT in dense excited 1MLCT and 3MLCT manifolds.11,15,39−41 The two lower energy electronic absorption bands in N34− at λ = 500 (20 000 cm−1) and 372 nm (26 880 cm−1) arise from 1MLCT excitations in which electron density with mixed Ru-(NCS)2 character is transferred to the
he metal-to-ligand charge-transfer (MLCT) excited states of transition-metal complexes are widely used for light harvesting and photocatalysis.1−4 Understanding the ensuing ultrafast relaxation dynamics of the initially excited singlet MLCT state to a triplet MLCT state via intersystem crossing (ISC) or to other singlet excited and ground MLCT states via internal conversion is crucial for developing better photosensitizers. Considerable research efforts are being directed at understanding the role of coupled vibrations, solvent dynamics, spin−orbit coupling, and the available excited density of states in determining the ultrafast photochemistry of transition-metal complexes.5−15 The newly developed 2D electronic−vibrational (2D EV) spectroscopy can provide a correlation map between electronic and vibrational degrees of freedom.16,17 Couplings between electronic and vibrational motion, or “vibronic couplings,” are required to observe the 2D EV signal.18 In this Letter, we use polarization-selective 2D EV spectroscopy to directly correlate specific 1MLCT states with vibrations in the 3MLCT state following the photoexcitation of one of the most efficient Ru-based solar cell dyes, [Ru(dcbpy)2(NCS)2]4−, [dcbpy = (4,4′-dicarboxy-2,2′-bipyridine)] or “N34−”. N3 belongs to a broader class of transition-metal compounds undergoing rapid and complex charge transfer (CT) dynamics, which can be dictated by structural rearrangement and solvent environment.6,19 The rich photoexcited dynamics reported for N3 in solution20−26 and on semiconductor substrates3,27−34 suggest that many states in the excited 1MLCT manifold participate in various CT and relaxation pathways available to N3. Evidence of nonergodic electron transfer and the involvement of nonthermalized vibronic states within the excited 1MLCT manifold has also been reported for N3.27,35,36 How might vibronic couplings © XXXX American Chemical Society
Received: September 6, 2018 Accepted: October 11, 2018 Published: October 11, 2018 6289
DOI: 10.1021/acs.jpclett.8b02752 J. Phys. Chem. Lett. 2018, 9, 6289−6295
Letter
The Journal of Physical Chemistry Letters π* orbital of the dcbpy ligand and the highest energy band at λ < 330 nm (30303 cm−1) is due to π → π* dcbpy intraligand transitions.39 The N34− electronic absorption bands are blueshifted with respect to the neutral N3 due to a higher energy LUMO for the deprotonated complex, leading to higher energy π−π* transitions.42,43 Many ground-to-nth excited state (S0 → Sn) transitions into 1MLCT states underlie these absorption bands (e.g., see the N34− absorption spectrum, Figure 1A).
dcbpy vibrational modes report on the charge-acceptor region, such as bipyridyl (bpy) ring modes (1400−1550 cm−1) and the carboxylate (COO) symmetric and asymmetric stretches at 1375 and 1596 cm−1, respectively.22 Density functional theory (DFT) calculations and time-dependent DFT (TDDFT) computed electronic excited state structures using the Gaussian electronic structure program44 confirm our spectral assignments. We note that this computational analysis, described in detail in SI Sections SI.IV and SI.V, is helpful for interpreting the experimental data, although neither explicit dynamical effects involving nonadiabatic processes nor matrix elements between vibronic wave functions are included here. The tIR spectra shown in Figure 2 reflect the CT from the Ru-(NCS)2 to the dcbpy ligands as expected. In the lowest
Figure 1. Linear absorption of N34− and laser pulse spectra. (A) Electronic absorption spectrum of N34− (black, solvent-subtracted) in basic, aqueous solution (pH 13) with the spectrally broadened UV pump spectrum (shaded blue) used in the tIR and 2D EV experiments overlaid to highlight the excitation region. The timedependent (TD)-B3LYP/def2-SVP calculated electronic absorption spectrum (red, a Gaussian broadening has been applied with an FWHM = 0.18 eV; intensities are normalized for comparison to the experiment; see the SI for further details) for the optimized N34− minimum geometry is reported along with S0 → Sn predicted oscillator strengths (red sticks). The transitions associated with the observed 1MLCT states are denoted by *; see the text for discussion. The computed spectrum is within ∼0.1 eV accuracy with respect to the experimental spectrum; N34− structure is inset. (B) Solventsubtracted FTIR spectra for both vibrational regions probed in the experiments; mid-IR probe spectra used in the experiments are overlaid (green).
Figure 2. Transient-IR (tIR) spectroscopy of N34−. (A) tIR spectra at τ2 = −1 (black), 2 (red), and 100 ps (yellow). The excited-state absorption (ESA) peaks of interest for the polarization-dependent 2D EV analysis are highlighted by green dashed arrows: ω3 = 1271 cm−1, bpy ring mode with CH wag (νBPY); ω3 = 1328 cm−1, COO symmetric stretch (νCOO); and ω3 = 2050 and 2070 cm−1, asymmetric and symmetric CN stretches, respectively (collectively νCN). The ground-state bleach (GSB) features are ω3 = 1375 cm−1, COO symmetric stretch; ω3 = 1405, 1435, and 1545 cm−1, bpy ring modes; ω3 = 1596 cm−1, COO asymmetric stretch; ω3 = 2120 cm−1, CN symmetric and asymmetric stretches. See the text for a discussion of tIR spectral shifts. (B) Calculated difference IR spectrum between the ground state, S0, and the lowest energy triplet state, T. Note that the calculated lowest energy triplet state is referred to as “T” and the experimental measurements refer to the lowest energy triplet state as “3MLCT”.
Our experiments excite the low-energy edge of the N34− electronic absorption band centered at 26 880 cm−1 with the second harmonic of a spectrally broadened Ti:sapphire fundamental output (Figure 1A, fwhm = 19 nm or 1210 cm−1; detailed description in SI Section SI.I) to populate as many 1MLCT states as possible. Vibrational modes reporting on both the Ru-(NCS)2 charge-donor segment and the dcbpy charge-acceptor segment on the N34− molecule are then probed with a mid-IR pulse to correlate as many 1MLCT states as possible with the excited state CT processes. The spectrally overlapped symmetric and asymmetric CN stretches of the NCS ligands compose the FTIR peak at ∼2120 cm−1 (Figure 1B) and report on the N34− charge-donor region. Several
energy 3MLCT, the CN stretches shift to lower frequency by 50−70 cm−1 to form the excited-state absorption (ESA) features collectively labeled νCN, which is consistent with bond weakening due to the removal of electron density from the charge-donor region. The arrival of electron density to the dcbpy ligands is reflected as the bpy ring modes all shift to higher frequencies by ∼45 cm−1, whereas the COO stretches lower in frequency by ∼50 cm−1 due to the increased aromaticity of the dcbpy ligands. The anharmonic coupling of the 3MLCT symmetric and asymmetric CN stretches is stronger than that in the ground state, which splits these ESA features, yet they are still not spectrally isolated in the vibrational domain, ω3. Whereas the COO asymmetric stretch 6290
DOI: 10.1021/acs.jpclett.8b02752 J. Phys. Chem. Lett. 2018, 9, 6289−6295
Letter
The Journal of Physical Chemistry Letters
Figure 3. Polarization-selective 2D EV spectra. (A) 2D EV spectra collected with pump and probe pulses with parallel (SZZZZ) and crossed (SZZYY) relative polarization; the color maps for spectra with the same ω3 region are comparable using the corresponding color bar. (B) Isotropic 2D EV spectrum (Siso = SZZZZ + 2SZZYY) with white boxes highlighting signals used in the dipole angle analysis described in the text (νCN = 1; νCOO = 2, 3; and νBPY = 4). Box 1 corresponds to 1MLCTC, box 2 corresponds to 1MLCTA, and boxes 3 and 4 correspond to 1MLCTB; see the text for a full discussion of these 1MLCT states. Excited state absorptions are negative (dashed contours) and ground-state bleaches are positive (solid contours) in all spectra; note the difference between color maps for the lower and upper panels. The color gradient changes every 5% of the max absolute value signal (of both SZZZZ and SZZYY in panel A) within the ω3 region indicated; the contour lines begin at ±10% for clarity and are plotted at 5% intervals thereafter.
(1596 cm−1) and dcbpy ring mode (1545 cm−1) features overlap in the 3MLCT, the COO symmetric stretch red-shifts into a spectrally isolated region (ESA, 1328 cm−1) and is labeled νCOO. The calculations show that νCOO consists of one dcbpy-localized COO symmetric stretching mode. An additional, spectrally isolated bpy ring mode (ESA, 1271 cm−1) localized to one dcbpy ligand blue-shifts into the probe window and is labeled νBPY (see Figure SI.7). The νCOO and νBPY modes are spectrally isolated, direct reporters on the N34− charge-accepting region and will be the focus of the polarization-selective 2D EV analysis below. The tIR dynamics of N34− reported here are consistent with reports of sub-100 fs ISC and the formation of the vibrationally excited 3MLCT manifold that relaxes over picoseconds.3 The picosecond (ps) relaxation dynamics are debated in the literature.3,45 Typically, three decay components are reported in the 1 to 2 ps, 7−10 ps, and >50 ps ranges for the N3−TiO2 system. Interligand electron transfer has been reported to have a 20 ps time scale in solvated N3.28 The tIR spectra (Figure 2A) at 2 and 100 ps delays demonstrate that most molecules probed are at, or near, the 3MLCT minimum by 2 ps (see SI Section SI.III for fitted tIR time traces). The calculated difference spectrum (S0−T, Figure 2B) provides strong qualitative agreement with the measured tIR spectra and supports our vibrational assignments in S0 and 3MLCT. We note that explicit treatment of hydrogen-bonded water molecules solvating the carboxylates can partially account for the discrepancy between calculation and measurement (see Figures SI.3 and SI.4).
The tIR data highlight the difference in N34− IR-active vibrations between S0 and the 3MLCT state, but the information regarding which S0 → Sn excitation is correlated with the observed changes in the vibrational structure is unresolved. The 2D EV experiment directly accesses this information. The vibrational modes assigned in the tIR experiment will be observed in the ω3 dimension of 2D EV spectra along with information in the ω1 dimension regarding the correlations of the assigned vibrations to the S0 → Sn excitations. Additional insight is obtained by collecting the polarization-selective 2D EV response.37,38 By measuring the 2D EV signal with the polarization of the pump and probe pulses parallel (SZZZZ) and perpendicular (SZZYY) to one another, the anisotropy parameter (r) can be measured. For a spectrally isolated signal, r is related directly to the angle θ between the transition dipole moments for the pumped and 1 probed transitions through r = 5 (3 cos2(θ ) − 1).46 We S measure the relative angle (θvibn ) between the initially excited S0 → Sn electronic transition dipole moments (μSSn0) and vibrational transition dipole moments (μvib) of spectrally isolated dcbpy vibrations in the 3MLCT using polarizationselective 2D EV spectroscopy. These measurements provide a static snapshot of the vibronic couplings present in N34− at a pump−probe delay time (τ2) of 2 ps, which is before rotational reorientation is expected to affect the anisotropy measurement in N3 and similar Ru compounds28,47 and after the vibrationally excited 3MLCT has substantially relaxed.3,45 S Agreement between calculated and measured θvibn allows for 6291
DOI: 10.1021/acs.jpclett.8b02752 J. Phys. Chem. Lett. 2018, 9, 6289−6295
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The Journal of Physical Chemistry Letters associating 2D EV peaks with different ω1 frequencies to different S0 → Sn excitations in N34−. Figure 3 shows polarization-selective 2D EV spectra. Clear ω1-dependent ESA signals for νCOO are measured, and two peaks are observed in the SZZZZ (ω1 = 24 480 cm−1, 25 110 cm−1), SZZYY (ω1 = 24 600 cm−1, 25 010 cm−1), and Siso (ω1 = 24 550 cm−1, 25 060 cm−1) spectra. The ω1 profile of the νBPY ESA is polarization-dependent with distinct features in the SZZZZ (ω1 = 24 410 cm−1, 25110 cm−1), SZZYY (ω1 = 24 940 cm−1), and Siso(ω1 = 25 000 cm−1) spectra, where the dominant ω1 peak is at ∼25 000 cm−1 with a weaker shoulder toward the lower ω1-frequency peak observed predominantly in SZZZZ. We note that the 50 cm−1 ω1 resolution and a slightly narrowed pump spectrum between the different experiments accounts for the small variation in SZZZZ and SZZYY peak positions. The ω1 line shapes for the νCOO and νBPY features suggest that the charge-acceptor region of the N34− is coupled to at least two excited 1MLCT states; we will refer to these two states as “1MLCTA” (excited in the range ω1≅ 24 400−24 700 cm−1) and “1MLCTB” (excited in the range ω1≅ 25 000− 25 300 cm−1) in the following discussion. The experimental spectra show that νCOO(νBPY) is strongly (weakly) coupled to 1 MLCTA and that νCOO and νBPY are both strongly coupled to 1 MLCTB. The Siso excitation frequency of the νCN ESA is centered at ω1= 24 870 cm−1 and reflects another 1MLCT state vibronically coupled to the N34− charge-donor region, which will be referred to as “1MLCTC”. The broad, mildly polarization-dependent electronic excitation observed for νCN is consistent with CT from the NCS ligands being delocalized over many excited 1MLCT states, likely including 1MLCTA and 1MLCTB. By contrast, the excitation profile of νBPY and νCOO suggests that there are at least two distinct 1MLCT states or two well-defined trajectories through the excited state manifolds participating in ISC. The correlation between three distinct ω1 bands and the vibrational signatures measured in the 3MLCT state at τ2= 2 ps indicates that at least three 1 MLCT states are directly coupled to the lowest energy 3 MLCT state through different N34− vibrations. In contrast with vibronic coupling information obtained from resonance Raman excitation profile analyses where the vibrations coupled to the S0 → Sn excitation are enhanced, the 2D EV spectra in Figure 3 highlight vibronic couplings between 3MLCT state vibrations and initially excited 1MLCT states. Comparison between experimental and calculated values of θSvibn are used to associate Sn states likely involved in the S0 → Sn transitions with the features corresponding to 1MLCTA, 1 MLCTB, and 1MLCTC in the 2D EV spectra (see Table 1 and comparison details in SI Section SI.IV). The calculated vectors for the S0 → Sn electronic transition dipole moments and the derivative of the calculated lowest energy triplet (T) electric dipole moment with respect to the assigned normal modes are used to obtain the calculated dipole angles discussed here. The angles measured for the νCOO and νBPY modes will be relatively unconvoluted because they are spectrally separated in both the electronic and vibrational frequency dimensions, which should provide accurate experimental measurement of θSvibn . The difference between the highest and lowest excitation energies for the associated S0 → Sn transitions should be ∼450−650 cm−1, as observed in the ω1 peak separation for excitations into 1MLCTA and 1MLCTB. The corresponding transition energies for Sn states associated with the coupled 1 MLCT states should also scale as 1MLCTA < 1MLCTC
