Probing Effect of Solvation on Photoexcited Quadrupolar Donor-Acceptor-Donor Molecule via Ultrafast Raman Spectroscopy

Ⅰ.   INTRODUCTION

During past decades, the growing interest in the synthesis of symmetric multipolar D-($\pi$-A)$_n$ or A-($\pi$-D)$_n$ molecules, where the A and D are electron-acceptor and electron-donor moieties, is mainly related to their potential applications in optics and optoelectronics, phototherapy, analytical and environmental sciences, photopolymerization, and micro-fabrication, etc. [1-8]. Because of the centrosymmetric structures, the multipolar molecules generally exhibit a nonpolar ground state, where the charge distributes evenly on multi-branches of the molecule [9-13]. After photoexcitation, however, a pronounced positive solvatochromic phenomenon usually occurs, similar to what occurs in the one-branch D-$\pi$-A analogues [14-16]. This striking spectroscopic feature has been explained in terms of the charge redistribution process from delocalized intramolecular charge transfer (ICT) to localized ICT in excited states [17-21].

To investigate such a striking phenomenon, considerable efforts have been devoted to the in-depth study of photophysical characteristics to try to understand the scientific basis for this observed behavior. Theoretical and experimental studies suggested that charge redistribution is primarily induced by antisymmetric vibrations and subsequent solvent and/or structural fluctuations, leading to further stabilization of the localized ICT state [22, 23]. Flash-photolysis time-resolved microwave conductivity measurements indicated that excited-state charge redistribution could result from solvent density and molecular structural fluctuation [24]. Time-resolved fluorescence anisotropy and femtosecond transient absorption (fs-TA) measurements confirmed that the initially delocalized photoexcitation may finally localize on one of the molecular branches due to solvation, leading to a polar emissive state [25-28]. By tracking the instantaneous S$_{1}$$\rightarrow$S$_{0}$ transition dipole moment ($\mu_{ \rm{em}}$) using femtosecond broadband fluorescence up-conversion spectroscopy, the direct evidence of charge redistribution could be obtained [29, 30]. Furthermore, by tracking specific spectral changes from unique vibrational modes (such as alkyne triple bond -C$\equiv$C- or cyano group -C$\equiv$N) of highly symmetric molecules using femtosecond IR spectroscopy (fs-IR)/femtosecond stimulated Raman scattering (FSRS) spectroscopy, the direct visual evidence of excited-state charge redistribution with suitable spectral resolution and femtosecond temporal resolution was observed [15, 16, 19, 21, 31]. In these studies, the specific vibrational markers of the analyzed quadrupolar molecules are distributed symmetrically on both sides of the central functional groups, while almost nothing is known if the characteristic functional groups are adjacent to each other at the central position of quadrupolar molecules. Here, questions to be raised are the following: (i) How do the vibrational spectra change during charge redistribution process? (ii) Is the excited-state dynamics sensitive to the polarity of solvents in a manner like the quadrupolar molecules with vibrational markers symmetrically distributed on both branches?

In this work, we addressed these questions from a detailed investigation of 4, 4'-(buta-1, 3-diyne-1, 4-diyl) bis($N$, $N$-bis(4-methoxyphenyl)aniline)(MeO-DATPA [32], the molecular structure is shown in Scheme 1) in different polar solvents by employing FSRS spectroscopy combined with quantum chemical calculations. The molecule comprises two central adjacent alkyne (-C$\equiv$C-) groups as electron-acceptors flanked by two separated, symmetric branches at each side that consist of $N$, $N$-bis(4-methoxyphenyl)aniline electron-donors linked via carbon-carbon single bond. The Raman modes of the central alkyne (-C$\equiv$C-) groups, which are well separated from those of other groups, can be probed specifically to track the temporal excited-state evolution [16, 19-21, 33]. As a result, access to the excited-state charge redistribution from delocalized ICT state to localized ICT state has been obtained by monitoring the excited state Raman spectra of the -C$\equiv$C- stretch region upon photoexcitation. It is found that in apolar cyclohexane (CHX), the excited-state Raman spectra remain constant during the probing time, suggesting an equal electronic density that is distributed on the two -C$\equiv$C- groups. However, a clear Raman peak shift is observed when it comes to medium polar tetrahydrofuran (THF), which means an uneven distribution of the excitation results from solvent disorder fluctuation. The strong polar dimethylformamide (DMF) accelerates this excited-state charge redistribution process because of the stronger solvation process relative to that in THF.

Scheme  Scheme 1.  The molecular structure of MeO-DATPA (dihedral angle refers to $\angle$1-2-3-4).

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Ⅱ.   MATERIALS AND METHODS

A   Materials

The synthesis and structural characterization of MeO-DATPA were described elsewhere [32]. Cyclohexane (CHX), dibutyl ether (C$_{8}$H$_{18}$O), tetrahydrofuran (THF), diethyl ether (C$_{4}$H$_{10}$O), butyl acetate (BuAcO), ethyl acetate (EtAcO), N, N-dimethylformamide (DMF), and acetone (ACE) were purchased from Beijing Chemical Work (analytic grade) and were used without further purification. 9, 10-Diphenylanthracene (9, 10-DPA) was purchased from Sigma-Aldrich.

B   Methods

The steady-state absorption and fluorescence spectra were collected by U-3010 and a F-4600 spectrometers (Hitachi, Japan). The fluorescence quantum yields were determined by comparative method, and the reference is the solution of 9, 10-DPA in CHX ($\phi_f$ = 0.955, excited at 366 nm) [34].

The fs-TA spectra were recorded by a home-built ultrafast pump-probe set-up with a time resolution about $\sim$100 fs, which was introduced in previous work [30]. The FSRS measurements are introduced in S1 part in Supplementary materials (SM). The concentration used for transient spectroscopic measurements is $\sim$1.5 × 10$^{-4}$ mol/L. There is no photodegradation phenomenon after fs-TA and FSRS experiments checked by the steady-state absorption spectra.

The quantum chemical calculations were conducted on Gaussian 09E.01 package by applying density functional theory (DFT) and time-dependent DFT (TD-DFT) [35]. The optimization of geometrical conformations was performed at the B3LYP functional with basis set 6-31G level [36]. The natural transition orbitals (NTO) analysis was performed by Multiwfn program based on the output results from Gaussian package [37].

Ⅲ.   RESULTS AND DISCUSSION

The steady-state absorption and fluorescence spectra of MeO-DATPA in three different polar solvents are shown in FIG. 1, and the solvent polarity-independent absorption spectra are dominated by the band around 370 nm, indicating symmetric and quadrupolar ground and Frank-Condon (FC) states [16, 21]. The fluorescence spectra show a significant solvatochromic spectroscopic feature with the fluorescence maximum shifting by $\sim$3381 cm$^{-1}$ upon going from CHX to DMF, which is typically indicative of an emissive state (relaxed S$_{1}$ state) with dipolar character in polar solvents [15-17, 28, 30, 38]. Besides, the fluorescence quantum yield of MeO-DATPA is extremely low in highly-polar DMF ($\sim$0.15, see Table S1 in SM). These results imply the occurrence of localized ICT state upon optical population of the S$_1$ state of MeO-DATPA in polar solvents [15, 16, 30, 31, 39].

Figure  1.  The normalized steady-state absorption and fluorescence spectra of MeO-DATPA in different polar solvents

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The fs-TA spectra and time-resolved fluorescence up-conversion spectra of MeO-DATPA (see FIG. S4 in SM) in different polar solvents collected from home-built set-ups by using 400 nm photoexcitation were described previously [30]. Despite useful information on the solvent polarity-dependent excited-state dynamics has been obtained by femtosecond transient absorption and femtosecond fluorescence up-conversion spectroscopy in our earlier study [30], here in this study, we now further carry out the FSRS to probe the solvent polarity tuned excited-state dynamics. By judging from the fs-TA spectra [30], a narrow bandwidth ($\sim$10 cm$^{-1}$) picosecond ($\sim$4 ps) laser at 600 nm is selected as Raman pump laser against resonance with the steady-state absorption of MeO-DATPA, also to avoid the dumping effect of excited states because there is less stimulated emission around 600 nm (see FIG. S4 in SM). This resonance-enhancement Raman spectra of the excited states ensure that the specific -C$\equiv$C- mode associated with the excited-state charge redistribution process was probed.

FIG. 2 displays the FSRS spectra of MeO-DATPA in different polar solvents, and the ground-state stimulated Raman spectra are presented in FIG. S1 in SM for comparison. In apolar CHX (FIG. 2(a)), the FSRS spectra exhibit one single and intense peak centered at $\sim$2042 cm$^{-1}$. As the time delay continues, the spectral profiles remain constant without significant band broadening or shifting. Furthermore, the last peak is an almost symmetric profile that strongly resembles the initial spectrum, suggesting the electronic density on both branches is almost the same (delocalized ICT state). In medium polar THF, the initial spectrum that consists of one band at $\sim$2041 cm$^{-1}$ undergoes significant peak upshift to $\sim$2066 cm$^{-1}$, simultaneously accompanied by the broader spectral profile change (relative to that in CHX) from symmetric to non-symmetric, indicating solvation induced charge redistribution process from delocalized ICT to localized ICT. The spectral evolution in DMF resembles that in THF, but the initial spectrum is much broader than those in CHX and THF, which is a direct evidence of strong frequency fluctuation in the polar environment induced by solute-solvent interaction [40]. To show the spectral profile change intuitively, the normalized initial and final spectra in different polar solvents are available in FIG. S6 in SM.

Figure  2.  The FSRS of MeO-DATPA in (a) CHX, (c) THF and (e) DMF at various delay time, and the evolution associated difference spectra (EADS) obtained from FSRS in (b) CHX, (d) THF and (f) DMF after global analysis.

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Generally, the Raman vibrational frequency can be roughly estimated theoretically by the Hooke's law [41, 42]:

where c is the velocity of light, K is force constant of the bond between atoms A and B, which is a measurement of bond strength, ${\mu}$ is the reduced mass of atoms A and B, $M_{ \rm{A}}$ and $M_{ \rm{B}}$ are the masses of atoms A and B, respectively. By employing Hooke's law, it is found that the vibrational frequency depends on the masses of the atoms, and the strength of the chemical bonds [41, 42]. The geometric arrangement, of course, exhibits the significant influence on the vibrational frequency [42]. As for MeO-DATPA, upon excitation, the excited-state planarization process from FC state (whose molecular conformation is the same as that of ground state) to relaxed S$_1$ state (FIG. S9 in SM) leads to the electronic density increase on the central alkyne bonds, thus the bond constant increases [43, 44]; the formation of localized ICT will further lead to the increase in electronic density of one of the alkyne bonds, thereby enhancing the force constant, leading to the blue-shift of -C$\equiv$C- stretch mode.} Besides, another contribution to the force constant could also be considered based on Coulomb's law [45]:

where $q$$_{ \rm{A}}$ and $q$$_{ \rm{B}}$ are the charges of atoms A and B in atomic charge units, and r is the distance. If the charges possess the same signs, then a positive contribution to the force constant is expected. For MeO-DATPA, the alkyne groups are not only the vibrational marker modes, but also the electron-acceptor. Upon solvation process in polar solvents, the localized electronic density of one of the two alkyne groups leads to a larger force constant. Therefore, the blue-shift of alkyne Raman mode in polar solvents can be explained in terms of excited-state charge redistribution process from delocalized ICT to localized ICT induced by fluctuation of solvent and/or structural fluctuation of MeO-DATPA itself.

However, the Raman spectral evolution in this work is significantly different from that in our previous work [21], in which a new peak is generated in polar solvents due to solvation-induced excited-state charge redistribution. The molecule in our previous work was an A-$\pi$-D-$\pi$-A type molecule with the two $\pi$-bridges (alkyne functional groups) separated by the electron-donor unit. In that case, the clear decay and rise of bands attributed to state-state conversion from delocalized ICT state to localized ICT state were observed. In this work, however, the two typical Raman markers (-C$\equiv$C-) are directly linked by a carbon-carbon single bond, and the two alkyne functional groups almost share the same electronic excitation at very initial time, the Raman spectra exhibit continuous band shifts during excited state charge redistribution. By using global analysis on FSRS data [46-49], the spectral signatures and lifetimes of each component were obtained, including the vibrational relaxation and charge redistribution process. FIG. 2 displays the evolution associated difference spectra (EADS) estimated by global analysis, and the component evolution profiles are shown in FIG. S8 in SM. The first EADS in FIG. 2(b) corresponds to FC state in CHX, which is a charge transfer state whereby the electron density is evenly distributed on both branches (delocalized ICT). After 22 ps vibrational relaxation process, the relaxed delocalized ICT state is formed, and no significant band shifts and broadens is observed, suggesting that the symmetric delocalized ICT state is maintained during the excited-state deactivation process. In medium polar THF, the first EADS resembles that in CHX but much broader because of solvent fluctuation effect. The second EADS exhibits a significant upshift due to solvation process from FC state (delocalized ICT) to localized ICT state with a time constant of 2.7 ps, after which the localized ICT state decays to the ground state. In highly-polar DMF, a strong solvation effect further accelerates spectral evolution with a time constant of 2.2 ps; it is observed that the non-symmetric characteristics of the spectral profile are more obvious than that in THF.

To show the solvation dependent spectral evolution dynamics as a comparison, the kinetics of Raman peak-shift of MeO-DATPA in different polar solvents are also shown in FIG. 3. The analysis of peak-shift kinetics provides additional information of structural dynamics, corresponding to the conformational change in excited state, the electronic density redistribution and vibrational energy relaxation [40, 44, 50]. In the case of MeO-DATPA in THF (FIG. 3(a)), the spectral evolution of the Raman band exhibits an upshift with a time constant of 2.6 ps, which is similar to that obtained from fs-TA and femtosecond broadband fluorescence up-conversion spectroscopy [30]. However, the time constant in DMF is $\sim$2.1 ps, which is faster than that in THF, indicating a faster solvation process in DMF. The kinetics of the vibrational mode of MeO-DATPA reveal a distinct behavior in THF and DMF. On ultrafast timescales, the electronic redistribution process from FC state to relaxed S$_1$ state will be strongly affected by solvent fluctuation. The time constants obtained herein are slightly larger than the reported solvation time constants [51], suggesting the excited-state conformational relaxation could also be included in the excited-state charge redistribution process [30].

Figure  3.  Kinetics of Raman peak-shift of MeO-DATPA in (a) THF and (b) DMF

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To illustrate the effects of conformational change on charge redistribution in excited states, theoretical calculations were performed. The optimized geometric structures of MeO-DATPA in ground and excited states are shown in FIG. S9 in SM. To simulate the conformational change effect, the dihedral angle between two phenyls of triphenylamine functional groups (see Scheme 1) are varied and the potential energy surfaces in ground and excited states against the dihedral angle are depicted in FIG. 4 [21, 31, 50, 52]. This predicts small dihedral angles of MeO-DATPA in both ground and excited states, and the energy barrier of torsion in S$_0$ state is significantly lower than that in S$_1$ state. Actually, the dihedral angle in ground state can rotate freely by ± 90$^{\circ}$, while the potential energy surface is much steeper in the exited state, where the torsional angle is confined within $\sim$ ± 20$^{\circ}$. Thus, it is confirmed that there is a planarization process from ground state to excited state. The natural transition orbitals (NTO) analysis of MeO-DATPA at different dihedral angles is also provided in FIG. S10 in SM, which clearly shows that a delocalized ICT state is observed as the dihedral angle increases. Based on the analysis above, the mechanism of solvent polarity modulated excited-state charge transfer redistribution is confirmed and shown in FIG. 5. In the apolar CHX, the solvent field perceived by the two branches of MeO-DATPA is equally weak, then the symmetric electric state is preserved because the photoexcitation delocalized distribute on the molecule, which is evidenced by the FSRS spectra in CHX that shows no obvious Raman peak shifts and broadens during the probing time. Due to the solvation process in medium polar THF, the solvent field around the two branches may differ significantly [16], and one of the branches is better solvated than the other. Therefore, the FC state will no longer remain in equilibrium, resulting in the change of surrounding solvent molecules' orientation to minimize the overall free energy to ensure that the solvent shell is in thermodynamic equilibrium with the solute [53, 54]. Consequently, a localized ICT state is energetically favored. As solvent polarity further increases, a stronger solvation process accelerates the conversion from delocalized ICT state to localized ICT state, acting as faster stabilization of localized ICT state (see FIG. 3).

Figure  4.  Computed energy of MeO-DATPA in ground and excited states as a function of the various dihedral angles ($k_ \rm{B}$ is Boltzmann constant, $T$ is room temperature (295.15 K))

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Figure  5.  The proposed mechanism of solvent polarity-dependent excited-state charge redistribution process. FL means fluorescence, GS means ground state, Non. means nonradiative decay

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Ⅳ.   CONCLUSION

In summary, this work successfully demonstrates the solvent polarity modulated excited-state charge redistribution process of MeO-DATPA upon photoexcitation. Steady-state spectroscopy reveals that the FC state is delocalized with quadrupolar electronic character, while the electronic structure of lowest excited state converts from delocalized quadrupolar to localized dipolar as solvent polarity increases. By applying FSRS spectroscopy, which is used to track the central two vibrational modes of carbonyl functional groups, the solvent polarity-dependent excited state dynamics is clearly observed. It is found that MeO-DATPA retains delocalized ICT state in apolar CHX, where the Raman spectra remain constant during probing time. In medium polar THF, a localized ICT state is formed from delocalized ICT state via solvation process. And this conversion process is obviously accelerated in polar DMF, suggesting that a polar environment is required for charge redistribution process. By applying DFT calculations, the structural fluctuation effect is also confirmed, which shows a significant role in the charge redistribution process.

Supplementary materials: The FSRS measurements, photophysical data, Lippert-Mataga equation, ultrafast spectroscopy, and theoretical calculations are provided.

Ⅴ.   ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (No.22133001, No.11774233, No.21773252, No.21773257, No.21827803), and the Project for high-grade, precision and advance in Beijing (BUPT). We sincerely thank Miquel Planells and Neil Robertson (University of Edinburgh) for provision of the MeO-DATPA molecule used in the study.