Correlated Molecular Structural Motions for Photoprotection after Deep

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Spectroscopy and Photochemistry; General Theory

Correlated Molecular Structural Motions for Photoprotection After Deep-UV Irradiation Longteng Tang, Yanli Wang, Liangdong Zhu, Che Lee, and Chong Fang J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b00999 • Publication Date (Web): 19 Apr 2018 Downloaded from http://pubs.acs.org on April 20, 2018

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The Journal of Physical Chemistry Letters

Manuscript for J. Phys. Chem. Lett. (2018)

Correlated Molecular Structural Motions for Photoprotection after Deep-UV Irradiation

Longteng Tang,† Yanli Wang,† Liangdong Zhu,† Che Lee,‡ and Chong Fang* Department of Chemistry, Oregon State University, Corvallis, Oregon 97331, United States

†

These authors contributed equally to this work.

‡

Current address: Department of Chemical Engineering and Materials Science, University

of Minnesota, Minneapolis, Minnesota 55455, United States

To whom correspondence may be addressed. *E-mail: [email protected]. Phone: 541-737-6704.

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ACS Paragon Plus Environment

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ABSTRACT. Exposure to ultraviolet (UV) light could cause photodamage to biomolecular systems and degrade optoelectronic devices. To mitigate such detrimental effects from the bottom up, we strategically select a photosensitive molecule pyranine and implement femtosecond electronic and Raman spectroscopies to elucidate its ultrafast photoprotection mechanisms in solution. Our results show that pyranine undergoes excited state proton transfer (ESPT) in water while this process is blocked in methanol regardless of excitation wavelengths (267, 400 nm). After 267 nm irradiation, the molecule relaxes from a higher lying electronic state into a lower lying singlet state with a 130 ps) is expected when significant excess energy is available from deep-UV excitation. Regarding a high-frequency marker band, we showed that HPTS in water with 400 nm excitation displays a blueshift (1527 to 1559 cm-1) of the ~1530 cm-1 mode (ring C=C stretching and phenolic COH rocking) as ESPT proceeds. However, the biexponential blueshift time constants are ~2.5 ps (34%) and 45 ps (66%), and the latter one is twice as fast as the main ESPT time constant of ~90 ps.23 Upon changing the pump wavelength to 267 nm in water, the blueshift trend (1500 to 1559 cm-1, see Figures 3a and 5c) holds but with a significantly increased magnitude. The excellent match of mode frequency (1559 cm-1) at 600 ps time delay after both excitation wavelengths confirms that the chromophore reaches the same PB* state. However, the retrieved time constants of ~1.3 ps (53%) and 25 ps (47%) are still faster than main ESPT reaction, suggesting that this mode frequency dynamics not only track ESPT but also reflect vibrational cooling which leads to the mode frequency blueshift in an anharmonic PES.25,45-47 As a control, the observed mode frequency dynamics in methanol corroborates our speculation. After 400 nm excitation,

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this mode shows a slight blueshift (1528 to 1530 cm-1, 2 cm-1), whereas after 267 nm excitation, the blueshift increases dramatically (1506 to 1525 cm-1, 19 cm-1). This makes sense because vibrational cooling should be more significant after the deep-UV (267 nm) excitation which has much more energy than the near-UV (400 nm) light. Along the same line, the observed blueshift time constants of the aforementioned 1530 cm-1 mode in water are shorter after deep-UV excitation than those after near-UV excitation (i.e., 1.3 ps < 2.5 ps, 25 ps < 45 ps), which confirm the increased “additive” contribution from vibrational cooling on various timescales48,49 (faster than the main ESPT time constant of ~90 ps) when more excess electronic energy is available. Notably, other high-frequency Raman bands at 1137 and 1204 cm-1 in methanol also exhibit a blueshift (Figure 3b), suggesting that a number of localized atomic motions could facilitate the energy relaxation process through vibrational cooling with at least two channels on the few ps and tens of ps timescales. They are likely associated with intramolecular vibrational relaxation/energy relaxation within the first solvation shell and thermal energy transfer to the bulk solvent, respectively.23,25,49 Such a wealth of spectral information allows us to gain deep insights into how the chromophore responds to photoexcitation with significant excess energy. In particular for HPTS in water, after the initial sub-ps relaxation out of the FC region toward a lower lying electronic state, the water molecular reorientation time within the first solvation shell becomes dominant to stabilize the chromophore after deep-UV excitation. This important process is revealed by the 1.2 ps time constant from the PA* 430 cm-1 mode intensity decay (Figure 3c), and substantiated by the 1.3 ps time constant from the 1530 cm-1 mode frequency blueshift (Figure 5c). The distinct solvation step with energy stabilization effect is in accord with the lower lying S1’ with respect to S1 (Figure 4), which also supports

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ultrafast vibrational cooling of the chromophore within the first solvation shell. In sharp contrast, after 400 nm excitation without a notable amount of excess energy, HPTS displays a time constant of 3 ps from the PA* 420 cm-1 mode intensity decay, corroborated by the time constant of 2.5 ps from the 1530 cm-1 mode frequency blueshift (Figure 5c).23 This specific process has been attributed to the formation of a contact-ion pair between HPTS and adjacent water molecules with charge transfer character, which is a preparatory step beyond the ~1 ps water reorientation time to facilitate efficient ESPT reaction from the PA* S1 state to the PB* fluorescent state (Figure 4). The overall importance of water motions along the energy dissipation pathways is consistent with a recent microsolvation study of a sunscreen chromophore, which forms a complex with water molecules after UVB excitation in promoting internal conversion back to the electronic ground state.7 As further kinetic analysis, after 400 nm excitation the same ~1520 cm-1 marker band of HPTS in methanol (Figure 5c) shows a single exponential time constant of ~13 ps, but bi-exponential time constants of ~370 fs (45%) and 34 ps (55%) after 267 nm excitation. Because ESPT is inhibited for HPTS in methanol, these time constants are associated with various energy relaxation channels including the FC dynamics within the HPTS-methanol H-bonding complex, methanol reorientation/solvation time, and thermal energy transfer to bulk solvent, on the order of increasing time constants.13,18,25 Moreover, the high-frequency modes likely contribute more to intramolecular and intermolecular vibrational cooling following photoexcitation, whereas the more delocalized low-frequency skeletal motions (both out-of-plane and in-plane) become increasingly susceptible to conformational dynamics as the vibrational mode frequency decreases further (e.g, Figure 5b to a). The site-specific addition or modification of functional groups (e.g., electron-withdrawing or

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donating, hydrophilic) on or near the chromophore rings could then be an effective strategy in controlling photoacidity, ESPT, fluorescence, and other energy relaxation pathways.33,37 In particular, the fluorescence QY decreases after UVC excitation due to an enhancement of chromophore conformational dynamics and vibrational cooling (see Figure 5) as these atomic motions and processes likely facilitate nonradiative decay in solution environment. In summary, this advanced spectroscopic work provides a holistic view of the energy relaxation pathways of an organic molecule after UV irradiation and uncovers its intrinsic photoprotection mechanism starting from photoexcitation time zero. Depending on its local environment and incident photon energy, the molecule could select or enhance a certain pathway for energy dissipation. From a comprehensive investigation of transient Raman marker bands upon electronic excitation, we reveal that the photochemical reaction (particularly ESPT), solvent molecular reorientation, solute structural distortion (particularly breaking the conjugated ring coplanarity), and vibrational cooling all play an intricate and functional role on ultrafast timescales. The main photochemical reaction coordinate of ESPT (yes or no) is not affected by the wavelength of the UV radiation in the same environment (water or methanol). However, the chromophore conformational change and vibrational cooling become prominent in both solvents after irradiation by more energetic UV photons as we tune the 400 nm pump to 267 nm. We also reveal after deepUV excitation, solvation of HPTS by adjacent water molecules on the ~1 ps timescale serves as an effective relaxation channel for the chromophore moving out of the FC region, which differs from the ~3 ps contact-ion formation after near-UV excitation, before the main ESPT step as the system undergoes further relaxation. These photoinduced processes are likely involved in photoprotection of biomolecules such as melanins and nucleobases.

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Moreover, our work highlights the multidimensional PESs from the first to higher lying electronic excited states. Molecular systems in nature evolve to protect themselves under many different circumstances by navigating alternative reaction paths on the nonequilibrium PESs after photoexcitation without accumulation of photodegraded species. For example, deep-UV irradiation pumps molecules to Sn, which could relax into a different S1 state or region to help dissipate energy more effectively therein instead of going through the same S1 state accessed by a near-UV or visible excitation. A higher starting point in the PES manifold likely allows molecules to undergo a different and efficient energy relaxation pathway versus that from a lower starting point, which are all governed by the light-matter interaction and molecular Hamiltonian including the system, bath, and their interactions. Fortunately, the combination of fs-TA and FSRS with wavelength tunability opens the door to track the molecular evolution along the non-equilibrium potential energy landscape. These findings lead to new knowledge about the photoprotection mechanism of molecules under UV radiation which could enrich the rational design principles to improve efficiency and durability of functional molecular machines like artificial light-harvesting complexes and organic solar cells.

Supporting Information. This material is available free of charge on the ACS Publications website at http://pubs.acs.org. Sample preparation, femtosecond transient absorption (fs-TA), femtosecond stimulated Raman spectroscopy (FSRS), computational method, discussion of significance, Figures S1 and S2 on fs-TA dynamics of HPTS in methanol, additional references, and the full authorship of Gaussian 09 software (PDF)

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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Phone: 541-737-6704. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is supported in part by the NSF CAREER grant (CHE-1455353), Oregon Medical Research Foundation New Investigator Grant and OSU Research Equipment Reserve Fund to C.F. We thank Cheng Chen and Taylor Krueger for helpful discussions.

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Figure 1. Steady-state electronic spectroscopy and schematic potential energy surfaces of HPTS in solution. 91x104mm (600 x 600 DPI)



Figure 2. Semilogarithmic contour plot of fs-TA spectra of HPTS in (a) H2O with 400 nm pump, (b) H2O with 267 nm pump, (c) MeOH with 400 nm pump, and (d) MeOH with 267 nm pump. 109x80mm (600 x 600 DPI)


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Figure 3. Time-resolved FSRS elucidates the photochemical/physical reaction coordinates. 138x131mm (300 x 300 DPI)



Figure 4. Schematic excited state pathways of HPTS in H2O after 267 nm photoexcitation. 57x42mm (600 x 600 DPI)


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Figure 5. Excited-state vibrational frequency dynamics of the (a) 430 cm-1, (b) 680 cm-1, and (c) 1520 cm-1 mode of HPTS in H2O and MeOH on ultrafast timescales after deep-UV (DUV) and near-UV excitation. 158x305mm (600 x 600 DPI)



TOC Graphic 50x50mm (600 x 600 DPI)


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Correlated Molecular Structural Motions for Photoprotection after Deep

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