Abstract
Methylation occurs in a myriad of systems with protective and regulatory functions. 8-methoxypyrene-1,3,6-trisulfonate (MPTS), a methoxy derivative of a photoacid, serves as a model system to study effects of methylation on the excited state potential energy landscape. A suite of spectroscopic techniques including transient absorption, wavelength-tunable femtosecond stimulated Raman spectroscopy (FSRS), and fluorescence quantum yield measurements via steady-state electronic spectroscopy reveal the energy dissipation pathways of MPTS following photoexcitation. Various solvents enable a systematic characterization of the H-bonding interaction, viscosity, and dynamic solvation that influence the ensuing relaxation pathways. The formation of a charge-transfer state out of the Franck–Condon region occurs on the femtosecond-to-picosecond solvation timescale before encountering a rotational barrier. The rotational relaxation correlates with the H-bond donating strength of solvent, while the rotational time constant lengthens as solvent viscosity increases. Time-resolved excited-state FSRS, aided by quantum calculations, provides crucial structural dynamics knowledge and reveals the sulfonate groups playing a dominant role during solvation. Several prominent vibrational motions of the pyrene ring backbone help maneuver the population toward the more fluorescent state. These ultrafast correlated electronic and nuclear motions ultimately govern the fate of the photoexcited chromophore in solution. Overall, MPTS in water displays the highest probability to fluoresce, while the aprotic and more viscous dimethyl sulfoxide enhances the nonradiative pathways. These mechanistic insights may apply robustly to other photoexcited chromophores that do not undergo excited-state proton transfer or remain trapped in a broad electronic state and also provide design principles to control molecular optical responses with site-specific atomic substitution.
Highlights
Methylation is a rather simple molecular substitution that has far-reaching implications in chemical and biological systems.[1,2] In synthetic chemistry, the reduced reactivity of the methyl substituent can protect against undesired reactions.[3]
A rotational phase was observed for HTPS and MPTS, but how does this component fit into energy dissipation pathways? Is it a nonradiative transition to the ground state or an excited-state relaxation? We address these questions by studying MPTS via an integrated experimental toolset including the steady-state electronic spectroscopy, femtosecond transient absorption (fs-Transient absorption (TA)),[26] and tunable femtosecond stimulated Raman spectroscopy (FSRS).[16,27,28]
Further experimental evidence to corroborate these findings can be found in the excited-state vibrational frequency dynamics (Fig. S10): the $1104 cmÀ1 mode of MPTS in water displays a dominant blueshift on the solvation timescale (1.5 ps) and a small blueshift on the rotational relaxation timescale (95 ps), while the adjacent 1050 cmÀ1 mode remains largely unshifted during the experimental time window of 900 ps
Summary
Methylation is a rather simple molecular substitution that has far-reaching implications in chemical and biological systems.[1,2] In synthetic chemistry, the reduced reactivity of the methyl substituent can protect against undesired reactions.[3]. The parent molecule 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS or pyranine) is a well-studied photoacid whose pKa drops from $7 to 0 from the electronic ground state (S0) to the first singlet excited state (S1).[6–13] The significant drop in pKa following photoexcitation is due to the phenolic hydroxy bond weakening upon electronic redistribution, which precedes a swift proton dissociation event Such an excited state proton transfer (ESPT) reaction, with strong implications in photosynthesis and energy harvesting, is one of the most fundamental reactions that plays important roles in chemical, biological, and energy-related systems.[14–17]. All three processes are intimately affected by the polarity, hydrogen (H-)bonding ability, and viscosity of the solvents used: water (H2O), methanol (MeOH), and dimethyl sulfoxide (DMSO) This investigation of MPTS reveals a more nuanced PES than that initially predicted, while providing insights into various roles played by rotational motions of a chromophore in solution
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