Nonradiative dynamics determined by charge transfer induced hydrogen bonding: a combined femtosecond time-resolved fluorescence and density functional theoretical study of methyl dimethylaminobenzoate in water

Abstract

As a case study of the interplay and the consequence of the interplay between intramolecular charge transfer (ICT) and intermolecular hydrogen (H)-bonding, a combined femtosecond time-resolved fluorescence (fs-TRF) and density functional theoretical (DFT) and time-dependent DFT (TDDFT) study has been conducted on methyl dimethylaminobenzoate (MDMABA) largely in a water solvent. Direct observation of the broadband spectra, anisotropy, and kinetic decays of fs-TRF from photo-excited MDMABA revealed a rapid ICT reaction occurring with a time constant of ∼0.7 ps from an initial locally excited (LE) state identified to have the Lππ* character; this produced a weakly emissive ICT state featuring radiative rate constant decreased by more than two orders of magnitude. The fluorescence of the ICT state is strongly quenched exhibiting a decay time of ∼49.7 ps, unusually faster than the nanosecond range lifetime in a polar aprotic solvent when intersystem crossing (ISC) is the major deactivation channel. This, according to the study of the solvent kinetic isotope effect, is identified to originate from an instantly enhanced strong solute-solvent H-bonding induced by the ICT reaction which allows elimination of the ISC, and enables the nonradiative decay to proceed almost entirely through the otherwise inaccessible internal conversion from the ICT state. The enhancement of H-bonding is verified by the calculation which presents theoretical evidence for not only the binding site and binding energy of the H-bonding configuration but also the electronic and structural characterization, lending support to the twisted ICT (TICT) description of the photo-excited MDMABA. This study contributes a prominent example for the extraordinary ability of water and a decisive role of ICT promoted H-bonding in offering a highly effective molecular mechanism for rapid elimination of the electronic excitation energy. The results contain an important insight for the in-depth understanding of the excited state H-bonding dynamics, and also have significant implication for clarifying the "sunscreen controversy" of the DMABA type of UVB sunscreen molecule.

Anti-Kasha’s Rule Emissive Switching Induced by Intermolecular H-Bonding

“…Upon addition of D 2 O into a DMSO- d 6 solution of TA , the formation of H-bonding − between water and TA brought in an integral shielding effect, which made an upfield shift (∼0.05 ppm, Figure a) of the aromatic protons on the TA . Meanwhile, the proton resonances of TA turned broad along with the formation of the H-bonding, suggesting that an aggregation effect may also be present.…”

QM/MM Studies on the Photophysical Mechanism of a Truncated Octocrylene Model

“…Full calculations on OCR is computationally costly, so a methyl group is used to replace its long aliphatic chain that is essentially unimportant for the photochemistry. This strategy has been used in many similar works before. − The MCDPA molecule is constructed manually. It is then solvated in a spherical methanol box with a radius of 50 Å (7772 methanol molecules).…”

“…But, the excited-state relaxation paths and photophysical mechanism of OCR remain ambiguous and not studied computationally. Herein, we have employed the combined QM­(CASPT2//CASSCF)/MM method to explore spectroscopic properties, excited-state electronic structures, and photoinduced relaxation pathways of a truncated OCR model, that is, methyl 2-cyano-3,3-diphenylacrylate (MCDPA) in methanol solution (the methyl group is used to replace the long aliphatic chain). − Finally, a possible photophysical mechanism is proposed to rationalize recent experimental observations …”

New Insights about the Photostability of DNA/RNA Bases: Triplet nπ* State Leads to Effective Intersystem Crossing in Pyrimidinones

“…Meanwhile, pyrimidinones now have to overcome the energy barrier before ISC can take place and the observed k ISC is smaller. These phenomena are in line with previous studies that high frequency vibrational modes of nucleobases can efficiently dissipate excess vibrational energy to high frequency solvent vibrations and this could also be the key reason for the observed difference in the ISC rate in other molecules. , …”