The 16th Femtochemistry Conference (FEMTO16)

Elucidating the Interplay between Ultrafast Internal Conversion, Intersystem Crossing, and Proton Transfer for Guiding New Photochemical Reactivities

Jun 25, 2025, 9:40 AM

20m

Contributed talk Session 9 - Solvation and reaction dynamics II

Speaker

Alexander Tarnovsky (Department of Chemistry and the Center for Photochemical Sciences, Bowling Green State University, USA)

Description

The mechanism of newly discovered photochemical reactions of $\beta$-enaminones and maleimide derivatives is a topic of interest and ongoing discussion [1,2]. We have examined excited-state dynamics in these systems following femtosecond UV excitation by means of ultrafast transient absorption spectroscopy with dispersed, broadband probing, complemented by the tools of computational photochemistry.
Excited-state relaxation of the $\beta$-enaminones in protic and aprotic solvents has been found to be within the 500 fs range and involve a tautomerization process. A fast sub-50 fs molecular descent from the Franck-Condon region leads to a flatter portion of the S$_1$ potential which region is defined by three degenerate potential energy surfaces forming a conical intersection and a singlet-triplet crossing.
For the substituted maleimides, the maleimide structure is modified by adding the hydroxy (-OH) functionality to introduce the possibility of excited-state intramolecular proton transfer (ESIPT). Maleimides where a proton source for ESIPT is not present were also studied. We have shown that, following UV excitation of these molecules into the weakly allowed $\pi$ to $\pi$ transition, they follow a major cascade-like ultrafast radiationless relaxation pathway via two lower-lying singlet n$\pi$** excited states into the ground S$_0$ state. In addition, the hydroxy-substituted maleimide displays a minor pathway with spectral and kinetic signatures consistent with excited-state proton-transfer reaction [3]. The maleimide system lacking the hydroxyl substituent reveals an intersystem crossing channel. The dynamics in these systems is compared with excited-state dynamics of unsubstituted maleimide.
This work is funded by the National Science Foundation, under grant numbers CHE-1955524, CHE-2350308, and CHE-2102619.

[1] S. K. Kandappa, L. K. Valloli, S. Jockusch, J. Sivaguru, J. Am. Chem. Soc. 2021, 143, 3677.
[2] J. Parthiban, D. Garg, S. Ahuja, S. Jockusch, A. Ugrinov, J. Sivaguru, ACS Catal. 2024, 14, 8794.
[3] D. Garg, A. N. Tarnovsky, J. Sivaguru, J. Phys. Chem. A 2025 (accepted).