Femtosecond to Microsecond Observation of Photochemical Pathways in Nitroaromatic Phototriggers Using Transient Absorption Spectroscopy.

The synthetic accessibility and tolerance to structural modification of phototriggered compounds (PTs) based on the ortho - nitrobenzene (ONB) protecting group have encouraged a myriad of applications including optimization of biological activity, and supramolecular polymerization. Here, a combination of ultrafast transient absorption spectroscopy techniques is used to study the multistep photochemistry of two nitroaromatic phototriggers based on the ONB chromophore, O -(4,5-dimethoxy-2-nitrobenzyl)-l-serine (DMNB-Ser) and O- [(2-nitrophenyl)methyl]-l-tyrosine hydrochloride (NB-Tyr), in DMSO solutions on femtosecond to microsecond time scales following the absorption of UV light. From a common nitro -S 1 excited state, the PTs can either undergo excited state intramolecular hydrogen transfer (ESIHT) to an aci -S 1 isomer within the singlet state manifold, leading to direct S 1 → S 0 internal conversion through a conical intersection, or competitive intersystem crossing (ISC) to access the triplet state manifold on time scales of (1.93 ± 0.03) ps and (13.9 ± 1.2) ps for DMNB-Ser and NB-Tyr, respectively. Deprotonation of aci -T 1 species to yield triplet anions is proposed to occur in both PTs, with an illustrative time constant of (9.4 ± 0.7) ns for DMNB-Ser. More than 75% of the photoexcited molecules return to their electronic ground states within 8 μs, either by direct S 1 → S 0 relaxation or anion reprotonation. Hence, upper limits to the quantum yields of photoproduct formation are estimated to be in the range of 13-25%. Mixed DMSO/H 2 O solvents show the influence of the environment on the observed photochemistry, for example, revealing two nitro -S 1 lifetimes for DMNB-Ser in a 10:1 DMSO/H 2 O mixture of 1.95 ps and (10.1 ± 1.2) ps, which are attributed to different microsolvation environments.