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 multi-step 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 timescales following absorption of UV light. From a common nitro-S₁ excited state, the PTs can either undergo excited state intramolecular hydrogen transfer (ESIHT) to an aci-S₁ isomer within the singlet state manifold, leading to direct S₁ → S₀ internal conversion through a conical intersection, or competitive intersystem crossing (ISC) to access the triplet state manifold on timescales of (1.93 ± 0.03) ps and (13.9 ± 1.2) ps for DMNB-Ser and NB-Tyr, respectively. Deprotonation of aci-T₁ 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 S₁ → S₀ relaxation or anion re-protonation. Hence, upper limits to the quantum yields of photoproduct formation are estimated to be in the range 13 – 25 %. Mixed DMSO/ H₂O solvents show the influence of the environment on the observed photochemistry, for example revealing two nitro-S₁ lifetimes for DMNB-Ser in a 10:1 DMSO/ H₂O mixture of 1.95 ps and (10.1 ± 1.2) ps, which are attributed to different micro-solvation environments.