Insight into the Amino-Type Excited-State Intramolecular Proton Transfer Cycle Using N-Tosyl Derivatives of 2-(2′-Aminophenyl)benzothiazole

Studies have been carried out to gain insight in to an overall excited state proton transfer cycle for a series of N-tosyl derivatives of 2-(2'-aminophenyl)benzothiazole. The results indicate that followed by ultrafast (<150 fs) excited-state intramolecular proton transfer (ESIPT), the titled compounds undergo rotational isomerization along the C-1-C-1' bond. For the model compound 2-(2'-tosylaminophenyl)benzothiazole (PBT-NHTs) the subsequent cis-trans isomerization process in both triplet and ground states are probed by nanosecond transient absorption (TA) and two-step laser-induced fluorescence (TSLIF) spectroscopy. Both TA and TSLIF results indicate the existence of a long-lived trans-tautomer species in the ground state with a lifetime of few microseconds. The experimental results correlate well with the theoretical approach, which suggests that PBT-NHTs proton transfer tautomer generated in the excited state undergoes intramolecular C-1-C-1' rotation to similar to 100 degrees between benzothiazole and phenyl moieties in which the energetics for the S-1 and T-1 states are nearly identical. As a result, the intersystem crossing between S-1 and T-1 states serves as a fast deactivation pathway for the excited-state cis-tautomer to channel into both cis and trans-tautomer in their respective T-1 states, followed by the dominant T-1-S-0 radiationless deactivation to populate the trans-tautomer in the ground state. The trans-tautomer species in the S, state proceeds with intermolecular double proton transfer to regenerate the cis-normal form. An overall proton-transfer cycle describing the amino-type ESIPT and the subsequent isomerization processes is thus depicted in detail.