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 C₁–C₁′ 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 C₁–C₁′ rotation to ∼100° between benzothiazole and phenyl moieties in which the energetics for the S₁ and T₁ states are nearly identical. As a result, the intersystem crossing between S₁ and T₁ states serves as a fast deactivation pathway for the excited-state cis-tautomer to channel into both cis- and trans-tautomer in their respective T₁ states, followed by the dominant T₁-S₀ radiationless deactivation to populate the trans-tautomer in the ground state. The trans-tautomer species in the S₀ 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

Harnessing Excited-State Intramolecular Proton-Transfer Reaction via a Series of Amino-Type Hydrogen-Bonding Molecules

A series of new amino (NH)-type hydrogen-bonding (H-bonding) compounds comprising 2-(2′-aminophenyl)­benzothiazole and its extensive derivatives were designed and synthesized. Unlike in the hydroxyl (OH)-type H-bonding systems, one of the amino hydrogens can be replaced with electron-donating/withdrawing groups. This, together with a versatile capability for modifying the parent moiety, makes feasible the comprehensive spectroscopy and dynamics studies of amino-type excited-state intramolecular proton transfer (ESIPT), which was previously inaccessible in the hydroxyl-type ESIPT systems. Empirical correlations were observed among the hydrogen-bonding strength (the N–H bond distances and proton acidity), ESIPT kinetics, and thermodynamics, demonstrating a trend that the stronger N–H···N hydrogen bond leads to a faster ESIPT, as experimentally observed, and a more exergonic reaction thermodynamics. Accordingly, ESIPT reaction can be harnessed for the first time from a highly endergonic type (i.e., prohibition) toward equilibrium with a measurable ESIPT rate and then to the highly exergonic, ultrafast ESIPT reaction within the same series of amino-type intramolecular H-bond system