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

Abstract

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.

A quantum-chemical insight into the tunable fluorescence color and distinct photoisomerization mechanisms between a novel ESIPT fluorophore and its protonated form

“…By using the nanosecond transient absorption (TA) and two-step laser-induced fluorescence (TSLIF) spectroscopies, they detected the existence of a long-lived trans-tautomer species in the ground state with a lifetime of few microseconds (~6 μs). According to the theoretical calculations, the peak at 590 nm observed by TSLIF, red-shifted by 1073 cm −1 in comparison to the steady state cis-tautomer emission (555 nm), can be ascribed to the trans-tautomer emission [49]. It is therefore apparent that such a similar phenomenon has also been found in the BTImP + species, as discussed above, which needs further experimental studies to confirm our theoretical predictions.…”

“…In addition, the trans-K-S 1 conformer can also undergo rotation to relax to the S 1 -T 1 intersection point through a barrierless way and then regenerate the cis-K-S 0 and trans-K-S 0 forms. In a recent study, Chou group reported a comprehensive photophysical behavior on the N-tosyl derivatives of 2-(2′-aminophenyl)benzothiazole to elucidate the relaxation dynamics in an overall proton transfer cycle [49]. By using the nanosecond transient absorption (TA) and two-step laser-induced fluorescence (TSLIF) spectroscopies, they detected the existence of a long-lived trans-tautomer species in the ground state with a lifetime of few microseconds (~6 μs).…”

Dormant acceptor activation of 10-hydroxybenzoquinline derivatives by excited-state intramolecular proton transfer

“…In recent articles [13,42,43] Chou et al [17] studied a new class of compounds where the ESIPT process occurs between two nitrogen atoms. The model compound 2-(2 0 -tosylaminophenyl) benzothiazole (PBT-NHTs) shows that the ESIPT rate strongly depends on the electron withdrawing power of the side groups R and R 0 .…”

“…The model compound 2-(2 0 -tosylaminophenyl) benzothiazole (PBT-NHTs) shows that the ESIPT rate strongly depends on the electron withdrawing power of the side groups R and R 0 . [13,42,43] when a Tosyl group is attached to the proton donor amino group the ESIPT process is ultrafast t PT < 150 fs. For other side groups the ESIPT process is rather slow or even not detected.…”

Theoretical study on the photooxygenation and photorearrangement reactions of 3-hydroxyflavone

“…2 The wavelength and intensity of the N* and PT* bands are sensitive to the local environment, which has led to the development of many 3HF based ratiometric uorescent probes aimed at studying membrane structures, 3,4 micellar assemblies, [5][6][7] biomolecular interactions, [8][9][10] and cellular processes 11,12 and for use as chemosensors for the detection of non-uorescent analytes. 13,14 Much research effort has also been devoted to the construction of novel type ESIPT compounds, recent examples for which are the benzoindolizine derivative showing proton transfer from a protonated amino group to a ring C-atom, 15,16 and the amino-benzothiazols 17,18 and amino-benzoquinolines 19 with proton transfer along their (amino)NH/N(ring) hydrogen bonds.…”