Ultrafast internal conversion pathway and mechanism in 2-(2′-hydroxyphenyl)benzothiazole: a case study for excited-state intramolecular proton transfer systems

Abstract

We study the ultrafast electronic relaxation of the proton transfer compound 2-(2'-hydroxyphenyl)benzothiazole (HBT) in a joint approach of femtosecond pump-probe experiments and dynamics simulations. The measurements show a lifetime of 2.6 ps for the isolated molecule in the gas phase in contrast to approximately 100 ps for cyclohexane solution. This unexpected decrease by a factor of 40 for the gas phase is explained by ultrafast internal conversion to the ground state promoted by an inter-ring torsional mode. The quantum chemical calculations based on multireference configuration interaction clearly demonstrate that a S(0)/S(1) conical intersection at a 90 degrees twisted structure exists and is responsible for the ultrafast decay. The reaction path leading from the keto form of HBT to this intersection is practically barrierless on the S(1) surface. The on-the-fly dynamics simulations using time-dependent density functional theory show that after electronic excitation to the S(1) state and after fast excited-state proton transfer (30-50 fs), HBT reaches the region of the S(1)/S(0) crossing within about 500 fs, which will lead to the observed 2.6 ps deactivation to the ground state. After the internal conversion, HBT branches in two populations, one that rapidly closes the proton transfer cycle and another (trans-keto) that takes approximately 100 ps for that step.

Ultrafast Dynamics of o-Nitrophenol: An Experimental and Theoretical Study

“…Taking into account previous studies, typical time constants for intramolecular proton transfer lie on a 10 fs time scale. 53,57 Thus, the observed fast shift in the onset of the photoelectron spectrum of 14 ± 6f s( section 3.2) in the gas phase can be attributed to proton migration to the aci-nitro form of o-NP. Because the proton transfer is accompanied by rotation around the C−N bond, as mentioned above, "full" outof-plane twist of the newly formed HONO group by ∼73°and return to the ground state is supposed to occur on a longer time scale.…”

Initial Processes of Proton Transfer in Salicylideneaniline Studied by Time-Resolved Photoelectron Spectroscopy

“…We note that this scheme does not contradict our interpretation of the initial steps in the proton transfer dynamics: Our second time constant could easily convolve torsional dynamics in both the enol and keto forms, as the two processes are thought to occur on similar time scales. 14,22 In addition to SA, ESIPT was studied in related species such as 2-(2′-hydroxyphenyl)benzothiazole (HBT) and 2-(2′-hydroxyphenyl)benzoxazole (HBO) in both the liquid 46−50 and gas 51 phases. However, in these molecules, the anilino group is fixed and cannot twist.…”

A Bistable Molecular Switch Driven by Photoinduced Hydrogen‐Atom Transfer

“…This theoretical prediction recently found experimental confirmation. [15,16] Recently, one of us proposed a reversible optically driven molecular switch based on the mechanistic principles of the ESIPT phenomenon. [17] The overall photophysical scheme of such systems is shown in Figure 1.…”