Abstract
A meso-mesityl-2,6-iodine substituted boron dipyrromethene (BODIPY) dye is investigated using a suite of computational methods addressing its functionality as photosensitizer, i.e., in the scope of light-driven hydrogen evolution in a two-component approach. Earlier reports on the performance of the present iodinated BODIPY dye proposed a significantly improved catalytic turn-over compared to its unsubstituted parent compound based on the population of long-lived charge-separated triplet states, accessible due to an enhanced spin-orbit coupling (SOC) introduced by the iodine atoms. The present quantum chemical study aims at elucidating the mechanisms of both the higher catalytic performance and the degradation pathways. Time-dependent density functional theory (TDDFT) and multi-state restricted active space perturbation theory through second-order (MS-RASPT2) simulations allowed identifying excited-state channels correlated to iodine dissociation. No evidence for an improved catalytic activity via enhanced SOCs among the low-lying states could be determined. However, the computational analysis reveals that the activation of the dye proceeds via pathways of the (prior chemically) singly-reduced species, featuring a pronounced stabilization of charge-separated species, while low barriers for carbon-iodine bond breaking determine the photostability of the BODIPY dye.
Highlights
The supply of renewable and sustainable energy sources is among the key scientific and technological endeavors of the 21st century [1,2,3,4,5]
The aim of the present computational study was to elucidate the photophysical and photochemical processes leading to the formation of the catalytically active species of a boron dipyrromethene (BODIPY)-based photosensitizer in the scope of light-driven hydrogen evolution, as well as to identify prominent excited-state relaxation channels associated to the photodegradation of the dye
The preliminarily performed benchmark of the excited states properties obtained by economical time-dependent density functional theory (TDDFT) calculations against state-of-the-art multiconfigurational simulations, namely MS-RASPT2, showed that the PBE0 functional is able to estimate the potential energy landscape in the ground state structure of the non-reduced and the singly reduced photosensitizer in reasonable agreement with MS-RASPT2
Summary
The supply of renewable and sustainable energy sources is among the key scientific and technological endeavors of the 21st century [1,2,3,4,5]. The present fully computationally study aims at elucidating the coupled photo-induced redox processes, namely the heavy atom mechanism vs the reductive mechanism (Scheme 1), associated with the generation of the reduced BODIPY dye enabling the catalytically active reduced palladium species, while SOCs are calculated to rationalize ISC processes. The quantum chemical simulations intend to determine the excited state relaxation channels associated to C-I bond cleavage and, the photodegradation of the PS and the decrease of the catalytic activity To this aim, and in contrast to the previous joint synthetic-spectroscopic-theoretical study, the excited state landscape of 1 is investigated in various oxidation and spin states using state-of-the-art multiconfigurational methods, i.e., multi-state restricted active space perturbation theory through second-order on a restricted active space self-consistent field reference (MS-RASPT2//RASSCF).
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