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Protonation‐State‐Driven Photophysics in Phenothiazinium Dyes: Intersystem Crossing and Singlet‐Oxygen Production
Author(s) -
RodriguezSerrano Angela,
Daza Martha C.,
Doerr Markus,
Tatchen Jörg,
Marian Christel M.
Publication year - 2017
Publication title -
chemphotochem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.13
H-Index - 18
ISSN - 2367-0932
DOI - 10.1002/cptc.201700069
Subject(s) - intersystem crossing , chemistry , thionine , photochemistry , protonation , singlet oxygen , triplet state , aqueous solution , singlet state , excited state , reaction rate constant , singlet fission , internal conversion , kinetics , oxygen , ion , molecule , atomic physics , organic chemistry , physics , electrode , quantum mechanics , astronomy , electrochemistry , spectral line
The impact of altering the solvent pH value on the photodynamic activity of thionine has been studied computationally by means of density functional theory and multi‐reference interaction methods. To this end, we have investigated the electronic structure of the ground and excited states of diprotonated (TH 2 2+ ) and neutral imine (T) forms of thionine (TH + ). It is well known experimentally that the T 1 state of TH + undergoes acid–base equilibrium reactions resulting in a pronounced pH effect for the efficiency of singlet‐oxygen ( 1 O 2 ) production. Our results show that the energy‐transfer reactions from the T 1 state of TH 2 2+ and T to 3 O 2 correspond to reversible equilibrium processes, whereas in TH + this process is very exothermic in a vacuum (−0.66 eV) and in aqueous solution (−0.49 eV). These facts explain the experimental observation of a much smaller efficiency of 1 O 2 production for TH 2 2+ than for TH + . Moreover, we found that the pH value significantly effected the intersystem crossing (ISC) kinetics impacting the concentration of triplet‐state species available for energy transfer. In very acidic aqueous solution (pH<2) where TH 2 2+ is the prevailing species, the ISC proceeds with a rate constant of ≈10 8 s −1 . In a basic medium where T is the dominant species, ISC decay occurs by means of a thermally activated channel (≈10 8 s −1 ) which competes with fluorescence (5.32×10 7 s −1 ). According to these results, maximum ISC efficiency is expected for intermediate acidic pH values (TH + , ≈10 9 s −1 ).