A quantum chemical investigation of the electronic structure of thionine
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Abstract
We have examined the electronic and molecular structure of 3,7-diaminophenothiazin-5-ium dye
(thionine) in the electronic ground state and in the lowest excited states. The electronic structure was
calculated using a combination of density functional theory and multi-reference configuration
interaction (DFT/MRCI). Equilibrium geometries were optimized employing (time-dependent) density
functional theory (B3LYP functional) combined with the TZVP basis set. Solvent effects were
estimated using the COSMO model and micro-hydration with up to five explicit water molecules. Our
calculated electronic energies are in good agreement with experimental data. We find the lowest excited
singlet and triplet states at the ground state geometry to be of p→p* (S1, S2, T1, T2) and n→p* (S3, T3)
character. This order changes when the molecular structure in the electronically excited states is relaxed.
Geometry relaxation has almost no effect on the energy of the S1 and T1 states (~0.02 eV). The
relaxation effects on the energies of S2 and T2 are moderate (0.14–0.20 eV). The very small emission
energy results in a very low fluorescence rate.While we were not able to locate the energetic minimum of
the S3 state, we found a non-planar minimum for the T3 state with an energy which is very close to the
energy of the S1 minimum in the gas phase (0.04 eV above). When hydration effects are taken into
account, the n→p* states S3 and T3 are strongly blueshifted (0.33 and 0.46 eV), while the p→p* states
are only slightly affected (<0.06 eV).
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