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A joint theoretical-experimental study on the optical properties of 5-N-succinimidyl-2.2 -bithiophene (NS-2T), a prototype system for a new class of biomarkers was conducted, and time-dependent density functional theory (TD-DFT) and approximate coupled-cluster single and doubles (CC2) calculations were performed in the ground and excited states. Results showed that CC2 and TD-DFT can very well describe the absorption and photoluminescence energies of the three systems, but the presence of several charge-transfer transitions in the TD-DFT spectrum of NS-2T required the use of a correlated method to validate the TD-DFT results.

Singlet and triplet excitation energies in oligothiophenes are analyzed using time-dependent density-functional theory (TD-DFT) with different exchange-correlation functionals. TD-DFT methods underestimate photoluminescence energies but the energy difference between singlet and triplet states shows a trends with the chain-length similar to coupled-cluster singles and doubles model (CC2).

We apply the frozen density embedding method, using a full relaxation of embedded densities through a freeze-and-thaw procedure, to study the electronic structure of several benchmark ground-state charge-transfer complexes, in order to assess the merits and limitations of the approach for this class of systems. The calculations are performed using both semilocal and hybrid exchange-correlation (XC) functionals. The results show that embedding calculations using semilocal XC functionals yield rather large deviations with respect to the corresponding supermolecular calculations. Due to a large error cancellation effect, however, they can often provide a relatively good description of the electronic structure of charge-transfer complexes, in contrast to supermolecular calculations performed at the same level of theory. On the contrary, when hybrid XC functionals are employed, both embedding and supermolecular calculations agree very well with each other and with the reference benchmark results. In conclusion, for the study of ground-state charge-transfer complexes via embedding calculations hybrid XC functionals are the method of choice due to their higher reliability and superior performance. [ABSTRACT FROM AUTHOR]

The intramolecular radiative and nonradiative relaxation processes of three thiophene-S,S-dioxide derivatives with different molecular rigidity are investigated in different solutions and inert matrix. It is shown that the fluorescence quantum efficiency and the relaxation dynamics are strongly dependent on the environment viscosity, whereas they are almost independent of the environment polarity.