Using time-dependent density functional theory (TD-DFT), configuration interaction single (CIS) method, and approximate coupled cluster singles and doubles (CC2) method, we investigated the absorption spectra of coumarin derivative dyes (C343, NKX-2388, NKX-2311, NKX-2586, and NKX-2677), which have been synthesized for efficient dye-sensitized solar cells. The CC2 calculations are found in good agreement with the experimental results except for the smallest coumarin dye (C343). TD-DFT underestimates the vertical excitation energy of the larger coumarin dyes (NKX-2586 and -2677). Solvents (methanol) are found to induce a red shift of the vertical excitation energies, and their effects on the molecular geometry and the electronic structure are examined in detail. The deprotonated form of coumarin is also investigated, where a blue shift of the vertical excitation energies is observed.

The surface electronic and molecular structure of a new organic chromophore useful for dye-sensitized nanostructured solar cells has been investigated by means of electron spectroscopy. Initially the use of a simple molecular system containing the polyene-diphenylaniline chromophore in a solar cell device was verified. The electronic and molecular surface structure of the functional dye-sensitized interface was then investigated in detail by a combination of core level spectroscopy, valence level spectroscopy, X-ray absorption spectroscopy, and resonant photoemission spectroscopy. The results indicate a dominating orientation of the molecule at the surface, having the diphenylaniline moiety pointing out from the surface. Valence level spectroscopy, X-ray absorption spectroscopy, and resonant photoemission spectroscopy were used to experimentally delineate the frontier electronic structure of the molecule, and the experimental spectra were analyzed against theoretical spectra, based on density functional theory. Together the investigation gives insight into energy matching of the molecular electronic states with respect to the TiO2 substrate as well as the localization of the frontier electronic states and the direction of the charge-transfer absorption process with regards to the TiO2 surface.

Unsymmetrical cyanine dyes are widely used in biomolecular detection due to their fluorogenic behavior, whereby fluorescence quantum yields can be very low in fluid solution but are significantly enhanced in conformationally restricted environments. Herein we describe a series of fluorinated analogues of the dye thiazole orange that exhibit improved fluorescence quantum yields and photostabilities. In addition, computational studies on these dyes revealed that twisting about the monomethine bridge beyond an interplanar angle of 60 leads to a dark state that decays nonradiatively to the ground state, accounting for the observed fluorogenic behavior. The effects of position and number of fluorine substituents correlate with both observed quantum yield and calculated activation energy for twisting beyond this critical angle.