Search

A comprehensive study of the photophysical properties of a series of monoaza5helicenes is presented on the basis of joint optical spectroscopy and quantum chemistry investigations. The molecules have been characterized by absorption and CW/time-resolved luminescence measurements. All quantities related to spin−orbit-coupling processes, such as intersystem crossing rates and radiative phosphorescence lifetimes, were found to depend strongly on the nitrogen position within the carbon backbone. Density functional theory and semiempirical quantum-chemical methods were used to evaluate the molecular geometries, the characteristics of the excited singlet and triplet states, and the spin−orbit coupling matrix elements. We demonstrate that the magnitude of spin−orbit coupling is directly correlated with the degree of deviation from planarity. The trends from the calculated photophysical quantities, namely, radiative fluorescence and phosphorescence decay rates and intersystem crossing rates, of the mono-aza-helicenes are fully consistent with experiment. [ABSTRACT FROM AUTHOR]

We discuss the electron-vibration coupling in mono-aza-5helicenes on the basis of a Franck−Condon analysis and density functional theory (DFT) calculations of the fluorescence and phosphorescence spectra measured in ethanol. The geometries of the initial states were obtained from time-dependent DFT (S1) and unrestricted DFT (T1) excited-state optimizations. In general, the position of the nitrogen atom has only a minor impact on the vibronic fine-structure in both absorption and emission. The shapes of the emission spectra from the lowest singlet and triplet states are found to be determined by contributions from multiple normal modes. The results of the calculations demonstrate how the interplay among these normal modes results in qualitatively and quantitatively different spectra for fluorescence and phosphorescence. [ABSTRACT FROM AUTHOR]