A comprehensive study of the photophysical properties of a series of monoaza[5]helicenes 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.

A systematic study on the synthesis and properties of aza- [5]helicenes bearing one or two nitrogen atoms in selected ring positions is reported for the first time. This photochemical approach can be conveniently applied to the preparation of either mono- or diaza[5]helicenes. The aza[5]helicenes were characterized by NMR spectroscopy, X-ray crystallography, emission spectroscopy, and luminescence lifetime. The extremely long triplet lifetime observed (in the range of seconds) makes these molecules promising candidates for practical applications in photo- and optoelectronics

Three closely related previously synthesized monoaza[5]helicenes have been resolved into their enantiomers via enantioselective HPLC using a cellulose-derivative Chiralcel OD column. Circular dichroism (CD) spectra of the enantiomerically enriched samples have been recorded and assigned. The spectra were analyzed as a function of time, and different rate constants were found in the kinetics of racemization for the three molecules. Ab initio DFT calculations for the ground electronic states were employed to determine minima and saddle point structures and to understand the racemization process. The theoretical geometries compared well with those from X-ray structures. CD spectra were calculated by TD-DFT ab initio methods, and compared with experimental data

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Circular dichroism (CD) spectra have been measured in the range of 400-200 nm on CH 3 OH solutions of both enantiomers for the almost complete series of monoaza [5]helicenes, namely the molecules where the hetero N atom occupies positions 1, 3, 4, 5, 6, and 7, respectively (the 2 isomer is missing due to difficulties in the synthesis). CD spectra recorded at controlled room temperature allow one to define precise racemization rates, that are nicely interpreted on the basis of DFT molecular orbital calculations. Time-dependent DFT methods provide us with calculated CD and UV spectra, that are compared with the corresponding experimental data. We discuss the role of the N atom in determining the height of the racemization barrier and in shaping the appearance of the CD spectra.

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