Isolated building blocks of photonic materials: high-resolution spectroscopy of excited states of jet-cooled push-pull stilbenes.

The lowest excited singlet states of 4-(dimethylamino)-4'-cyanostilbene (DCS), 4-(dimethylamino)-4'-nitrostilbene (DANS), and 4-di(hydroxy-ethyl)amino-4'-nitrostilbene (DANS-diol) have been investigated by high-resolutionfluorescence excitation and dispersed emission spectroscopy on samples seeded in supersonic expansions. Using ab initio calculations of the harmonic force fields of the electronic ground state as a starting point for the analysis of vibrational modes and frequencies, detailed vibrational assignments of the rich line structure of the S 1 ← S 0 transition in these three push-pull stilbenes are reported. From the experimental and theoretical analysis, it becomes apparent that the excitation spectra are dominated by low-frequency vibrational modes of the stilbene-like backbone, albeit that a characterization in terms of trans-stilbene vibrational terminology is not very appropriate. The fluorescence excitation and emission spectra show significant Franck-Condon factors for transitions involving these vibrational modes, in line with significant geometry changes in the stilbene moiety upon electronic excitation. Comparison of the spectroscopic data of the DANS chromophore in DANS and DANS-diol demonstrates that the inclusion of the diol auxiliary function has a significant effect on its spectroscopic properties. Similarly, the comparison between solution and gas-phase data leads to the conclusion that spectroscopic data obtained in solution for DCS allow for a reasonably good extrapolation to the isolated molecule but that the structure of DANS is, even in low-polarity solvents, already modified to a larger extent than would a priori be expected.