Knowledge Bank

Diethylaniline (DEA) exhibits prominent low frequency activity in its vibrationally resolved $S1\leftarrow S0$ fluorescence excitation spectrum. We report here studies at full rotational resolution of the first three hands in this spectrum which appear within $200cm-1$ of the origin. The spectra yield rotational constants of the $S0$ and $S1$ states, from which it is shown that the inertial defect $(|\Delta I|)$ increases on $S1\leftarrow S0$ excitation, that $|\Delta I|$ increases with increasing vibrational energy in the S1 state, and that the orientation of the $S1\leftarrow S0$ transition moment is a function of excitation energy, as well. We will deduce from these results and related theoretical calculations the preferred conformation of DEA in its $S0$ and $S1$ states, the nature of the vibrational modes that appear in the $S1\leftarrow S0$ spectra, and the possible relevance of this motion to the TICT character of the $S1$ states of aniline, DEA, diethylaminobenzonitrile (DEABN), and related molecules.