Van der Waals clusters of pyridazine and isoquinoline: The effect of solvation on chromophore electronic structure

Abstract

Van der Waals clusters of pyridazine and isoquinoline with CH4, NH3, H2O, and CH3OH are generated in a supersonic molecular jet expansion and investigated by two-color time-of-flight mass spectroscopy. As is the case for the other diazine systems, no spectra could be observed for pyridazine (H2O)n or (CH 3OH)n clusters. Both chromophore molecules are reported to have close lying, vibronically coupled S1 and S2 excited states: nπ* for pyridazine and nπ* (S1) and ππ* (S2) for isoquinoline. Cluster spectra for pyridazine methane and ammonia clusters do not favor the presence of two nπ* transitions in the S1←S0 transition region but rather suggest that the "S2 origin" is a vibronic feature of the S1←S0 transition. Isoquinoline clusters that are only weakly or not at all hydrogen bonded (CH4 and NH3) display a complicated spectrum indicative of S1 (nπ*)-S 2 (ππ*) vibronic coupling and not the usual shifted isolated molecular spectrum. Isoquinoline clusters with substantial hydrogen bonding (H2O and CH3OH) display relatively simple spectra indicative of only a single electronic transition S2 (ππ*)←S0 in the region and no interstate vibronic coupling. These results are compared and contrasted with each other and the spectra of the other diazine clusters. Potential energy calculations are also employed to help understand the clustering in these systems. © 1987 American Institute of Physics.