Formation of heterogeneous clusters in superfluid helium nanodroplets: phthalocyanine and water

Clusters consisting of a single phthalocyanine molecule and a single water molecule are investigated by means of electronic spectroscopy in superfluid helium droplets. A recent spectroscopic study of those clusters [J. Fischer, F. Schlaghaufer, E.-M. Lottner, A. Slenczka, L. Christiansen, H. Stapelfeldt, M. Karra, B. Friedrich, T. Mullan, M. Schütz and D. Usvyat, J. Phys. Chem. A, 2019, 123, 10057-10064] which all exhibit a water induced electronic shift to the red is now complemented by the corresponding clusters exhibiting a water induced shift to the blue. These clusters will be analyzed by means of fluorescence excitation spectra, dispersed emission spectra, and additional experimental observations made feasible by helium droplets as cryogenic reactor. Together with the blue shifted clusters a total number of at least 6 isomeric variants could be identified in helium droplets. This contrasts to a number of only three isomeric variants obtained from quantum chemical calculations [J. Fischer, F. Schlaghaufer, E.-M. Lottner, A. Slenczka, L. Christiansen, H. Stapelfeldt, M. Karra, B. Friedrich, T. Mullan, M. Schütz and D. Usvyat, J. Phys. Chem. A, 2019, 123, 10057-10064] disregarding the helium environment and to a single isomer identified in a molecular beam experiment [J. Menapace and E. Bernstein, J. Chem. Phys., 1987, 87, 6877-6889]. The discrepancy in the number of isomers provides evidence of a profound involvement of helium in clustering. Moreover, the discrepancies between the gas phase experiment and quantum chemical calculations similarly reveal the influence of the dynamics of cluster formation on the population of global and local minima that are accessible as isomeric variants.