Journal of Physical Chemistry A, Vol.111, No.39, 9644-9648, 2007
Mass spectrometry and photoelectron spectroscopy of tetracene cluster anions, (Tetracene)(n)(-) (n=1-100): Evidence for the highly localized nature of polarization in a cluster analogue of oligoacene crystals
Photoelectron spectroscopy of tetracene cluster anions, (tetracene), (n = 1 - 100), reveals the coexistence of two types of isomers, designated as isomers I and II-1 (n = 10-50) or isomers I and II-2 (n > 60), in a wide size range. The vertical detachment energies (VDEs) of isomer I increase persistently due to polarization and structural relaxation effects, where a monomeric anion core is encompassed with geometrically reorganized neutral molecules. Conversely, a characteristic ion distribution in the mass spectrum of (tetracene),, ensues from the two-dimensional (2D) herringbone-type ordering of isomer II-1, whose VDEs remain constant at 1.80 eV for n >= 14. Also, isomer II-2, presumably adopting multilayered structural motifs, exhibits invariable VDEs of 2.0 eV, a manifestation of significant charge screening effects in these isomers. The invariable nature of the VDEs of isomers II-1 and II-2 unambiguously demonstrates a largely localized nature of polarization induced by the excess charge residing in microscopic crystal-like environments. Surprisingly, only 14 tetracene molecules within a 2D herringbone-type layer including an excess charge can provide the charge stabilization energy corresponding to similar to 80% of that of the crystal, and the rest of the energy is provided by polarization of neutral molecules in adjacent layers.