Accessing the Intrinsic Nature of Electronic Transitions from Gas-Phase Spectroscopy of Molecular Ion/Zwitterion Complexes.

A molecule's color is governed by the nature of its electronic transitions. Herein we show that the degree of charge transfer can be assessed by measuring the change in absorption induced by complexation with the betaine zwitterion. Our approach benefits from direct formation of complexes by electrospray of a mixture solution, followed by photodissociation action spectroscopy. We explored two ion groups: 1) No permanent dipole moment due to even charge delocalization (e.g. MnO4- ) and 2) Non-even charge distribution but where the charge according to resonance forms is either delocalized (e.g. oxyluciferin) or located at one site (e.g. m-nitrophenolate, mNP). The maximal shift for ions from (1) was <0.05 eV but as large as 0.3 eV and 0.2 eV for mNP and oxyluciferin. Hence our work supports the notion that oxyluciferin undergoes CT, and that the microenvironment can account for large variation in light emission from insects, ranging from green to red (shift of 0.3 eV).