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

Abstract

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. MnO ) 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).

Counterion-controlled spectral tuning of the protonated Schiff-base retinal

“…The absorption by the 2 + B complex has a maxiumum at 460 nm, and is shifted by approximately 0.6 eV as compared with the bare chromophore. This is an anomalously large blue shift, being a factor of two larger than the largest blue shift due to betaine complexation measured so far [27,28]. The absorption band of the 1 + B complex is considerably wider and peaked at 410 nm.…”

“…more than six times larger than that of water. It has recently been shown that electrospraying a solution containing the chromophore of choice and betaine results in the production of mixed complexes [27,28]. The structure of these complexes is expected to be one where the negative part of the betaine is located close to the positive charge in the chromophore, inducing a strong local electric field.…”

“…Thus, by measuring the photo-fragmentation spectrum of the complex it is possible to deduce the effect of an electric field on the chromophore's absorption band. Moreover, it has been shown that betaine complexation can be used to determine the character of the electronic transition [27].…”

“…This is due to the difference in interaction energy between the external charges and the charges on the chromophore in the electronic ground state and in the excited state. On the other hand, in ions in which the electronic transition is not of CT character, the shift due to betaine complexation is limited [27]. In the case of RPSB, the electronic transition is expected to have a high degree of CT, and thus a large blue shift is expected.…”

Intrinsic Photophysics of Light‐harvesting Charge‐tagged Chlorophyll a and b Pigments

“…To obtain the intrinsic properties, the molecule has to be studied isolated in the gas phase. After the absorption spectrum of the bare Chl is established, perturbations can be included by complexation of the pigment with various adducts to study the effect of individual interactions …”