Infrared Spectra of Charge Transfer Complexes of Biocytin With Organic Acceptors

Abstract

The infrared (IR) spectra of the well-known biomolecules called biocytin and their charge transfer complexes with standard organic acceptors such as TCNQ (7,7,8,8-tetracyano-p-quinodimethane), TCNE (tetracyano-p-ethylene), DDQ (2,3- dichloro-5,6-dicyano-p-benzoquinone), chloranil, and iodine have been studied in the present work. The spectrum of biocytin contains a beta density at high frequency and a slightly asymmetric triangular distribution as well as a U-shaped distribution at low frequency, the latter being associated with anisotropic disorder. Weak charge transfer from TCNE and iodine reduces the width of U-shaped distribution and makes the triangular peak more asymmetric. Strong CT interactions with TCNQ and DDQ make the triangular peak in absorption become a Gaussian distribution. The strongest interaction with chloranil leads to oscillations in the density of states with square-root singularities. Toward the high frequency side, the interband transition do occur but with either spin-orbit splitting or exciton-phonon coupling. There is correlated hopping in biocytin-iodine complex.