Spectroscopic studies of solid phthalocyanines and their charge transfer salts

Abstract

<jats:p> We present the polarized reflection spectra of several MPcs ( M ≡ Co , Ni , Cu , Zn , Pb ) in the Q-band region and interpret them based on conventional exciton theory. We compare the polarized reflection spectra of the phthalocyanine radical salts NiPc ( AsF <jats:sub>6</jats:sub>)<jats:sub>0.5</jats:sub>, H <jats:sub>2</jats:sub> Pc ( AsF <jats:sub>6</jats:sub>)<jats:sub>0.67</jats:sub> and LiPc and interpret the new absorption band near the Q-band using the relationship between the degree of oxidation and the intensity of this new band. Based on the pressure dependence of this new band and the diagnostic phonon modes, we prove a pressure-induced charge transfer in NiPc ( AsF <jats:sub>6</jats:sub>)<jats:sub>0.5</jats:sub>. A metal–insulator phase transition is predicted from the analysis of the plasmon absorption and is confirmed by the high-pressure electrical resistivity. We propose the origin of the pressure-induced charge transfer and the mechanism of the metal–insulator transition. We find the optical transition associated with the 3 d <jats:sub>z</jats:sub>2 band in the reflection spectrum of CoPc ( AsF <jats:sub>6</jats:sub>)<jats:sub>0.5</jats:sub>, which is proved by comparison with the mixed crystals Co <jats:sub>x</jats:sub> Ni <jats:sub>1 − x</jats:sub> Pc ( AsF <jats:sub>6</jats:sub>)<jats:sub>0.5</jats:sub>. A new weak intermolecular optical transition is found through the resonance enhancement of a local phonon in the mixed crystals Co <jats:sub>x</jats:sub> Ni <jats:sub>1 − x</jats:sub> Pc ( AsF <jats:sub>6</jats:sub>)<jats:sub>0.5</jats:sub>. </jats:p>