Electroabsorption study of vacuum-evaporated films of Pt(II)octaethylporphyrin

Abstract

Abstract Electric-field modulated absorption spectra in the range 270–580 nm have been studied in solid films of Pt(II)octaethylporphyrin (PtOEP) formed by vapour deposition onto room-temperature quartz substrates. The electroabsorption (EA) spectra were measured at first (1ω) and second (2ω) harmonic of fundamental frequency of modulating field (ω) applied to the sample in a sandwich cell arrangement with aluminum electrodes, Al/PtOEP/Al/quartz. The (2ω)EA spectra are interpreted in terms of the standard Stark spectroscopy analysis using a direct comparison of the (2ω)EA spectrum with first and second absorption derivatives. Both main absorption porphyrin bands (B and Q) give the first absorption derivative-like EA signals from molecular (Frenkel) excitons responding to an electric field with the similar values of an average polarizability change, Δ p ¯ ≅ 20 ± 5 A 3 / f 2 (f = 1.7 in the Lorentz field approximation). This result has been rationalized employing Gouterman’s “four orbital model”. A weakly resolved structure observed around 440 nm suggests a contribution of an intermolecular charge transfer state. The presence of an internal electric field, less than 105 V/cm, has been deduced from the relation between (1ω)EA and (2ω)EA spectra.