Thermochromism of Water-Soluble Cationic 5, 10, 15, 20-Tetrakis (4-N-Trimethylphenylanilinium) Porphyrin Studied by Resonance Raman Spectroscopy

Abstract

Some meso-tetraarylporphyrins having strong electron-donating groups (−OH, 73x2212;NH₂ or −NMet₂) at para-positions on the phenyl rings have been shown to exhibit thermochromism and solvatochromism [1, 2]. Recently, this phenomenon has been observed with a cationic 5, 10, 15, 20-tetrakis(4-N-trimethylphenylanilinium) porphyrin (H₂TMAP) in buffered aqueous solutions and in the 0 — 80 °C temperature range. As shown in Figure 1 for H₂TMAP at pH 3.4, a decrease of temperature induces dramatic spectral changes in the Soret- as well as Q-band regions that are characteristic for acidbase transitions induced by pH decrease [1, 2], e.g. increasing ratio of red-shifted Soret band of diacidic H₄TMAP²⁺ (∼ 433 nm) to the neutral free-base H₂TMAP (∼ 413 nm). UV- vis absorption spectra allowed dependencies of porphyrin pK_a to be determined as a function of temperature in buffers at various pH. The presence of distinct isobestic points in the absorption spectra suggests that temperature-induced transition of H₂TMAP can be considered, like the pH-induced one, as occurring between two separate electronic states—neutral free-base H₂TMAP and diacidic H₄MAP²⁺ species.