Tuning linear dichroism/birefringence (LDLB) contributions in VCD spectra of tartaric acid-TPPS4 porphyrin films

Abstract

Vibrational circular dichroism (VCD) is employed to investigate chiral-templation of D and L tartaric acids (TA) which, according to electronic circular dichroism (ECD) studies, drive tetrakis-(4-sulfonatophenyl)porphyrin (TPPS₄) to self-assemble into chiral J-aggregates. TPPS₄/TA solutions, prepared in both H₂O and D₂O, are drop cast on CaF₂ plates. VCD spectra of corresponding films are not affected by rotating the samples around the incident beam, but undergo dramatic changes, even sign reversal, by flipping the samples from “front” to “rear” position, depending on acid content. The observed IR absorption features are mainly due to vibrational transitions of TA. The dissymmetry ratio g for the observed features is of the order of 10⁻², which is quite high for vibrational phenomena: the semi-sum spectra in the front-rear positions can be considered as representative of “intrinsic” VCD phenomena, the semi-difference is associated with linear dichroism-linear birefringence components (LDLB term). In this study, we show that TPPS₄ and TA, when mixed appropriately following the specified protocols, yield films in which the VCD and LDLB components can be effectively modulated.