Multivariate curve resolution – Alternating least squares applied to the investigation of ultrafast competitive photoreactions

Abstract

The main advantage of multivariate curve resolution – alternating least squares method (MCR–ALS) is the possibility to act as multiset analysis method, combining data coming from different experiments to provide a complete and more accurate description of a chemical system. Exploiting the multiset side, the combination of experiments obtained from two photo-active systems with complementary pathways and monitored by femtosecond UV–vis transient absorption spectroscopy is presented in this work. A multiset hard- and soft-multivariate curve resolution model (HS-MCR) was built allowing the description of the spectrokinetic features of the entire system. Additionally, reaction quantum yields were incorporated in the hard-model in order to describe branching ratios for intermediate species.The photodynamics of salicylidene aniline (SA) was investigated as a case study. The overall reaction scheme involves two competitive and parallel pathways. On the one hand, a photoinduced excited state intramolecular proton transfer (ESIPT) followed by a cis–trans isomerization leads to the so-called photochromic form of the molecule, which absorbs in the visible. The formation of the photochromic species is well characterized in the literature. On the other hand, a complex internal rotation of the molecule takes place, which is a competing reaction. The rotation mechanism is based on a trans–cis isomerization. This work aimed at providing a detailed spectrokinetic characterization of both reaction pathways for SA. For this purpose, the photodynamics of two molecules of identical parent structures and different substituent patterns were investigated in femtosecond transient absorption spectroscopy. For SA, the mechanism described above involving the two parallel pathways was observed, whereas for the derivative form of SA, the photochromic reaction was blocked because of the replacement of an H atom by a methyl group. The application of MCR approaches enabled to obtain transient spectra for the different intermediate species involved and rate constants for the photochromic reaction, thus contributing to a comprehensive description of the photodynamics of SA.