Procedures for Converting Electronic Absorption Spectra into Higher-Order Derivatives to Examine Photoinduced Spectral Changes

The practices to convert electronic absorption spectra into the fourth- and eighth-order derivatives are described to stress their usefulness in quantitative analysis, keeping in mind the applicability to elucidate photochemical behavior of versatile materials. The simulation was carried out in two ways. The first study disclosed that the higher-even-order derivatives are an efficient approach to resolve elementary bands due to rotational energy level transitions (RLT) and, in particular, vibrational energy level transitions (VLT). Electronic absorption bands are determined by full width at half-maximum (FWHM) of and distance (Delta lambda(v)) between adjacent VLT bands, while there is a critical FWHM as well as Delta lambda(v) for the sufficient resolution of VLT bands in derivative spectra. The subsequent simulation was performed with aid of Savizky-Golay smoothing of synthetic noisy spectra to know how polynomial order (s) and filter width (p) affect derivative-spectral shapes. It was shown that p should be smaller than 1 + 2 x (Delta lambda(v) - 1), whereas Delta lambda(v) is of an electronic absorption band to be analyzed. The simulation results were substantiated by spectral changes due to the photoisomerization of azobenzene in hexane as a model compound, displaying that Delta lambda(v) values of electronic transitions of S-0-S-1 and S-0-S-2 are quite different, respectively.