Evidence for excited state intramolecular charge transfer in benzazole-based pseudo-stilbenes

Abstract

Two azo compounds were obtained through the diazotization reaction of aminobenzazole derivatives and N,N-dimethylaniline using clay montmorillonite KSF as catalyst. The synthesized dyes were characterized using elemental analysis, Fourier transform infrared spectroscopy, and (13)C and (1)H NMR spectroscopy in solution. Their photophysical behavior was studied using UV-vis and steady-state fluorescence in solution. These dyes present intense absorption in the blue region. The spectral features of the azo compounds can be related to the pseudo-stilbene type as well as the E isomer of the dyes. Excitation at the absorption maxima does not produce emissive species in the excited state. However, excitation around 350 nm allowed dual emission of fluorescence, from both a locally excited (LE, short wavelength) and an intramolecular charge transfer (ICT, long wavelength) state, which was corroborated by a linear relation of the fluorescence maximum (ν(max)) versus the solvent polarity function (Δf) from the Lippert-Mataga correlation. Evidence of TICT in these dyes was discussed from the viscosity dependence of the fluorescence intensity in the ICT emission band. Theoretical calculations were also performed in order to study the geometry and charge distribution of the dyes in their ground and excited electronic states. Using DFT methods at the theoretical levels BLYP/Aug-cc-pVDZ, for geometry optimizations and frequency calculations, and B3LYP/6-311+G(2d), for single-point energy evaluations, the calculations revealed that the least energetic and most intense photon absorption leads to a very polar excited state that relaxes non-radioactively, which can be associated with photochemical isomerization.

Homoleptic Lanthanide Complexes Containing a Redox-Active Ligand and the Investigation of Their Electronic and Photophysical Properties

“…Emission is not observed upon excitation of the free papl ligand in the lowest energy absorption band ( Figure S14). This observation was previously reported for azo-type compounds and it was explained by the generation of a very polar excited state relaxing non-radiatively, associated with photochemical isomerization [19]. However, excitation at higher energy leads to dual emission at room temperature (Figure 8 (orange line) and Figures S13 and S14): a high energy band (325-425 nm) (with vibronic structure of similar energy (λ max = 365, 384, 419, and 442 nm), and a lower energy band (425-600 nm) also featuring vibronic structure of similar energy (λ max = 490, 517, and 560 nm).…”

“…This very interesting and very complex emission profile of the free ligand results from a combination of multiple functionalities in its structure (e.g., azoaryl, phenanthrol and pyridyl moieties, including keto-enol forms), each of which may display a specific characteristic behavior upon excitation. Based on related systems [19], the high energy band (325-425 nm) is a locally excited (LE) state together with radiative relaxation from a mixing of higher S and T. The emission of complex 1 confirms the triplet emission energy (see below). The lower energy band (425-600 nm) is assigned to intramolecular change transfer states.…”

Relay proton transfer triggered twisted intramolecular charge transfer

“…[1][2][3] Though several mechanisms were propsed to demonstate formation of the ICT state, the twisted ICT (TICT) model proposed by Grabowski et al is widely accepted. [4][5][6][7][8][9][10][11][12] A close look reveals that other than polarity and viscosity, the hydrogen bonding also strongly influence the TICT process of numerous systems. 4,5 In other words, that the donor is perpendicular to other part of the molecule in the ICT state.…”

Ground and excited state properties of chalcogenol esters: a combined theoretical and experimental study

“…In the ground state, regarding the absorption maxima, no linear correlation could be observed ( R 2 = 0.50 and R 2 = 0.24 for 1c and 1f , respectively). However, for both dyes, taking the emission maxima into account, the observed solvatochromic shifts versus Δ f fit a linear plot ( R 2 = 0.92 and R 2 = 0.98 for 1c and 1f , respectively), which corroborates the presence of an intramolecular charge transfer mechanism in the excited state, as already described . It must be noted that only these two compounds presented such a linear correlation.…”