Optical properties and aggregation of 1-N-methylamino-4'-nitroazobenzene in various environments

Abstract

Aggregation of 1-N-methylamino-4'-nitroazobenzene (RED-H) was studied by UV-vis spectroscopy in solution, using different methanol: water mixtures as solvents, at different concentrations. In 100 % methanol, methanol: water 80:20 and 60:40, RED-H exhibited an maximum absorption wavelength at $\lambda $ = 477 nm and no aggregation was observed. By contrast, in mixtures of methanol: water 40:60 and 20:80 the appearance of an additional blue shifted band around $\lambda $ =430 nm and a red shift of the absorption band to $\lambda $ = 500 nm followed by a long tail confirmed the presence of both H-and J-aggregates in these solvents. Theoretical estimations of aggregate stability carried out at the LMP2/aug-cc-PVTZ(-f)//MP2/6-31G level of theory showed that the H-aggregate is the more stable one (-14.9 kcal/mol) stabilized mostly by electronic correlation while the J-aggregate is much less stable, being stabilized by electrostatic interactions (-3.4 kcal/mol). Theoretical estimation of the absorption spectra of RED-H, H- and J-aggregates carried out using the TD-B3LYP method reproduces the experimentally observed spectra. All electronic transitions show a strong charge-transfer component.