The effects of environmental properties on the linear and nonlinear response of Bromothymol blue (BTB) were studied. The role of the optical band gap and dielectric constant, as well as thermal conductivity, on the optical properties of the BTB molecule and the BTB complex with cetyl trimethyl ammonium bromide (CTAB) were discussed using a Z-scan instrument with a wavelength of 532 nm and power of 80 mW, a spectrophotometer, and FT-IR spectra. The orders of the nonlinear absorption coefficient, β, and the nonlinear refractive index, n2, of BTB were obtained as 10− 7 cm/W and 10− 12 cm2/ W, respectively. The main factor in the appearance of β is reverse saturable absorption. It was observed that an enhancement of up to 5 times in the dielectric constant of the environment caused a reduction of about 6 times in the nonlinear absorption and about 2 times in the nonlinear refraction coefficient of the solution. Moreover, the CTAB complex can reduce the optical band gap of BTB by 0.7 eV and also absorbs light in the 600 to 700 nm wavelength.Therefore, it is a promising candidate for photodynamic therapy. Results of DFT calculations performed via the B3LYP method and the 6-311G(d,p) basis show an enhancement of the dipole moment of BTB with an increase in the dielectric constant of the environment.

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