Investigation of the nonlinear absorption spectrum of all-trans retinoic acid by using the steady and transient two-photon absorption spectroscopy

Abstract

This work investigates the two-photon absorption (2PA) spectrum of all-trans retinoic acid (ATRA) in DMSO solution, employing the wavelength-tunable Z-scan and white-light pump-probe techniques with femtosecond pulses. Our results showed that ATRA presents two 2PA allowed bands at 280 nm (2hν = 560 nm) and 365 nm (2hν = 730 nm) with 2PA cross-section values of 34 GM and 40 GM, respectively. The 2PA band at 280 nm was ascribed to the transition from ground to high energy excited states with contribution from the real intermediate excited state (1¹B_u⁺-like). The 2PA band at 365 nm was attributed to an overlap of 1¹B_u⁺-like and 2¹A_g⁻-like excited states, which are essential to describe the photochemical processes that this class of organic materials play in nature. Through solvatochromic measurements and by using the two-energy level approach, we found that the 1¹B_u⁺-like and 2¹A_g⁻-like states contribute, respectively, 48% and 52% of the lowest energy 2PA band of ATRA in DMSO solution. From femtosecond transient spectroscopy we verified that when ATRA is excited at 775 nm (2PA excitation) its excited state absorption (ESA) spectrum presents a red-shift of ∼5.5 nm in comparison to the same spectrum excited at 387.5 nm (1PA excitation), corroborating the interpretation about the 2PA spectrum for the ATRA in DMSO.