Effect of spirocyclization of xanthene dyes on linear and nonlinear optical properties by considering D-π-A and D-A-D Systems: DFT and TD-DFT approach

Spirocyclization, a unique feature of xanthene dyes, makes it a promising candidate for developing fluorescent ratiometric probes for sensing, imaging, tracking, and labeling. How will this feature of xanthene dyes influence the linear and nonlinear optical properties? To examine the effect of spirocyclization of xanthene dyes, we have selected both the open-close form of rhodamine B and π-extended xanthene dyes substituted with thienyl and thieno[3,2-b]thienyl group at position 3 and 6. Here, rhodamine B will serve as the (Donor)D-π-A(Acceptor) system and thiophene-substituted xanthene dyes will serve as the D-A-D system. The geometry optimization of the close-open form of xanthene dyes at B3LYP/6–311++G(d,p) and CAM-B3LYP/6–311++G(d,p) revealed that the open form is energetically more stable in the S₀ state than the closed form. The vertical excitation energy (ΔE), from Time-Dependent-Density Functional Theory (TD-DFT) calculation at B3LYP/6–311++G(d,p), CAM-B3LYP/6–311++G(d,p), ωB97XD/6–311++G(d,p) revealed that (ΔE)_open < (ΔE)_close. Further, the open form of xanthene dyes displays red-shifted absorption compared to the closed form. The λ_Vt of xanthene dyes (open-close forms) is mainly assigned to HOMO → LUMO transition (S₀ → S₁) with % orbital contribution for open form ∼ 90 % and close form ∼ 60 %. The oscillator strength of xanthene dyes is obtained in the range of 0.01 – 1.74. The λ_Vt of xanthene dyes is in agreement with experimental absorption. The static polarizability (α₀), first-order hyperpolarizability (β₀), second-order hyperpolarizability (γ), molecular hyperpolarizability (µβ₀), and frequency-dependent hyperpolarizability (β¹⁰⁶⁴), of the open form of xanthene dyes, were found to be higher than the close form. Thus, the open form of xanthene dyes will show superior linear and nonlinear optical properties than the closed form. The thienyl[3, 2-b] thieno substituted xanthene dye with red-shifted absorption shows higher α₀, β₀, γ, µβ₀, β¹⁰⁶⁴, and γ values, show better linear and NLO properties than thienyl and diethylamine substituted xanthene dyes.