Linear Photophysics and Femtosecond Nonlinear Spectroscopy of a Star-Shaped Squaraine Derivative with Efficient Two-Photon Absorption

Abstract

The synthesis, comprehensive linear photophysical and photochemical study, two-photon absorption (2PA) spectrum, ultrafast relaxation kinetics in the excited states, and efficient superluminescence properties of a new symmetrical three-armed star-shaped squaraine derivative (1) are presented. The steady-state spectral parameters of 1 in a number of organic solvents, including fluorescence excitation anisotropy spectra, revealed a weak interaction between the squaraine branches and the effect of symmetry breaking in the ground electronic state. The degenerate 2PA spectrum of 1 was obtained over a broad spectral range with a maximum cross section of ∼8000 GM using the open aperture Z-scan technique. The nature of the fast dynamic processes in the excited electronic states of 1 was investigated by the femtosecond transient absorption pump−probe method, revealing characteristic relaxation times of ∼3−4 ps. The efficient superluminescence emission of 1 was observed in relatively low concentration solution (≈ 2.3·10 −4 M) under femtosecond transverse pumping. A quantum-chemical study of 1 was performed using ZINDO/S//DFTB theory levels. Simulated 1PA and 2PA absorption spectra were found to be in a good agreement with experimental data. The figure of merit for 1 is ∼10 11 GM, 1 one of the highest values ever reported for two-photon fluorescence molecular probes, suggesting strong potential for its application in two-photon fluorescence microscopy and bioimaging.

Far‐Red‐ to NIR‐Emitting Adamantyl‐Functionalized Squaraine Dye: J‐Aggregation, Dissociation, and Cell Imaging

“…Moreover, AM‐1 was synthesized under similar reaction conditions by condensation of C and squaric acid. [2a] 1 H NMR, 13 C NMR, and MALDI‐TOF mass spectroscopy confirmed the molecular structures and purities of the related compounds. Synthesis and molecular characterization details are presented in the Supporting Information.…”

“…[8b] Because of the twisted molecular structure and lower conjugation efficiency related to the intermolecular charge transfer interactions, it was reasonable to expect a modest 2PA cross section for AM‐1′ . [2a], , …”

“…As a class of organic dyes, squaraine derivatives exhibit sharp and intense absorption ( ε max > 10 5 m –1 cm –1 ) and fluorescence emission which can be tailored from the far‐red to near‐infrared spectral region . In recent years, squaraine dyes have been investigated for applications such as nonlinear optics, fluorescence microscopy bioimaging, dye‐sensitized solar cells and photosensitizers for photodynamic cancer therapy . In general, squaraines have the tendency to undergo H‐/J‐aggregation in aqueous media and the solid state, which may restrict their optical applications to some extent due to strong quenching of the fluorescence emission .…”

Electronic Nature of Neutral and Charged Two-Photon Absorbing Squaraines for Fluorescence Bioimaging Application

“…The main ESA bands were observed in the short wavelength region at ∼450–530 nm (for 1 ) and ∼500–570 nm (for 2 ) that are typical for symmetrical squaraines with λ ab max at ∼600–700 nm. 33,34,37 It should be mentioned that gain (i.e., optical amplification via stimulated emission) is not possible in the spectral range λ < 600 nm, and saturable absorption (SA) is negligible because of a weak linear absorption at λ < 570 nm for both molecules. Observed negative changes in the optical densities (i.e., Δ D < 0) in the spectral range of 580–680 nm for 1 and 600–700 nm for 2 are related to SA processes and some possible inputs of gain in the corresponding fluorescence contours (Figure 6c,d, curves (3)).…”

“…The degenerate 2PA spectra of 1 and 2 were obtained in THF ( 1 ) and ACN ( 2 ) at room temperature in 1 mm path length quartz cuvettes with molecular concentrations C ≈ 10 –3 M, using an open aperture Z -scan method. 59 Transient absorption pump–probe measurements 34,60 were performed in DCM ( 1 ) and ACN ( 2 ) at room temperature in a 1 mm quartz flow cell to minimize thermo-optical distortion and photodecomposition processes. Experimental details of the femtosecond laser system and process employed were comprehensively described previously.…”