Femtosecond Spectroscopy of Superfluorescent Fluorenyl Benzothiadiazoles with Large Two-Photon and Excited-State Absorption

Abstract

New symmetrical fluorene-containing derivatives with a benzothiadiazole (BTD) central core, 4,7-bis(5-(9,9-didecyl-7-(phenylsulfonyl)-9H-fluoren-2-yl)thiophen-2-yl)benzo[c][1,2,5]thiadiazole (1) and 4,7-bis(5′-(9,9-didecyl-7-(phenylsulfonyl)-9Hfluoren-2-yl)-2,2′-bithiophen-5-yl)benzo [c][1,2,5]thiadiazole (2), were prepared, and multifarious linear photophysical and nonlinear optical properties, potentially attractive for a number of emerging applications, were investigated. Comprehensive photophysical and photochemical characterization was performed in a number of organic solvents at room temperature, including linear, one-photon absorption (1PA), steady-state fluorescence, excitation anisotropy, and fluorescence lifetime properties. Fast relaxation processes in the excited states of 1 and 2 with characteristic times of ∼0.2−3 ps were revealed by a femtosecond transient absorption pump−probe method. Large two-photon absorption (2PA) cross sections of 1 and 2 (up to 2500−2700 GM) were obtained by an open aperture Z-scan technique. Efficient two-photon optical power limiting (OPL) behavior with a figure of merit (FOM) ≈ 15.4 and superfluorescence properties were demonstrated in solution under femtosecond excitation. The electronic structure and optical parameters of 1 and 2 were analyzed by quantum chemical calculations using the TD-DFT method. Simulated linear absorption spectra were found in good agreement with experimental data while 2PA cross sections were overestimated via computational simultation and a possible explanation is provided. Good fluorescence quantum yields, long wavelength absorption, far-red to near-IR emission, efficient OPL and superfluorescence properties, large 2PA cross sections, and extremely high photochemical stability make these new materials good candidates for emerging nonlinear optical applications, including optical sensor protection, stimulated emission depletion microscopy, and two-photon fluorescence microscopy deep tissue bioimaging.

Synthesis of Near-Infrared Fluorescent Two-Photon-Absorbing Fluorenyl Benzothiadiazole and Benzoselenadiazole Derivatives

“…Figure 4 also shows the 2PA spectra and excitation anisotropy spectra of dyes 2 – 5 . 49 The fluorescence maximum of 3 was red-shifted 36 nm (to 683 nm) relative to that of 2 , as an extra thiophenyl group was introduced at each side of the benzothiadiazole core. While this red shift is desirable for NIR imaging; unfortunately there was a significant decrease in the fluorescence quantum yield, from 0.60 for 2 to 0.27 for 3 .…”

“…Preliminary investigation for the potential of one of these dyes for 2PFM NIR imaging is disclosed. 49…”

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

“…18,30 It is worth mentioning that some of the specific nonlinear optical properties such as superluminescence 36,37 (or amplified spontaneous emission) and lasing abilities were recently reported for symmetrical squaraines, 11 and can be employed in the development of a new generation of fluorescent labels with increased brightness and high spectral resolution. 38,39 Here we report the synthesis and comprehensive characterization of the linear spectroscopic, photochemical, and nonlinear optical properties of a three arm star-shaped squaraine derivative (1), including excitation anisotropy, two-photon absorption (2PA), femtosecond pump−probe spectroscopy, superluminescence, and symmetry breaking phenomena. The nature of the electronic characteristics of the molecular structure of 1 was also investigated by quantum-chemical calculations at DFTB 40 and ZINDO/S 41 levels of theory.…”

Superior optical nonlinearity of an exceptional fluorescent stilbene dye

“…15,16 Regarding the detailed 2PA-ESA process (due to the absorption of a singlet excited state or a triplet excited state), femtosecond pumped transient absorption and nanosecond flash photolysis experiments remained desirable. 17,18 Even so, the good optical limiting behaviors under the excitation of femtosecond and nanosecond pulses suggested the great potentials in many applications, such as reshaping and optical stabilization. 19 To elucidate the potential application of stilbene chromophore as a biomarker, two-photon confocal microscopic studies of stilbene chromophore in HeLa cells were performed.…”