New Two-Photon Absorbing Squaraine Derivative with Efficient Near-Infrared Fluorescence, Superluminescence, and High Photostability

Abstract

The nature of linear photophysical and nonlinear optical properties of a new squaraine derivative 2,4-bis[4-(azetidyl)-2-hydroxyphenyl]squaraine (1) with efficient near-infrared (NIR) emission was comprehensively analyzed based on spectroscopic, photochemical, and two-photon absorption (2PA) measurements, along with quantum chemical analysis. The steady-state absorption, fluorescence, and excitation anisotropy spectra of 1 and its fluorescence emission lifetimes revealed the multiple aspects of the electronic structure of 1, including the relative orientations of the main transition dipoles, effective rotational volumes in solvents of different polarities, and a maximum molar extinction of 1.35 × 10 −5 M −1 •cm −1 , which is unusually small for similar symmetric squaraines. The degenerate 2PA spectrum of 1 was obtained over a broad spectral range under femtosecond excitation, using standard open-aperture Z-scan and two-photon induced fluorescence methods, revealing maximum 2PA cross sections of ∼400 GM. Squaraine 1 exhibited efficient superluminescence emission in the polar solvent (dichloromethane) at room temperature under femtosecond pumping conditions. Quantum chemical analysis of the electronic structure of 1 was performed using the DFT/ TD-DFT level of theory and found to be in good agreement with experimental data. The new squaraine derivative 1 displayed high fluorescence quantum yield, efficient NIR superluminescence, large 2PA cross sections, and high photostability with a photodecomposition quantum yield ∼4 × 10 −6 , suggesting its potential for applications in two-photon fluorescent bioimaging and lasing.

Intramolecular Charge Transfer and Ultrafast Nonradiative Decay in DNA-Tethered Asymmetric Nitro- and Dimethylamino-Substituted Squaraines

“…The indolenine-based squaraines at the focus of the study are shown in Figure A. Squaraine dyes, which are of great interest in nonlinear optics, − bioimaging, , sensing, − and supramolecular assembly, ,− were chosen due to their chemical tunability, structural stability, and large transition dipole moment amplitude. ,,− Unsubstituted symmetric squaraine 1 was used as a control. The synthesis and characterization of 1 has been reported previously .…”

Tunable Non-Kasha Behaviors and Excited-State Dynamics of Quadrupolar Squaraine Aggregates

“…It is noted that one meaningful aspect of 2PA different from one-photon absorption (1PA) is the distinct selection rules for the electronic dipole transitions dependent upon the molecular symmetry (Figure c). ,, Therefore, dominant 2PA transitions close to the 1PA maxima rather than the 2-fold relation could be elucidated using the three-level energy model. Small detuning energy (Δ E ∼ 0.118 eV) and vibronic coupling effects lead to a resonance-enhanced excitation condition for the S 0 → S n two-photon transition when the transition energy matches well with 2 times the laser power (Figure c). , Interestingly, degenerate δ 2PA values up to 9520 GM at 850 nm, 8100 GM at 900 nm, and 7520 GM at 950 nm were investigated for the SQC6 dimer in toluene (Figure b). The longer wavelength spectral shift and 38.4-fold enhancement at 950 nm in comparison to that of 196 GM for the SQC6 monomer revealed high potentials for manifold nonlinear optical applications.…”

“…Small detuning energy (ΔE ∼ 0.118 eV) and vibronic coupling effects lead to a resonance-enhanced excitation condition for the S 0 → S n two-photon transition when the transition energy matches well with 2 times the laser power (Figure 5c). 38,39 Interestingly, degenerate δ 2PA values up to 9520 GM at 850 nm, 8100 GM at 900 nm, and 7520 GM at 950 nm were investigated for the SQC6 dimer in toluene (Figure 5b). The longer wavelength spectral shift and 38.4-fold enhancement at 950 nm in comparison to that of 196 GM for the SQC6 monomer revealed high potentials for manifold nonlinear optical applications.…”

Structure–activity relationships of aniline-based squaraines for distinguishable staining and bright two-photon fluorescence bioimaging in plant cells

“…[11][12][13] Compared to some small-molecule organic fluorophores, multipolar squaraines (SQs) possessing relatively sharp and intense absorption, long-wavelength emission, and outstanding photostability have been demonstrated as promising NIRemitting candidates for broad biological applications. [14][15][16][17][18] Long-wavelength excitation and emission of SQs have attractive advantages of deep tissue penetration, minimum photodamage, and minimum interference from auto-fluorescence in biological systems, as compared with some commercial fluorophores illustrated in Scheme S3 (ESI †). While zwitterionic SQs have the tendency to undergo H-/J-aggregation in aqueous media, which may restrict their optical applications to some extent due to the fluorescence emission quenching.…”