Aza‐boron dipyrromethenes (aza‐BODIPYs) presenting a benzothiadiazole substitution on upper positions are described. The strong electron‐withdrawing effect of the benzothiadiazole moiety permits enhancement of the accepting strength and improves the delocalization of the aza‐BODIPY core to attain a significant degree of electronic communication between the lower donating groups and the upper accepting groups. The nature of the intramolecular charge transfer is studied both experimentally and theoretically. Linear spectroscopy highlighted the strongly redshifted absorption and emission of the synthesized molecules with recorded fluorescence spectra over 1000 nm. Nonlinear optical properties were also investigated. Strong enhancement of the two‐photon absorption of the substituted dyes compared with the unsubstituted one (up to 4520 GM at 1300 nm) results in an approximately 15–20 % improvement of the optical power limiting performances. These dyes are therefore a good starting point for further improvement of optical power limiting in the short‐wave IR range.

The electronic properties of neutral 2,4-bis(4-bis(2-hydroxyethyl) amino-2-hydroxy-6-(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)phenyl)squaraine (1) and charged 2-((3-octadecylbenzothiazol-2(3H)-ylidene)methyl)-3-oxo-4-((3-(4-(pyridinium-1-yl)butyl)benzo-thiazol-3-ium-2-yl)methylene)cyclobut-1-enolate iodide (2) squaraine derivatives were analyzed based on comprehensive linear photophysical, photochemical, nonlinear optical studies (including two-photon absorption (2PA) and femtosecond transient absorption spectroscopy measurements), and quantum chemical calculations. The steady-state absorption, fluorescence, and excitation anisotropy spectra of these new squaraines revealed the values and mutual orientations of the main transition dipoles of 1 and 2 in solvents of different polarity, while their role in specific nonlinear optical properties was shown. The degenerate 2PA spectra of 1 and 2 exhibited similar shapes, with maximum cross sections of ∼300–400 GM, which were determined by the open aperture Z-scan method over a broad spectral range. The nature of the time-resolved excited-state absorption spectra of 1 and 2 was analyzed using a femtosecond transient absorption pump–probe technique and the characteristic relaxation times of 4–5 ps were revealed. Quantum chemical analyses of the electronic properties of 1 and 2 were performed using the ZINDO/S//DFT theory level, affording good agreement with experimental data. To demonstrate the potential of squaraines 1 and 2 as fluorescent probes for bioimaging, laser scanning fluorescence microscopy images of HeLa cells incubated with new squaraines were obtained.

An optical power limiter (OPL) device can change its optical properties depending on the input light intensity: it is ideally transparent at low incident power and presents a decrease in transmission when the incident intensity increases. This phenomenon can be used for protection of optical sensors. This paper presents the design of a functionalized aza-BODIPY chromophore, combining good nonlinear two-photon absorption and exited-state absorption in the short-wave infrared range (1300−1600 nm). The aza-BODIPY chromophores also show excellent thermal and photochemical stabilities. Their covalent grafting to a sol−gel matrix leads to the formation of class II sol−gel materials that allows for extraordinary doping concentrations of up to 280 mol•L −1 (40 wt %), which results in highly efficient OPL performances at 1550 nm.

The fast relaxation processes in the excited electronic states of functionalized aza‐boron‐dipyrromethene (aza‐BODIPY) derivatives (1–4) were investigated in liquid media at room temperature, including the linear photophysical, photochemical, and nonlinear optical (NLO) properties. Optical gain was revealed for nonfluorescent derivatives 3 and 4 in the near infrared (NIR) spectral range under femtosecond excitation. The values of two‐photon absorption (2PA) and excited‐state absorption (ESA) cross‐sections were obtained for 1–4 in dichloromethane using femtosecond Z‐scans, and the role of bromine substituents in the molecular structures of 2 and 4 is discussed. The nature of the excited states involved in electronic transitions of these dyes was investigated using quantum‐chemical TD‐DFT calculations, and the obtained spectral parameters are in reasonable agreement with the experimental data. Significant 2PA (maxima cross‐sections ∼2000 GM), and large ESA cross‐sections ∼10−20 m2 of these new aza‐BODIPY derivatives 1–4 along with their measured high photostability reveal their potential for photonic applications in general and optical limiting in particular.

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.