Graphene quantum dots (GQDs) are promising semiconducting materials for practical applications. Chemical functionalization of GQD edges is a strategy to increase their solubility and processability and to modify their optical properties. Such functionalization has a twofold effect on the electronic structure, both attributed to electronic spatial localization. The first is relevant to the introduction of electronegative species, whereas the second effect modifies the electron wave function spatial distribution through the conformational changes of geometry of the otherwise planar GQD backbone. Our computational study shows that functionalization with a weakly electronegative chlorine functional group induces small red-shifts due to the localization of excitonic states around the defect site. Groups with strong electron inducting ability such as nitro introduce stronger red-shifts. In both cases, the planar geometry of the pristine sheet is preserved when functionalization is performed at only a single site. However, functionalizing at multiple sites around the edge results in warping of the geometry of the GQD, leading to stronger electronic localizations and the largest red-shifts. This observation suggests the utility of controlling the planar geometry as a new strategy for chemically modifying 2-dimensional nanomaterials toward optical tuning.

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.

No abstract