Spectroscopic Characterization and Modeling of Quadrupolar Charge-Transfer Dyes with Bulky Substituents

Abstract

Joint experimental and theoretical work is presented on two quadrupolar D-π-A-π-D chromophores characterized by the same bulky donor (D) group and two different central cores. The first chromophore, a newly synthesized species with a malononitrile-based acceptor (A) group, has a V-shaped structure that makes its absorption spectrum very broad, covering most of the visible region. The second chromophore has a squaraine-based core and therefore a linear structure, as also evinced from its absorption spectra. Both chromophores show an anomalous red shift of the absorption band upon increasing solvent polarity, a feature that is ascribed to the large, bulky structure of the molecules. For these molecules, the basic description of polar solvation in terms of a uniform reaction field fails. Indeed, a simple extension of the model to account for two independent reaction fields associated with the two molecular arms quantitatively reproduces the observed linear absorption and fluorescence as well as fluorescence anisotropy spectra, fully rationalizing their nontrivial dependence on solvent polarity. The model derived from the analysis of linear spectra is adopted to predict nonlinear spectra and specifically hyper-Rayleigh scattering and two-photon absorption spectra. In polar solvents, the V-shaped chromophore is predicted to have a large HRS response in a wide spectral region (approximately 600-1300 nm). Anomalously large and largely solvent-dependent HRS responses for the linear chromophores are ascribed to symmetry lowering induced by polar solvation and amplified in this bulky system by the presence of two reaction fields.

Electron-donating strength dependent symmetry breaking charge transfer dynamics of quadrupolar molecules

“…[7][8][9][10][11][12][13][14][15] The ICT process, which takes place readily in dipolar molecules, is easily influenced by the polarity of the surrounding environments, and leads to fluorescence solvatochromism or a decrease in fluorescence quantum yield along with an increase in static dipole moment in the excited state. 16- 18 When ICT occurs in molecules with D--A--D or A--D--A quadrupolar, or D--A3, A--D3 octupolar symmetry structures, [19][20][21][22][23][24][25] their ground-state absorption usually presents little solvent dependence because of symmetry characteristics but emission shows a strong solvatochromism effect, indicative of quadrupolar or octupolar ground state but dipolar emission state. [26][27][28][29][30][31][32][33][34] This phenomenon was rationally interpreted by symmetry breaking charge transfer (SBCT), which is triggered by the fluctuation of molecular structure upon photoexcitation or the local field generated by the surrounding solvents.…”

Experimental Charge Density Analysis and Electrostatic Properties of Crystalline 1,3-Bis(Dimethylamino)Squaraine and Its Dihydrate from Low Temperature (T = 18 and 20 K) XRD Data

“…It is well documented that the role of electrostatics is essential to interpret the structural features of squaraine aggregates, as well as to understand unusual properties of centrosymmetric squaraines and the features of materials including squaraine frameworks [12][13][14], particularly the so-called quadrupolar dyes. Experimental as well as theoretical and computational work on these materials is published regularly [15][16][17][18][19].…”

Dipolar vinyl sulfur fluorescent dyes. Synthesis and photophysics of sulfide, sulfoxide and sulfone based D–π–A compounds

“…The 1 H NMR spectra were recorded at 200/400 MHz and the 13 C NMR spectra were measured at 50/100 MHz, in CDCl 3 as solvent with TMS as reference. Coupling constants (J) are reported in Hertz.…”

“…[6][7][8][9][10][11] These molecular systems have also been extended to quadrupolar (D-p-A-p-D or A-p-D-p-A) compounds, which also have promising technological applications. [12][13][14][15][16][17] In general, optical materials take important roles in optoelectronics, as part of transistors, OLEDs, solar cells, lightening agents and light modulators. 18,19 These usual D-p-A molecular architectures play an important role in their photophysics because they not only show high uorescent quantum yields owing to the effective radiative decay of their excited intramolecular chargetransfer (ICT) state but also possess impressive bipolar chargetransporting properties for their constitutive hole-and electrontransporting moieties, [20][21][22][23][24][25] that can be modulated through tuning the groups electron donor and/or acceptor strengths.…”