Three Donor-p-Acceptor-p-Donor type styryl dyes (5a-c) with different secondary donors are synthesized and characterized to study their nonlinear and linear optical properties. The structure-property relationships of the dyes are described in the light of systematic photophysical and theoretical investigations. The photophysical characteristics of 5a-c are influenced by the polarity of the medium, with an appreciable bathochromic shift in emission (5b = 81 nm) and large Stoke shifts (5b = 104-173 nm) in polar solvents. 5a-c showed intramolecular charge transfer characteristics recognized with the help of emission solvatochromism, solvent polarity graphs, natural bond orbital analysis and HOMO-LUMO energy difference. The optimized geometry and frontier molecular orbitals reveal that the electron donation takes place from secondary donors and not from a fixed donor (triphenylamine) which is more twisted. The nonlinear optical properties obtained using solvent induced spectral shift and computational methods are found within the limiting values. Z-scan results reveal saturable kind of behavior for 5a, 5b and 5c, whereas 5a and 5b show reverse saturable kind of behavior in acetone and ethanol and hence give optical limiting values. The two-photon absorption cross section described by two-level approximation is highest for 5b (251-300 GM).
We have designed and synthesized two dye molecules viz. MMR1 ((E)‐2‐cyano‐3‐(5‐((E)‐1‐cyano‐2‐(4′‐(dimethylamino)‐[1,1′‐biphenyl]‐4‐yl)vinyl)thiophen‐2‐yl)acrylic acid) and MMR2 (2‐((Z)‐5‐((5‐((E)‐1‐cyano‐2‐(4′‐(dimethylamino)‐[1,1′‐biphenyl]‐4‐yl)vinyl)thiophen‐2‐yl)methylene)‐4‐oxo‐2‐thioxothiazolidin‐3‐yl)acetic acid) with D‐π‐A’‐π‐A frameworks. The synthesized interfacial modifiers have dimethyl aniline as the donor group whereas cyanoacetic acid and rhodanine‐3‐acetic acid as the major acceptor groups in MMR1 and MMR2 respectively. The photophysical study in solution medium shows red shifted absorption maxima (λmax) for MMR2 vs MMR1. However, the molar extinction coefficient (ϵmax) for MMR2 is found to be lower than MMR1. The photovoltaic characterization of these molecules was carried out using iodide/triiodide and cobalt (II/III) based redox electrolytes for comparision. The molecules showed improved efficiency with the cobalt based electrolytes where MMR1 showed the highest power conversion efficiency (PCE) of 3.25% (Jsc=7.28 mA/cm2, Voc=0.713 V, FF=0.63). Further to have a deep insight into these synthesized molecules DFT studies were performed. The polarizability and hyperpolarizability values for the interfacial modifiers are calculated using DFT computation and are found to be correlated to the PCE.
Two difluoroboron curcumin analogs and their phenylated versions were studied for their photophysical and non-linear optical properties based on comprehensive density functional theory approach. The ground state geometry optimization, vertical excitation, and the first excited state optimization were carried out using B3LYP and CAMÀ B3LYP functionals with 6-31G(d) basis set. The geometry of the difluoroboron curcumin analogs was found to remain planar in both the ground and excited states. The computed vertical excitation and emission values obtained with global hybrid functional are in good correlation with the observed values. The phenylated analogs show enhancement in the photophysical properties. The linear and non-linear optical properties were computed using two global hybrids functionals (B3LYP and BHHLYP) and a range separated hybrid functional (CAMÀ B3LYP) with 6-31 + G(d,p) basis set. The difluoroboron curcumin analogs without the phenyl substituent have larger polarizability and hyperpolarizabiliy parameters than the phenylated analogs. The polarizability and hyperpolarizabiliy were found to be comparable with the BHHLYP and CAMÀ B3LYP functionals.[a] M. M. Raikwar, Prof. N. Sekar
This article reports design and synthesis of three new fluorophores (5 a‐c) with the donor‐π‐acceptor‐π‐donor motif, where dicyano vinylene group is the central acceptor, N‐ethyl carbazole group is a fixed donor and varying the N‐substituted secondary donors so as to study their photophysical behavior. Based on the systematic photophysical and theoretical reconnaissances, the fluorophores structure‐property relationships are defined. Their spectral and photophysical behaviors are affected by the solvent polarity, with a significant bathochromic shift in emission and large Stoke shifts being observed in polar solvents. Contrary to dipolar asymmetric analogue this system shows an efficient intramolecular charge transfer as observed from various solvent polarity plots. For 5 a, 5 b, and 5 c, the viscosity induced emission studies show respectively 3.78, 6.53, and 6.82 times enhancement in their fluorescence intensity and hence act as a fluorescent molecular rotor (FMR). The structure and electronic properties of these fluorophores were evaluated using density functional theory computations using B3LYP/6‐311++G(d,p). The frontier molecular orbitals and the optimized geometry reveal that secondary donors in each fluorophore donate the electron and not the N‐ethyl carbazole group which is more twisted. The non‐linear and linear optical properties were determined employing solvatochromic and computational methods in solvents of varying polarities with results falling within the expected limiting values. The linear and non‐linear optical properties of the fluorophores change with the strength of different secondary donors used and solvent polarity. The two photon absorption cross‐section described using two level approximation is highest for 5 b (115‐172 GM).
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