Search

Red emitting dyes are of interest in various technological applications. Coumarins, though being an important class of fluorescent molecules, those with red emission, have been rarely studied theoretically. The structural and electronic aspects of three novel red emitting coumarins were studied using DFT and TD-DFT methods. The functionals employed were the hybrid functionals B3LYP, CAM-B3LYP, PBE0 and the highly parameterized empirical functional M06. The geometry at ground state reveals the electron donor N,N-diethylamino group is coplanar with the chromophoric system and the nitrile group induces a red shift to the absorption and emission. The electronic energies and dipole moments were solvent dependent. The basis sets and functionals were benchmarked for their performance with these molecules. B3LYP has been proved to be more efficient in computations whereas the basis sets do not have noticeable effect on the electronic properties. However, adding a polarization function to the basis set has improved the calculation of vertical excitation. The B3LYP functional gives maximum absolute deviation of 0.20 eV in calculating the vertical excitations and 0.18 eV for emission.

The structure of 3-iodobenzaldehyde (3IB) was characterized by FT-IR, Raman and single-crystal X-ray diffraction techniques. The conformational isomers, optimized geometric parameters, normal mode frequencies and corresponding vibrational assignments of 3IB were examined using density functional theory (DFT) method, with the Becke-3-Lee-Yang-Parr (B3LYP) functional and the 6-311+G(3df,p) basis set for all atoms except for iodine. The LANL2DZ effective core basis set was used for iodine. Potential energy distribution (PED) analysis of normal modes was performed to identify characteristic frequencies. 3IB crystallizes in monoclinic space group P21/c with the O-trans form. There is a good agreement between the theoretically predicted structural parameters, and vibrational frequencies and those obtained experimentally. In order to understand halogen effect, 3-halogenobenzaldehyde [XC6H4CHO; X=F, Cl and Br] was also studied theoretically. The free energy difference between the isomers is small but the rotational barrier is about 8kcal/mol. An atypical behavior of fluorine affecting conformational preference is observed., (Copyright © 2015 Elsevier B.V. All rights reserved.)

The photophysical and electrochemical properties of eleven polycyclic benzimidazole fused organic pigments (four based on phthalic anhydride, four based on naphthalic anhydride and three based naphthalene tetra carboxylic acid dianhydride) were investigated using density functional theory and time dependent density functional theory methods (B3LYP/6-31G(d) and M06/6-31G(d)). The predicted geometries are comparable using both functionals. The electrochemical properties are in good agreement with the experimental results. However, the experimental absorption-emission properties are closer to the values computed using the M06/6-31G(d) method. Both the methods perform equally well in explaining the intramolecular charge transfer characteristics. This work can help to understand the modern age functional materials at molecular level and to design new molecules.

The syn- and anti-conformers of 2-tellurophenecarboaldehyde are studied in the gas phase. A transition state is also modelled for the syn-anti isomerisation. Computations are done using different methods namely HF, DFT/B3LYP, MP2 and CCSD(T). The basis set used for all atoms is 6-311++G(d,p) except that LanL2DZ ECP is used for tellurium atom only. The optimised molecular structures and related structural parameters of these conformers are reported. The energy differences between the syn- and anti-conformers, associated rotational barriers and thermodynamic parameters are derived from the computations. The infrared frequencies of these conformers are also reported with appropriate assignments. This study is extended to include solvent effect and the conformers are fully optimised (DFT/B3LYP) using the integral equation formalism in the Polarisable Continuum model. In the gas phase, the theoretical rate constant for the unimolecular conversion, anti conformer to transition state, is reported for the first time; DFT/B3LYP (4.82×10(30) s(-1)) and MP2 (7.81×10(30) s(-1)). It is found that the structures are not much affected by the solvents but energy difference increases and rotational barrier decreases. The results indicate that there is a close agreement with the predictions from the different theoretical methods. The results obtained are critically analysed and compared with the furan, thiophene and selenophene analogues. A major factor affecting conformational preference and the mole fraction is the charge on the chalcogen heteroatom in the ring. An interesting outcome of this work is that in both the gas phase and solutions, the syn conformer is more stable and exists almost exclusively., (Copyright © 2010 Elsevier B.V. All rights reserved.)