Tuning optoelectronic properties of carbazole through substituent effect: TD-DFT investigation

Abstract

π-conjugated organic materials such as carbazoles have attracted much attention because of their applications in electronic devices such as OLED’s, solar cells and sensors. Due to their optoelectronic properties, high charge carrier mobility, suitable band gaps and orbital energies, carbazoles have received immense attention to serve as a potent photosensitizer for obtaining high performance in DSSC. The present QSPR study of substituted carbazole and carbazole anion with electron donor (NH2) and acceptor (NO2) substituents at various positions is done by the density functional theory (DFT) calculations at B3LYP/6-31G(d,p) level. The longest maximum absorption wavelengths (λmax) in vacuum as well as in polar (acetonitrile) and non-polar (benzene) solvents are studied by using the time-dependent density functional theory (TD-DFT). For the singly substitution of NH2 (at C3) and NO2 (at C4) groups to the carbazole and its anion, the λmax values are observed to be shifted to the longest wavelength from 290.56 nm to 325.73 and 375.77 nm than the other respective positions. Whereas, the respective λmax values for carbazole-anion are found at 384.28 and 590.53 nm respectively. For disubstituted carbazole with NH2 and NO2 at C3 and C4 positions, the longest λmax shifts to 477.15 nm and for carbazole-anion the longest λmax obtained at C4 (NH2) and C5 (NO2) positions with λmax value 694.61 nm. Similar observations are also found in acetonitrile and benzene solvents. Further, it is observed that as the λmax increases, the HOMO-LUMO energy gap (EHL) value decreases accordingly which can be attributed to intramolecular charge transfer from NH2 to NO2 groups. A very good correlation of λmaxwith EHL is observed with correlation coefficient between 0.91 to 0.95 in vacuum as well as in acetonitrile and benzene solvents. Present study may provide valuable guidelines for the choice of suitable substituent to design carbazole moieties as efficient photosensitizer in DSSC. Copyright © 2017 VBRI Press.