Comparing molecular and electronic properties of perylene tetracarboxylic diimides decorated at 1,6- and 1,7-bay-positions using DFT/TDDFT method

Abstract

Geometrical structures and singlet vertical excitations of perylene tetracarboxylic diimides decorated at 1,6- and 1,7-bay positions investigated using density functional theory and its time dependent formalism. Such compounds are attracting much attention in the field of organic functional materials science. While 1,7-PDI regioisomers are synthetically accessible in pure form, it is still a challenging task to obtain 1,6-regioisomers in pure form. Ground state equilibrium geometries were optimized using the B3LYP functional with triple-ζ quality TZVP basis set. Introduction of either bromine or amino substituent groups at 1,6(7)-positions distorts the planarity of perylene core. Core twisting is larger for the derivatives substituted at 1,6-positions. For computing singlet vertical excitation energies, global hybrids B3LYP, PBE0 and range-separated hybrid CAM-B3LYP density functionals tested. B3LYP computed the most accurate excitation energies against experimental absorption maximum (λmax), with a mean absolute deviation (MAD) of 0.050 eV when solvent effects included. N-alkyl substituents at the bay positions shift the absorption maxima towards the ∼700 nm region. Red shifting is more efficient with 1,7-substitution, while 1,6-amino substituted PDIs display an additional absorption peak resulting from HOMO-1 to LUMO electronic transition appearing as extension of lowest energy absorption band and covers a broader region of visible spectrum. Such electronic transition is hardly detectable for 1,7-regioisomers. Calculations reveal that amino substituted 1,6-regioisomers display lower-lying HOMOs compared to the respective 1,7-regioisomers. This observation is in line with the NBO analysis, suggesting higher contribution of side groups to HOMO levels in 1,7-regioisomers. Plotted frontier molecular orbital isodensity surfaces reveal that electron density distribution is affected markedly from the position of amino substituents on PDI core.