Theoretical investigation of the substituent effects on the electronic and optical properties of 6-substituted coumarin derivatives

The electronic structures, absorption and emission spectra of selected 6-substituted coumarins were investigated and compared with unsubstituted coumarin by using density functional theory (DFT) and time-dependent DFT (TD-DFT) methods with Gaussian 09 software package. Results revealed that the presented absorption and emission spectra were affected by the substituent groups apparently. The maximum absorption wavelength of a coumarin molecule can be shifted to a longer one by introducing a large conjugated substituent, an electron donating group or a group which can form a rigid structure with the parent moiety at 6-position. The lowest energy emission of a 6-substituted coumarin can be greatly red-shifted by modifying the substituent to one which can be strongly conjugated with the phenyl ring. A group which has a strongly electron-withdrawing effect may cause a large excitation intensity and a short wavelength in emission. Most selected molecules are predicted to give the lowest-energy emissions in the purple or blue light region, which are supposed to be selected and modified as purple or blue luminescent materials. 6-nitrocoumarin is predicted as an invisible fluorescent material. 6,7-benzocoumarin, which forms a rigid conjugated ring with the parent moiety, is expected to be modified as a green luminescent material. Many of these 6-substituted coumarins are expected to be potential candidates as large Stokes shift dyes for multicolor labeling and fluorescence-activated cell sorting (FACS), especially those molecules, which bear -CONH2, -CN, and -CH3 respectively. (C) 2015 Elsevier B.V. All rights reserved.