Photophysical And Charge Transport Properties Of Fluorene-thiophene Conjugated Oligomers: A Theoretical Insight Into Structure-Property Relationships

At the quantum-chemical level, we characterize some important parameters that control photoelectronic properties of threeπ-conjugated oligomers:2,7-di(2-thienyl)-9,9-dihexylfluorene (FT), 2,5-bis-(9H-fluorene-2-yl)-thieno[3,2-b]t-hioph- ene (BFTT) and 2,7-bis[5-(1,1-dicyanovinyl)thiophen-2-yl]-9,9-di-n-hexylfluo-rene (FTCN). The discrete cross-like packing of FT crystal, the well-known herringbone packing of the molecule BFTT and theπ-stack in a slipped cofacial manner FTCN were studied by density functional methods (DFT) and Marcus charge transport theory. These results show that (i) because of the discrete cross-like packing of FT crystal which reduces the charge mobility, the calculated hole and electron mobilities are all smaller than other molecules. Whereas, the emission spectrum is 423 nm lying in the blue regions, which confirms that is useful blue OLED material; (ii) The effects of extend conjugated length and electron-withdrawing dicyanovinyl group all obviously reduce the energy gap and excitation energies, making the maximum absorption and emission spectra red-shift; and (iii) the chemical modifications by changing linkage mode or introducing the electron- withdrawing group could improve the charge transfer, especially for FTCN. The calculated hole and electron mobilities reach up to 1.91 and 0.62 cm2 V^-1 s^-1. Meanwhile, it is found that the packing effect weakens the emission intensity to some extent according to the simulations of photoluminescences of dimers.