Theoretical study on photophysical properties of bis-dipolar diphenylamino-endcapped oligoarylfluorenes as light-emitting materials

Bis-dipolar emissive oligoarylfluorenes, OF(2)Ar-NPhs(2), bearing an electron affinitive core, 9,9-dibutylfluorene, as conjugated bridges and diphenylamino as endcaps, show great potential for application in organic light-emitting diodes. The various electron affinitive central aryl cores that include thiophene S,S'-dioxide, dibenzothiophene S,S'-dioxide, 2,1,3-benzothiadiazole, 4,7-dithien-2-yl-2,1,3-benzothiazole, dibenzothiophene, and dibenzofuran produce a remarkable influence on their optical and electronic properties. In this contribution, we apply quantum-chemical techniques to investigate a series of bis-dipolar diphenylamino-endcapped oligoaryfluorenes, OF(2)Ar-NPhs(2). The geometric and electronic structures in the ground state are studied using density functional theory (DFT) and the ab initio HF, whereas the lowest singlet excited states are optimized with ab initio CIS. The maximal absorption and emission wavelengths are investigated by employing time-dependent density functional theory (TDDFT). As a result, HOMOs, LUMOs, energy gaps, ionization potentials, electron affinities, and reorganization energies are affected by varying the electron affinitive cores in OF(2)Ar-NPhs(2). The absorption and emission spectra of this series of bis-dipolar oligoarylfluorenes also exhibit red shifts to some extent due to the electron-withdrawing property and the conjugated length of the electron affinitive cores. Remarkably, their calculated emission spectra can cover the full UV-vis spectrum (from 412 to 732 nm). Also, the Stokes shifes are unexpectedly large, ranging from 34 to 234 nm, resulting from a more planar conformation of the excited state between the two adjacent units in the oligoarylfluorenes. All the calculated results show that the oligoarylfluorenes can be used as hole and electron transport/injection materials in organic light-emitting diodes.