Optical properties of organic semiconductor thin films: 2,6,9,10-tetrakis(phenylethynyl)anthracene

Two kinds of anthracene derivative thin films, 2, 6-bis((4-hexylphenyl)ethynyl)-9, 10-bis(phenyl ethynyl)anthracene (B-ant-THB) and 9, 10-bis((4-hexyl phenyl)ethynyl)-2, 6-bis (phenyl ethynyl) anthracene (HB-ant-TB), were synthesized to investigate their optical properties. The difference between the two anthracene derivatives consisted of the position of the 1-ethynyl-4-hexylbenzene group substitution into an anthracene ring. We measured the optical properties of the anthracene derivatives by using photoluminescence (PL), transmittance spectroscopy, and spectroscopic ellipsometry. The dielectric functions of the two films were similar with respect to transition energies, but were different in terms of amplitudes. The optical band gap energies of the B-ant-THB and the HB-ant-TB film states were estimated to be 2.45 eV and 2.34 eV, respectively, whereas density functional theory (DFT) calculations of isolated molecular states showed the same value of 2.42 eV. The large bathochromic shift of the HB-ant-TB film state compared to the DFT calculation is attributed to strong intermolecular coupling between the HB-ant-TB molecules in the crystalline film because the HB-ant-TB film has higher crystallinity than the B-ant-THB film. The symmetric structures of the transmittance and PL spectra of the anthracene derivative films were observed as distinct peaks having similar vibrational energy levels for the singlet ground state and the first excited state.

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