Green-emitting Polyfluorenes Containing Hexylthiophen-dibenzothiophene-S,S-dioxide Unit with Large Two-photon Absorption Cross Section

Abstract

Green light-emitting polyfluorenes containing 3,7-bis(4-hexylthiophen-2-yl)dibenzo[b,d]thiophene 5,5-dioxide (DHTSO) unit were synthesized. All the resulted polymers show high thermal stability with the decomposition temperatures (Td) over 420 °C and the glass transition temperatures (Tg) over 75 °C. The polymers exhibit the enhanced highest occupied molecular orbital (HOMO) energy levels and the depressed lowest unoccupied molecular orbital (LUMO) energy levels with the increase of DHTSO unit in polymers. The photoluminescence (PL) spectra of the polymers show positive solvatochromism in solution with the variation of solution polarities, indicating remarkable intramolecular charge transfer (ICT) effect in the polymers containing DHTSO moiety. The fluorescence quantum yields (ΦPL) are in the range of 34%−67% for PF-DHTSOs in film. All polymers possess two photon absorption (TPA) properties, and the TPA cross sections (δ2) are enhanced with increasing DHTSO unit in polymers. The highest δ2 is 2392 GM for PF-DHTSO15 in chloroform solution upon 740 nm excitation. The device of PF-DHTSO15 shows green emission with the Commission Internationale de L’. Eclairage (CIE) coordinates of (0.26, 0.59), and the maximum luminous efficiency (LEmax) of 10.8 cd·A−1 with the configuration of ITO/PEDOT:PSS/EL/CsF/Al. These results indicate that introducing DHTSO unit into polyfluorene backbone could be a promising molecular design strategy for TPA and effective green-light emission.