Theoretical investigation of oligo(fluorene) derivatives containing electron-deficient unit 2-pyran-4-ylidene-malononitrile moiety

A novel series of conjugated fluorene-based polymers containing a 2-pyran-4-ylidene-malononitrile (PM) moiety show great potential for application in organic light-emitting diodes (OLEDs). In this contribution, we investigate the electronic structures and optical properties of these copolymers: poly{(2,7-fluorene)-alt-(2-{2,6-bi-[2-(4-phenylene)vinyl] pyran-4-ylidene}-malononitrile)} and poly{(2,7-fluorene)-alt-(2-{2,6-bi-[2-(2-methoxyl-5-phenylene)vinyl]pyran-4-ylidene}-malononitrile)}, namely (FPM)n and (FOPM)n (n = 1,2,3,4), respectively. The geometric structures in the ground-state and lowest singlet excited-state are optimized by B3LYP and ab initio CIS methods, respectively. The absorption and emission spectra are calculated by employing time-dependent density functional theory (TD-DFT). The results show that the HOMOs, energy gaps (ΔEHOMO-LUMO), ionization potentials (IPs) of (FPM)n and (FOPM)n (n = 1-4) are affected by PM units, especially for the LUMOs and electron affinities (EAs). Thus the abilities of electron injection of them are significantly improved. Furthermore, the hole and electron reorganization energies (λ) of (FPM)n (n = 1-4) close with each other, which means (FPM)n (n = 1-4) have a good charge-carrier transport balance. Thus, we infer that (FPM)n can be used as a single-layer electroluminescent material with high efficient luminescence.