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A solution processible organic conjugated molecule 2-[2,6-bis-(2-{4-[2-(4-diphenylaminophenyl)vinyl]phenyl}vinyl)pyran-4-ylidene]malononitrile (TPA−DCM−TPA) was designed and synthesized for application in organic solar cells (OSCs). The molecule consists of two electron-rich triphenylamine moieties and one electron-deficient 2-pyran-4-ylidenemalononitrile unit linked with conjugated bridges. The optical and electrochemical properties, the light-induced electron spin resonance (LESR), and the electronic ground state configuration of the compound were characterized. TPA−DCM−TPA film showed a broad absorption band covering from 350 to 650 nm. The bulk-heterojunction OSCs with the device structure of ITO/PEDOT:PSS/TPA−DCM−TPA:PCBM/LiF/Al or Ba/Al were fabricated, in which TPA−DCM−TPA was used as donor and PCBM as acceptor material. The open-circuit voltage, short-circuit current, and power-conversion efficiency of the optimized OSC with Ba/Al as cathode, TPA−DCM−TPA:PCBM = 1:3 (w/w), and ca. 85 nm thickness of the active layer reached 0.9 V, 2.14 mA/cm2, and 0.79%, respectively, under the illumination of AM1.5, 100 mW/cm2. The results indicate that TPA−DCM−TPA is a promising photovoltaic organic molecule.

The photophysical properties of two newly synthesized photoactive compounds with asymmetrical D-π-A structure and symmetrical D-π-A-π-D structure are investigated in different aprotic solvents by steady-state and femtosecond fluorescence depletion measurements. It is found that the asymmetrical DA compound has larger dipole moment change than that of the symmetrical DAD compound upon excitation, where the dipole moments of the two compounds have been estimated using the Lippert-Mataga equation. Furthermore, the steady-state spectral results show that increasing solvent polarity results in small solvatochromic shift in the absorption maxima but a large red shift in the fluorescence maxima for them, indicating that the dipole moment changes mainly reflect the changes of dipole moment in excited-state rather than in ground state. The redshifted fluorescence band is attributed to an intramolecular charge transfer (ICT) state upon photoexcitation, which could result in a strong interaction with the surrounding solvents to cause the fast solvent reorganization. The resulting ICT states of symmetrical compounds are less polar than the asymmetrical compounds, indicating the different extents of stabilization of solute-solvent interaction in the excited state. Femtosecond fluorescence depletion measurements are further employed to investigate the fast solvation effects and dynamics of the ICT state of these two novel compounds. The femtosecond fluorescence depletion results show that the DA compound has faster solvation time than that of DAD compound, which corresponds to the formation of relaxed ICT state (i.e., a final ICT state with rearranged solvent molecules after solvation) in polar solvents. It is therefore reasonably understood that the ICT compounds with asymmetrical (D-π-A) structure have better performance for those photovoltaic devices, which strongly rely on the nature of the electron pushpull ability, compared to those symmetrical compounds (D-π-A-π-D).