Direct Evidence of Solvent Polarity Governing the Intramolecular Charge and Energy Transfer: Ultrafast Relaxation Dynamics of Push-Pull Fluorene Derivative

Photoinduced charge and energy transfer are significant photophysical processes controlling the efficiency of the photosynthesis and molecular electronics. Here the influence of solvent polarity and excitation wavelength on the dynamics of excited state relaxation pathways of push–pull chromophore (PXFCN), where phenoxazine and cyano fluorene acted as donor and acceptor respectively, are investigated in detail by using steady state spectroscopy, nanosecond and femtosecond transient absorption spectroscopy and picosecond emission spectroscopy. In acetonitrile (ACN), the steady state emission spectra of PXFCN exhibited three maxima at around 330, 405 and 620 nm covering the complete continuum range (CIE coordinates of 0.32, 0.40) with absolute quantum yield of ~ 0.12. The aggregation induced emission with an increase quantum yield of ~ 0.32 was observed in tetrahydrofuran and water mixture due to the formation of nano-aggregates. Interestingly the steady state and time resolved emission spectra of PXFCN in ACN obtained by exciting at different wavelengths exhibited the occurrence of both the intramolecular charge and energy transfer processes, whereas in cyclohexane (CHX) the emission originated mainly from the local excited state and revealing efficient intramolecular energy transfer. The femtosecond transient absorption spectra in the polar solvent, ACN shows that the excited state relaxation pathway is controlled by solvent stabilized twisted intramolecular charge transfer dynamics limiting the formation of triplet state. However in the case of CHX, the charge transfer state formed upon photoexcitation decayed to the triplet state by geminate charge recombination. The nanosecond transient absorption spectra manifest the dominant feature of triplet state and the charge transfer state in CHX and ACN respectively and their complete dynamics were obtained. Thus based on the transient absorption and emission spectra, it is inferred that the intramolecular charge transfer occurring along with the energy transfer is controlled by the polarity of the solvent through conformational changes leading to favourable position yielding the charge and energy transfer between the donor and acceptor moieties.