ABSTRACT
A comprehensive investigation into the spectral properties and excitation energy transfer in di- or tribranched dithienyldiketopyrrolopyrrole (DPP) molecules with meta-substituted benzene as a central core (TADPP2-TT and TADPP3), by means of steady-state and transient measurements and quantum chemical calculations, is reported. Excitation in the meta-substituted chromophores is localized on one of the DPP units in the branching molecules owing to the disruption of conjugation by meta substitution for both TADPP2-TT and TADPP3. Weak electronic couplings result from the long distance between the DPP chromophores and the steric effects from the attached side chains on DPP. The attachment of the acetylene linker in each branch, which has a very low twisting barrier, could also play an important role in reducing the interaction between DPP chromophores. The dynamics of the excited states show that excitation energy transfer occurs on the picosecond scale, which is attributed to the incoherent hopping mechanism.
ABSTRACT
Excited state solvation plays a very important role in modulating the emission behavior of fluorophores upon excitation. Here, the solvation effects on the local micro-environment around a fluorophore are proposed by investigating the fantastic emission behavior of a novel amyloid fibril marker, NIAD-4, in different alcoholic and aprotic solvents. In alcoholic solvents, high solvent viscosity causes an obvious enhancement of fluorescence because of the restriction of torsion of NIAD-4, where the formation of a non-fluorescent twist intramolecular charge transfer (TICT) state is suppressed. In aprotic solvents, high solvent polarity leads to a remarkable redshift of the emission spectra suggesting strong solvation. Surprisingly, an abnormal fluorescence enhancement of NIAD-4 is observed with increasing solvent polarity of the aprotic solvents, whereas solvent viscosity plays little role in influencing the fluorescence intensity. We conclude that such an abnormal phenomenon is originated from a solvation induced micro-viscosity enhancement around the fluorophore upon excitation which restricts the torsion of NIAD-4. Femtosecond transient absorption results further prove such a micro-viscosity increasing mechanism. We believe that this solvation induced micro-viscosity enhancement effect on fluorescence could widely exist for most donor-π-acceptor (D-π-A) compounds in polar solvents, which should be carefully taken into consideration when probing the micro-viscosity in polar environments, especially in complex bioenvironments.
ABSTRACT
We report the excited-state intramolecular charge transfer (ICT) characteristics of four tetrahydro[5] helicene-based imide (THHBI) derivatives with various electron-donating substitutes in different polarity of solvents using steady-state, time-resolved transient absorption (TA) spectroscopy. It is found that, the small bathochromic-shift of the absorption spectra but large red shift of the emission spectra for all dyes with increasing solvent polarity indicates the larger dipole moment of the excited state compared to ground state. The results of theoretical calculations exhibit the charge transfer from the terminal donors to helical backbone, which accounts for the degrees of red shift of the emission spectra from different extent of ICT nature. Time-resolved TA spectra recorded as a function of electron-donating substitutes and solvent polarity show the dye with stronger donors (THHBI-PhNPh2) in more polar solvent behaves faster excited-state ICT relaxation, leading to the formation of solvent-stabilized ICT state (ICT' state) from the excited ICT state; The dyes (THHBI-Ph, THHBI-PhCF3 and THHBI-PhOMe) with relative weaker donors show weaker dependence on solvent polarity, and instead of that intersystem crossing (ISC) becomes possible from ICT state to triplet state.
ABSTRACT
The photophysical properties of three octupolar chromophores containing planar triazatruxene (TAT) as the central electron donor with different electron-withdrawing groups in the tribranched arrangement have been systematically investigated by means of steady state and transient spectroscopy. The multidimensional intramolecular charge transfer (ICT) properties of these tribranched chromophores related to the observed two-photon absorption (TPA) properties are explored by estimating the TPA essential factors (Mge and Δµge). Besides the large Stokes shift between steady state absorption and fluorescence spectra in different polar solvents, photoinduced ICT was further demonstrated by quantum-chemical calculations and transient absorption measurements. Both quantum calculations and spectral experiments show that a multidimensional ICT occurs from the electron-rich core to the electron-deficient periphery of these TAT derivatives. The results of solvation effects and the dynamics of the excited states show that the excited states of these three chromophores tend to exhibit an excitation localization on one of the dipolar branches, which is beneficial to achieve large Mge and Δµge, thus leading to enhanced TPA properties.
ABSTRACT
The photophysical properties of intramolecular charge transfer (ICT) in a novel tribranched donor-π-acceptor chromophore, triphenoxazine-2,4,6-triphenyl-1,3,5-triazine (tri-PXZ-TRZ), with thermally activated delayed fluorescence character was investigated in different aprotic solvents by steady-state spectroscopy and femtosecond and nanosecond transient absorption spectroscopy measurements. Increasing the solvent polarity led to a significant increase in the Stokes shift. The large Stokes shift in highly polar solvents was attributed to ICT properties upon excitation; this resulted in a strong interaction between the tri-PXZ-TRZ molecule and the surrounding solvent, which led to a strong solvation process. Quantum-chemical calculations and changes in the dipole moment showed that this compound has a large degree of ICT. Furthermore, an apolar environment helped to preserve the symmetry of tri-PXZ-TRZ and to enhance its emission efficiency. The femtosecond and nanosecond transient absorption spectroscopy results indicated that the excited-state dynamics of this push-pull molecule were strongly influenced by solvent polarity through the formation of a solvent-stabilized ICT state. The excited-state relaxation mechanism of tri-PXZ-TRZ was proposed by performing target model analysis on the femtosecond transient absorption spectra. In addition, the delayed fluorescence of tri-PXZ-TRZ was significantly modulated by a potential competition between solvation and intersystem crossing processes.
