Photophysical Properties of Intramolecular Charge Transfer in Two Newly Synthesized Tribranched Donor−π−Acceptor Chromophores

Abstract

The nature of optical excitation and the degree of intramolecular charge transfer (ICT) as well as the dynamics of excited ICT states of two new tribranched donor-pi-acceptor molecules with acceptor-terminated (DA(3)) and acceptor-centered (AD(3)) geometries have been investigated by steady-state and femtosecond time-resolved stimulated emission fluorescence depletion (FS TR-SEP FD) measurements in different polar solvents. The interpretation of the experimental results is based on the comparative investigation of the two D-pi-A compounds with respect to the model monomer counterpart (DA). The larger solvatochromic effects and stronger solvent dependence of spectral properties of DA(3) than that of AD(3) indicate that the excited ICT state of DA(3) possesses higher polarity and larger dipole moments compared to those of AD(3). The similarity of absorption and strong solvent-dependent fluorescence spectra of DA(3) and DA reveals that the excited-state properties of DA(3) are identical to that of the model DA, which localized on one of the branches in DA(3). In contrast to DA(3), the large red shift in the absorption and the small Stokes shift of AD(3) suggest the formation of a delocalized ICT state to a certain extent in the excited state of AD(3). The dynamic behavior of excited ICT states for all three compounds are also investigated by femtosecond time-resolved stimulated emission depletion (FS TR-SEP FD) measurements, where the excited-state relaxations are highly dependent on both solvent polarity and the polar degree of the excited ICT states. Furthermore, the steady-state fluorescence excitation anisotropy shows that the intramolecular excitation transfer among the three disorder-induced localized ICT states with nondegenerate transition dipole moments is involved within DA(3). Compared to DA(3), a substantial red shift in the absorption of AD(3) results from the formation of a delocalized ICT state, where the specific excitation anisotropy spectrum shows that the excitation energy is mainly redistributed between the localized ICT state and the delocalized ICT state.

Twisted Intramolecular Charge Transfer State for Long-Wavelength Thermally Activated Delayed Fluorescence

“…Such delocalization of the ICT state over the multiple D−A structures stabilizes it. 16 The energy of the delocalized ICT excited state is changed by the torsional disorder between the PXZ donor units and the TRZ acceptor unit, which lengthen the lifetime (τ 2 were 1.33 and 1.10 μs for bis-and tri-PXZ-TRZ, respectively). However, τ 2 of tri-PXZ-TRZ was shorter than that of bis-PXZ-TRZ.…”

Solvent-dependent intramolecular charge transfer delocalization/localization in multibranched push-pull chromophores

“…All the compounds show two intense absorption bands in the near-UV around 320 nm and visible spectral regions from 400 to 500 nm, and the low-energy absorption around 450 nm for all the chromophores is ascribed to the charge-transfer transition, while the high energy absorption band arises from π − π * transition. 7,8,15 Typically, the slightly red-shift ICT state absorption of trimer compared to that of the monomer compound indicates that the three branches in trimer compound are mutually conjugated though the common core, giving rise to a delocalized electron system in two dimensions. Furthermore, as shown in Fig.…”

“…1-4 It is well known that local environments, especially solvents, have profound effects on the emission behavior of ICT type compounds, [5][6][7][8][9][10][11] where the surrounding solvent molecules respond by slightly modifying their translational and rotational motion and then eventually dissipate the excitation energy of excited ICT chromophore. [12][13][14] The degree of intramolecular charge transfer in multibranched chromophores can be estimated from the dynamics of solvent-coupled excited state relaxation.…”

Localized Emitting State and Energy Transfer Properties of Quadrupolar Chromophores and (Multi)Branched Derivatives

“…Since the depolarization of the emission through rotational diffusion of molecules itself is strongly suppressed by the dry polystyrene film, an intramolecular excitation energy transfer mechanism from high-energy to low-energy ICT states should be involved in explaining the observed depolarization. 12,33,66,69,70 By taking a closer look at the anisotropy spectra of chromophores 2 and 3 from Figure 3a, it is found that, similarly with chromophore 1, anisotropy values are relatively low in the region from 400 to 440 nm. As shown in Scheme 2 and Table S2 (Supporting Information), in this region, the second excited state |2 1q ⟩ is split into |3 2q ⟩ and |4 2q ⟩ for chromophore 2, and into |4 3q ⟩, |5 3q ⟩, and |6 3q ⟩ for chromophore 3.…”