To gain new insights into ligand-to-ligand charge-transfer (LLCT) dynamics, we synthesised two heteroleptic Ir³⁺ complexes: (Ir(dfppy)₂(tpphz)) and (Ir(dfppy)₂(dpq)), where dfppy, tpphz, and dpq are 2-(4,6-difluorophenyl)pyridine, tetrapyrido[3,2-a:2′,3′-c:3′′,2′′-h:2′′′,3′′′-j]phenazine, and 2,3-bis-(2-pyridyl)-quinoxaline, respectively. The tpphz and dpq ligands have longer π-conjugation than dfppy. Therefore, the excited ligand-centred (LC) state and the metal-to-ligand charge transfer (MLCT) state of dfppy are higher than those of tpphz and dpq. The LLCT dynamics from dfppy to tpphz (or dpq) was probed using femtoscond transient absorption (TA) spectroscopy after the selective excitation of dfppy. The TA band for the LC/MLCT state of dfppy is observed at 480 nm. Because of the LLCT process, the TA bands related to the MLCT states of tpphz and dpq ligands increased, whereas those of dfppy decreased. The time constants for the LLCT process were 17 ps for Ir(dfppy)₂(tpphz) and 5 ps for Ir(dfppy)₂(dpq). The MLCT emission of Ir(dfppy)₂(tpphz) showed strong temperature dependence, indicating that the LLCT process has a significant energy barrier. In comparison, the temperature weakly influenced the emission of Ir(dfppy)₂(dpq), and thus, its LLCT process may have a smaller barrier. The anomalous rigidochromism and photodynamic behaviours can be explained in terms of the barrier between cyclometalating and ancillary ligands.

A series of donor–acceptor–donor (D–A–D) pyrene (Py) 2,7-position-based compounds (CN, F, H, Me, and OMe) were designed and synthesized to demonstrate facile intra- and intermolecular charge transfer control by an electron push–pull effect for an efficient mechanofluorochromic (MFC) material. An examination of the photophysical properties of CN–OMe revealed that the locally excited (LE) and intramolecular charge transfer (ICT) state of the compounds were modulated finely by the electron push–pull substituent effect. In addition, it is noteworthy that the CN emission originated only from the LE state despite the D–A–D molecular system. Moreover, we confirmed that the selective LE and/or ICT state modulation affects intra- and intermolecular charge transfer control in the solid-state emission. In particular, this intra- and intermolecular charge transfer control is directly dominated by MFC phenomena, which means that the molecular electron push–pull substituent effect is dependent on MFC behavior. As a result, this study shows that as the electron-withdrawing group ability increases, the intramolecular interactions become insufficient and induce strong intermolecular D–A interactions for stabilizing molecules, leading to high MFC efficiency that is reversibly repeated several times.

The molecular geometry effects of ortho, meta, and para-terphenyl based donor–π–acceptor (D–π–A) dyads on intramolecular charge transfer (ICT) were studied to investigate structure-ICT relationships. Terphenyl based D–π–A dyads were prepared by two-step palladium catalyzed, Suzuki–Miyaura coupling reactions, in which triphenylamine (TPA) was used as the electron donor and 1,2-diphenyl-benzimidazole (IMI) as the electron acceptor. The photophysical and electrochemical properties of terphenyl backbone-based ortho (O), meta (M), and para (P) dyads were compared. In steady state absorption spectra, a red-shift of CT band was observed in the order O < M < P, which was attributed to terphenyl isomer conjugation effects and agreed well with density functional theory (DFT) based calculations. In particular, the emission spectra of the three terphenyl D–π–A dyads produced showed similar emission maxima at ∼475 nm and a bathochromic shift property was observed in order to increase the solvent polarity, indicating the ICT process. From Lippert–Mataga plots, excited-state dipole moment changes (Δμ) were estimated to be 31.5 Debye (D) for O, 62.9 D for M, and 51.5 D for P. For M isomer, a large Δμ and the markedly reduced quantum yield was shown, as well as photo-induced electron transfer (PET) was expected in the excited state, but no radical species were observed by femtosecond transient absorption (TA) measurements. Based on experimental results, we conclude that all three terphenyl based D–π–A dyads, including non-conjugated ortho- and meta-terphenyl dyads, exhibit partial charge transfer rather than unit-electron transfer.