Enhanced Intersystem Crossing, Yet Still Fluorescence Upon Introduction of Intermediate Charge-Transfer States in Hemicaged [Zn(bpy)3]2.

Photoactive zinc(II) complexes typically undergo fluorescence from the singlet excited state as the dominant radiative pathway, as the operative spin-orbit coupling is usually very small and phosphorescence from the triplet state is strongly forbidden. Although dicationic zinc(II) tris(bipyridine) strictly follows this scheme with fluorescence at λem = 326 nm, constructing the ligand sphere as a hemicage was reported to lead to quantitative intersystem crossing (ISC) and subsequent fast phosphorescence with λem = 485 and a short radiative lifetime of ca. 1 µs. Surprised by this finding, we reinvestigated [Zn(bpy)3]2+ and its hemicage derivative in great detail, including variable temperature and time-resolved photophysical measurements in solution and solid state as well as high-level theoretical calculations to resolve their excited state behavior. Our investigations suggest that both compounds undergo fluorescence at room temperature with significantly different radiative rate constants of kr = 2 × 108 and 1.2 × 106 s-1, respectively, and only weak phosphorescence on the millisecond time scale at low temperatures. The major difference is the occurrence of additional charge-transfer states within the ligand scaffold of the hemicage, which accelerate the ISC to the 3LC(bpy) state from 350 s down to 82 ns and reduce the fluorescence rate constant.

Mechano-Stimulus and Environment-Dependent Circularly Polarized TADF in Chiral Copper(I) Complexes and Their Application in OLEDs.

Molecular emitters that combine circularly polarized luminescence (CPL) and high radiative rate constants of the triplet exciton decay are highly attractive for electroluminescent devices (OLEDs) or next-generation photonic applications, such as spintronics, quantum computing, cryptography, or sensors. However, the design of such emitters is a major challenge because the criteria for enhancing these two properties are mutually exclusive. In this contribution, we show that enantiomerically pure {Cu(CbzR)[(S/R)-BINAP]} [R = H (1), 3,6-tBu (2)] are efficient thermally activated delayed fluorescence (TADF) emitters with high radiative rate constants of kTADF up to 3.1 × 105 s-1 from 1/3LLCT states according to our temperature-dependent time-resolved luminescence studies. The efficiency of the TADF process and emission wavelengths are highly sensitive to environmental hydrogen bonding of the ligands, which can be disrupted by grinding of the crystalline materials. The origin of this pronounced mechano-stimulus photophysical behavior is a thermal equilibrium between the 1/3LLCT states and a 3LC state of the BINAP ligand, which depends on the relative energetic order of the excited states and is prone to inter-ligand C-H···π interactions. The copper(I) complexes are also efficient CPL emitters displaying exceptional dissymmetry values glum of up to ±0.6 × 10-2 in THF solution and ±2.1 × 10-2 in the solid state. Importantly for application in electroluminescence devices, the C-H···π interactions can also be disrupted by employing sterically bulky matrices. Accordingly, we have investigated various matrix materials for successful implementation of the chiral copper(I) TADF emitters in proof-of-concept CP-OLEDs.

Ultrafast photoinduced relaxation dynamics of the indoline dye D149 in organic solvents.

The relaxation dynamics of the indoline dye D149, a well-known sensitizer for photoelectrochemical solar cells, have been extensively characterized in various organic solvents by combining results from ultrafast pump-supercontinuum probe (PSCP) spectroscopy, transient UV-pump VIS-probe spectroscopy, time-correlated single-photon counting (TCSPC) measurements as well as steady-state absorption and fluorescence. In the steady-state spectra, the position of the absorption maximum shows only a weak solvent dependence, whereas the fluorescence Stokes shift Δν̃(F) correlates with solvent polarity. Photoexcitation at around 480 nm provides access to the S(1) state of D149 which exhibits solvation dynamics on characteristic timescales, as monitored by a red-shift of the stimulated emission and spectral development of the excited-state absorption in the transient PSCP spectra. In all cases, the spectral dynamics can be modeled by a global kinetic analysis using a time-dependent S(1) spectrum. The lifetime τ(1) of the S(1) state roughly correlates with polarity [acetonitrile (280 ps) < acetone (540 ps) < THF (720 ps) < chloroform (800 ps)], yet in alcohols it is much shorter [methanol (99 ps) < ethanol (178 ps) < acetonitrile (280 ps)], suggesting an appreciable influence of hydrogen bonding on the dynamics. A minor component with a characteristic time constant in the range 19-30 ps, readily observed in the PSCP spectra of D149 in acetonitrile and THF, is likely due to removal of vibrational excess energy from the S(1) state by collisions with solvent molecules. Additional weak fluorescence in the range 390-500 nm is observed upon excitation in the S(0)→S(2) band, which contains short-lived S(2)→S(0) emission of D149. Transient absorption signals after excitation at 377.5 nm yield an additional time constant in the subpicosecond range, representing the lifetime of the S(2) state. S(2) excitation also produces photoproducts.