Emission Quenching in Tetraphenylfuran Crystal: Why This Propeller-Shaped Molecule Does Not Emit in the Condensed Phase?

Due to their substantial fluorescence quantum yields in the crystalline phase, propeller-shaped molecules have recently gained significant attention as potential emissive materials for optoelectronic applications. For the family of cyclopentadiene derivatives, light-emission is highly dependent on the nature of heteroatomic substitutions. In this paper, we investigate excited state relaxation pathways in the tetraphenyl-furan molecule (TPF), which in contrast with other molecules in the family, shows emission quenching in the solid-state. For the singlet manifold, our calculations show nonradiative pathways associated with C-O elongation are blocked in both vacuum and the solid state. A fraction of the population can be transferred to the triplet manifold and, subsequently, to the ground state in both phases. This process is expected to be relatively slow due to the small spin-orbit couplings between the relevant singlet-triplet states. Emission quenching in crystalline TPF seems to be in line with more efficient exciton hopping rates. Our simulations help clarify the role of conical intersections, population of the triplet states and crystalline structure in the emissive response of propeller-shaped molecules.

Application of time-resolved electron paramagnetic resonance spectroscopy in the mechanistic study of thermally activated delayed fluorescence (TADF) materials

Triplet exciton harvesting is crucial in organic light emitting diodes (OLEDs), because the triplet states produced by electron and hole recombination is up to 75% of the total excitons, whereas...

Temperature Dependent Emission Properties of ReI Tricarbonyl Complexes with Dipyrido-Quinoxaline and Phenazine Ligands

In this work, the emission properties of fac-[Re(CO)3(NN)(py)]+, NN = 1,10-phenanthroline (phen), dipyrido[3,2-f:2’,3’-h]quinoxaline (dpq) and dipyrido[3,2-a:2’3’-c]phenazine (dppz); py = pyridine were investigated in different temperatures, ranging from 80 to 300 K, and in different solvent mixtures and in polymethyl methacrylate. The changes observed in the emission quantum yields were rationalized based on a two-level excited state model, in which the nonemissive upper state is thermally populated and decays faster than the lowest lying emissive state. fac‑[Re(CO)3(dpq)(py)]+ is a metal-to-ligand charge transfer (MLCT) emitter as the complex with phen but exhibits smaller emission quantum yields, being more sensitive to the solvent. This behavior was rationalized by quantum-mechanical calculations including the spin-orbit coupling matrix elements, revealing that intersystem crossing from the lowest singlet excited state in fac- [Re(CO)3(dpq)(py)]+ likely occurs to triplet states lying at higher energies. Similar behavior were observed for fac-[Re(CO)3(dppz)(py)]+, although the later exhibits intraligand emission that are strongly quenched in fluid solutions by low-lying MLCT states. The fundamental studies carried out here provide new insights on the excited state dynamics of ReI complexes with dipyridoquinoxaline and phenazine ligands and can contribute for further advances on their application as luminescent probes.

Searching Building Blocks with Large Excited-State Gaps in Five-memebered Heterocyclic Rings by Aromaticity Strategy

Organic molecules with large gap between the excited singlet/triplet states can be applied in hot exciton emission and singlet fission to beat the spin statistics limit in optoelectronic devices. Herein, a novel strategy is proposed for achieving large triplet-triplet gap (E(T1T2)) and singlet-triplet gap (E(S1T1)) by manipulating the aromaticity of the low-lying excited states. Partially conjugated five-membered heterocyclic rings are found to naturally have low E(T1) owing to high aromaticity obeying Baird’s Rule. Utilizing such ring (pyrazoline) as a bridge and selecting various donor and acceptor moieties, numbers of derivatives have been theoretically designed with tunable emission colors, significantly large E(T1T2) and E(S1T1), and satisfying E(T2)>E(S1)≥2E(T1). The ultrafast spectroscopy and sensitization measurements for one of them with blue fluorescence (named TPA-DBPrz) confirm the calculated prediction. This work opens a new avenue and provides molecular units to develop high-efficiency optoelectronic materials.

Migration of Excitation Energy in Furocoumarins

The migration of excitation energy of a number of psoralen compounds has been studied. For this, the methods of induced absorption spectroscopy, stationary electron spectroscopy, fluorescence and phosphorescence, as well as quantum chemistry were used. A comparative photostability of psoralen was achieved by exposure to a XeCl excilamp irradiation (emission wavelength λem = 308 nm) with parameters Δλ = 5–10 nm, Wpeak = 18 mW/cm2, p = 8.1 J/cm3, f = 200 kHz, pulse duration 1 μs. It was found that the singlet-triplet transition played a major role in the migration of excitation energy into triplet states. Among all tested compounds, substances with an OCH3-group in the structure have the strongest effect on the spectral-luminescent characteristics.

Formation characteristics of aerosol triplet state and coupling effect between the separated components with different polarity

Atmospheric aerosols contain organic matter that can form triplet state (3C*) excited by sunlight, which plays a critical role in the aging process of aerosols. In order to understand the triplet state reaction mechanism of complex aerosol components, the formation characteristics of 3C* in the aerosol components with different polarity, i.e., the highly polar water-soluble matter (HP-WSM), humic-like substances (HULIS) and methanol-soluble matter (MSM) were investigated. The coupling effect of generation of 3C* and reactive oxygen species (ROS) between different aerosol components was also examined. The results show that the 3C* generation characteristics is strongly dependent on the polarity of these components. HULIS has the strongest generation ability of 3C*, and the MSM contribute the most to the total generation of 3C*. It is found that the high-energy triplet states (ET ≥ 250 kJ mol−1) of HULIS and HP-WSM components account for up to 80 %. These 3C* has an important contribution to the photochemically generation of ROS, and the generated ROS of different components are also different, which is determined by the chromophore composition of complex organic matter. Tyrosine-like chromophore is the main substance leading to the formation of water-soluble 3C*, whilethe highly oxidized HULIS chromophore plays a leading role in the water-insoluble component. This study also found that there is a coupling effect between HP-WSM and HULIS on 3C* and ROS generation. The 3C* generation rate increases by about 40 % after mixing, but the generation of 1O2 is severely reduced. Overall, this study provides deep insights into the generation characteristics of the triplet state of atmospheric aerosols.

Phosphorescent Organometallic Knots of Gold(I)-Bis(acetylide) Strands Directed by Copper(I) π-Coordination

Through elaborate ligand design to create knotted structures with specific topologies is a major challenge for chemists. In this work, the self-assembly between U-shape 3,6-di-tert-butyl-1,8-diethynyl-9H-carbazole (H2L) and Au+ through gold(I)-bis(acetylide) linkages under π-bonded Cu+ template gives rise to complex 1 with two interlocked metallostrands as well as complexes 3 (n = 3) and 4 (n = 4) with [(AuL)n]n- metallostrands showing trefoil knot topology. Upon incorporating two [Au(dppb)Au]2+ (dppb = Ph2P(CH2)4PPh2) moieties through bis(Au-acetylide) coordination bonds, the interlocked structure (1) is fully closed to form a figure-eight knotted structure in complex 2. The folding and threading of metallocyclic strings are directed by Cu+, which are π-ligated to two or three acetylides to generate double-folding or triple-folding cross points. Complexes 1-4 show intense phosphorescence in both solutions and solid states at ambient temperature, originating from admixture of metal centered 3[d®p/s], 3IL (intraligand), and 3[p (L) ® s/p (Au/Cu)] 3LMCT triplet states.