Red fluorescent molecules suffer from large, non-radiative internal conversion rates (kIC) governed by the energy gap law. To design efficient red thermally activated delayed fluorescence (TADF) emitters for organic light-emitting diodes (OLEDs), a large fluorescence rate (kF) as well as a small energy difference between the lowest singlet and triplet excited states (ΔEST) is necessary. Herein, we demonstrated that increasing the distance between donor (D) and acceptor (A) in intramolecular-charge-transfer molecules is a promising strategy for simultaneously achieving small ΔEST and large kF. Four D-Ph-A-Ph-D-type molecules with an anthraquinone acceptor, phenyl (Ph) bridge, and various donors were designed, synthesized, and compared with corresponding D-A-D-type molecules. Yellow to red TADF was observed from all of them. The kF and ΔEST values determined from the measurements of quantum yield and lifetime of the fluorescence and TADF components are in good agreement with those predicted by corrected tim...

Anthraquinone-Based Intramolecular Charge-Transfer Compounds: Computational Molecular Design, Thermally Activated Delayed Fluorescence, and Highly Efficient Red Electroluminescence

This report describes new, readily accessible copper(I) complexes that can exhibit unusually long-lived, high quantum yield emissions in fluid solution. The complexes are of the form [Cu(NN)(POP)]+ where NN denotes 1,10-phenanthroline (phen), 2,9-dimethyl-1,10-phenanthroline (dmp) or 2,9-di-n-butyl-1,10-phenanthroline (dbp) and POP denotes bis[2-(diphenylphosphino)phenyl] ether. Modes of characterization include X-ray crystallography and cyclic voltammetry. The complexes each have a pseudotetrahedral coordination geometry and a Cu(II)/Cu(I) potential upward of +1.2 V vs Ag/AgCl. In room-temperature dichloromethane solution, charge-transfer excited states of the dmp and dbp derivatives exhibit respective emission quantum yields of 0.15 and 0.16 and corresponding excited-state lifetimes of 14.3 and 16.1 μs, respectively. Despite the fact that coordinating solvents usually quench charge-transfer emission from copper systems, the photoexcited dmp (dbp) complex retains a lifetime of 2.4 μs (5.4 μs) in methanol.

Simple Cu(I) complexes with unprecedented excited-state lifetimes.

Luminescent complexes beyond the platinum group: the d10 avenue

Cu(I) complexes and clusters are the largest class of compounds of relevant photochemical and photophysical interest based on a relatively abundant metal element. Interestingly, Nature has given an essential role to copper compounds in some biological systems, relying on their kinetic lability and versatile coordination environment. Some basic properties of Cu(I) and Cu(II) such as their coordination geometries and electronic levels are compared, pointing out the limited significance of Cu(II) compounds (d 9 configuration) in terms of photophysical properties. Well-established synthetic protocols are available to build up a variety of molecular and supramolecular architectures (e.g. catenanes, rotaxanes, knots, helices, dendrimers, cages, grids, racks, etc.) containing Cu(I)-based centers and exhibiting photo- and electroluminescence as well as light-induced intercomponent processes. By far the largest class of copper complexes investigated to date is that of Cu(I)-bisphenanthrolines ([Cu(NN)2]+) and recent progress in the rationalization of their metal-to-ligand charge-transfer (MLCT) absorption and luminescence properties are critically reviewed, pointing out the criteria by which it is now possible to successfully design highly emissive [Cu(NN)2]+ compounds, a rather elusive goal for a long time. To this end the development of spectroscopic techniques such as light-initiated time-resolved X-ray absorption spectroscopy (LITR-XAS) and femtosecond transient absorption have been rather fruitful since they have allowed us to firmly ground the indirect proofs of the molecular rearrangements following light absorption that had accumulated in the past 20 years. A substantial breakthrough towards highly emissive Cu(I) coordination compounds is constituted by heteroleptic Cu(I) complexes containing both N- and P-coordinating ligands ([Cu(NN)(PP)]+) which may exhibit luminescence quantum yields close to 30% in deaerated CH2Cl2 solution and have been successfully employed as active materials in OLED and LEC optoelectronic devices. Also copper clusters may exhibit luminescence bands of halide-to-metal charge transfer (XMCT) and/or cluster centered (CC) character and they are briefly reviewed along with miscellaneous Cu(I) compounds that recently appeared in the literature, which show luminescence bands ranging from the blue to the red spectral region.

Photochemistry and Photophysics of Coordination Compounds: Copper

1,10-Phenanthroline: a versatile building block for the construction of ligands for various purposes

MLCT excited states of cuprous bis-phenanthroline coordination compounds

Cuprous bis-phenanthroline compounds possess metal-to-ligand charge transfer, MLCT, excited states. Phenanthroline ligands coordinated to Cu(I) that are disubstituted in the 2- and 9-positions with alkyl or aryl groups, abbreviated CuI(phen′)2+, have long-lived excited states at room temperature. The parent CuI(phen)2+ compound is non-emissive under the same conditions with a short excited state lifetime, τ<10 ns. Disubstitution in the 2,9-positions stabilizes the Cu(I) state and increases the energy gap between the MLCT and the ground state. The prototypical and most well studied compound is CuI(dmp)2+, where dmp is 2,9-(CH3)2-1,10-phenanthroline. In dichloromethane solution at room temperature, CuI(dmp)2+ displays broad MLCT absorption with λmax=454 nm, a broad unstructured emission with λmax=730 nm, and an excited state lifetime of 85 ns. The emission arises from two closely spaced MLCT excited states, separated in energy by 1800 cm−1, that behave as one state at room temperature. CuI(dmp)2+* excited states are quenched in the presence of Lewis bases and coordinating solvents. A 5-coordinate excited state complex, or exciplex, is proposed to account for temperature dependent quenching data. The substantial inner-sphere reorganizational energy changes that follow light excitation are novel features of these MLCT excited states. This review attempts to cover all the existing data reported on CuI(phen′)2+ excited states and contrast it with well-known MLCT behavior of (dπ)6 transition metal compounds.

MLCT Excited States of Cuprous Bis‐phenanthroline Coordination Compounds

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MLCT excited states of cuprous bis-phenanthroline coordination compounds

MLCT Excited States of Cuprous Bis‐phenanthroline Coordination Compounds