Short Excited-state Lifetime Research Articles

Fabrication and Deterministic Transfer of High-Quality Quantum Emitters in Hexagonal Boron Nitride

Color centers in solid state crystals have become a frequently used system for single photon generation, advancing the development of integrated photonic devices for quantum optics and quantum communication applications. In particular, defects hosted by two-dimensional (2D) hexagonal boron nitride (hBN) are a promising candidate for next-generation single photon sources, due to its chemical and thermal robustness and high brightness at room temperature. The 2D crystal lattice of hBN allows for a high extraction efficiency and easy integration into photonic circuits. Here we develop plasma etching techniques with subsequent high temperature annealing to reliably create defects. We show how different fabrication parameters influence the defect formation probability and the emitter brightness. A full optical characterization reveals the higher quality of the created quantum emitters, represented by a narrow spectrum, short excited state lifetime and high single photon purity. We also investigated the photostability on short and very long timescales. We utilize a wet chemically-assisted transfer process to reliably transfer the single photon sources onto arbitrary substrates, demonstrating the feasibility for the integration into scalable photonic quantum information processing networks.

Optical properties of silicon-vacancy color centers in diamond created by ion implantation and post-annealing

Silicon-vacancy (SiV) color centers have been created in diamond by ion implantation and post annealing at LABEC (Florence). A wide range of implantation depths (0–2.4 μm) and fluences (108–1015 cm−2), along with a variety of substrates (single and poly-crystals) have been explored. The photoluminescence properties of the SiV centers have been studied at room temperature, including their single-photon emission characteristics. Single-photon emitters have been obtained at the lower-end of the implantation fluences range. They exhibit a short excited-state lifetime (~1 ns), a strong zero-phonon transition with a narrow linewidth (~1.6 nm) and a very small inhomogeneous broadening (0.015 nm), features that qualify them for application in quantum optical technologies. The activation yield of SiV centers has been assessed under different experimental conditions. It has been found to be independent of the implantation energy and in the range of 3% after thermal annealing.

Excited-State Behaviors of M1Au24(SR)18 Nanoclusters: The Number of Valence Electrons Matters

Doping is a quite useful strategy for probing the structure and properties of metal nanoclusters, but the effect of doping on the photodynamical properties is still not fully understood. Here, we reveal that the number of valence electrons plays a major role in determining the photodynamics of M1Au24(SR)18 nanoclusters. By carrying out temperature-dependent optical absorption, it is found that Cd doping enhances electron–phonon coupling while Hg doping does not significantly alter the coupling. Moreover, the relaxation dynamics of [M1Au24(SR)18]0 (M = Hg/Cd) nanoclusters show similar features to that of the negatively charged Au25 nanocluster. Specifically, the 8-electron M1Au24 (M = Cd/Hg) nanoclusters show a long excited-state lifetime (50−200 ns) and a weak picosecond relaxation, similar to the case of the anionic [Au25]− nanocluster. On the other hand, the non-8-electron MAu24 (M = Pd/Pt) nanoclusters show much more significant picosecond relaxation and thus much shorter excited-state lifetimes, which...

Unusually Short-Lived Solvent-Dependent Excited State in a Half-Sandwich Ru(II) Complex Induced by Low-Lying 3MC States.

A ruthenium complex with a half-sandwich geometry ([(p-cymene)Ru(Cl)(curcuminoid)]) was synthesized, characterized, and investigated regarding its ultrafast photophysics. These photophysical investigations of the complex revealed a weak and short-lived emission from the initially populated 1MLCT state and solvent-dependent photoinduced dynamics, where the secondarily populated 3MC state is stabilized by nonpolar solvents. Overall the decay of the 3dd-MC state to the ground state is completed within picoseconds. This short excited-state lifetime is in stark contrast to the typically observed long-lived 3MLCT states with lifetimes of nanoseconds or microseconds in unstrained, octahedral ruthenium complexes but is in good agreement with the findings for distorted octahedral complexes. This is pointing to the half-sandwich geometry as a new and easy approach to study these otherwise often concealed dd states.

Low-Threshold Light Amplification in Bifluorene Single Crystals: Role of the Trap States.

Organic single crystals (SCs) expressing long-range periodicity and dense molecular packing are an attractive amplifying medium for the realization of electrically driven organic lasers. However, the amplified spontaneous emission (ASE) threshold (1-10 kW/cm2) of SCs is still significantly higher compared to those of amorphous neat or doped films. The current study addresses this issue by investigating ASE properties of rigid bridging group-containing bifluorene SCs. Introduction of the rigid bridges in bifluorenes enables considerable reduction of nonradiative decay, which, along with enhanced fluorescence quantum yield (72-82%) and short excited state lifetime (1.5-2.5 ns), results in high radiative decay rates (∼0.5 × 109 s-1) of the SCs, making them highly attractive for lasing applications. The revealed ASE threshold of 400 W/cm2 in acetylene-bridged bifluorene SCs is found to be among the lowest ever reported for organic crystals. Ultrafast transient absorption spectroscopy enabled one to disclose pronounced differences in the excited state dynamics of the studied SCs, pointing out the essential role of radiative traps in achieving a record low ASE threshold. Although the origin of the trap states was not completely unveiled, the obtained results clearly evidence that the crystal doping approach can be successful in achieving extremely low ASE thresholds required for electrically pumped organic laser.

Direct detection of the photoinduced charge-separated state in a Ru(ii) bis(terpyridine)-polyoxometalate molecular dyad.

Observation of photoinduced intramolecular charge-separation is difficult for photosensitizer-POM dyads because of rapid backward electron transfer. We report here for the first time on a long-lived charge-separated state (τ = 470 ns) observed in a Ru(ii) bis(terpyridine)-based dyad. Charge-separation occurs despite virtually no driving force and the short intrinsic excited-state lifetime of the photosensitizer.

Fine control of chlorophyll-carotenoid interactions defines the functionality of light-harvesting proteins in plants

Photosynthetic antenna proteins can be thought of as “programmed solvents”, which bind pigments at specific mutual orientations, thus tuning the overall energetic landscape and ensuring highly efficient light-harvesting. While positioning of chlorophyll cofactors is well understood and rationalized by the principle of an “energy funnel”, the carotenoids still pose many open questions. Particularly, their short excited state lifetime (<25 ps) renders them potential energy sinks able to compete with the reaction centers and drastically undermine light-harvesting efficiency. Exploration of the orientational phase-space revealed that the placement of central carotenoids minimizes their interaction with the nearest chlorophylls in the plant antenna complexes LHCII, CP26, CP29 and LHCI. At the same time we show that this interaction is highly sensitive to structural perturbations, which has a profound effect on the overall lifetime of the complex. This links the protein dynamics to the light-harvesting regulation in plants by the carotenoids.

Anomalously Long-Lasting Blue PhOLED Featuring Phenyl-Pyrimidine Cyclometalated Iridium Emitter

Summary Simply replacing pyridine in cyclometalating ligands in FIrpic with pyrimidine affords the complex MS 2 , which has a slight red-shifted emission but shorter excited-state lifetime and the capability to form a highly efficient blue phosphorescent organic light-emitting device (OLED) with long operation lifetime ( T 50 > 2,200 hr). Further replacement of the picolinate-based ancillary ligand of MS 2 with strong-field CF 3 -containing pyridine-azole ancillary ligands leads to bluer emitters ( MS 17 and MS 19 ) with nearly unitary photoluminescence quantum yield and preferential horizontal emitting dipole orientations (with horizontal dipole ratios of 75%–77%) beneficial for OLED optical out-coupling. OLEDs using MS 17 and MS 19 as emitters gave bluer emission and very high external quantum efficiency exceeding 31% yet with comparably short device lifetimes as FIrpic -based devices. Theoretical analysis indicates that the distinct stability of the MS 2 -based device might be attributed to its relatively large energy difference between 3 MC (metal center d-d transition) and 3 MLCT states.

Excited-State Dynamics of a Benzotriazole Photostabilizer: 2-(2'-Hydroxy-5'-methylphenyl)benzotriazole.

A large number of common photostabilizers are based on the 2-(2'-hydroxyphenyl)benzotriazole structure. One common example is 2-(2'-hydroxy-5'-methylphenyl)benzotriazole, or TINUVIN-P. The excited-state dynamics of this molecule have been extensively characterized by ultrafast spectroscopies. These experiments have established that upon photoexcitation TINUVIN-P exhibits excited-state proton transfer followed by a remarkably fast internal conversion. We simulate the excited-state dynamics using ab initio multiple spawning (AIMS) and a complete active space configuration interaction (CASCI) wave function with a correction from density functional theory (DFT) to generate the potential energy surfaces. We predict ultrafast proton transfer on the order of 20 fs followed by simultaneous twisting and pyramidalization until a seam of conical intersection is reached. Near the intersection seam population transfer to the ground state is highly efficient. The process is best described as ballistic wavepacket motion from the Franck-Condon point along a barrierless coordinate leading to the seam of intersection. Internal conversion is primarily mediated by a minimum-energy conical intersection (MECI) with a high degree of pyramidalization. We posit that the presence of a nitrogen atom in the bond linking the phenyl to the benzotriazole allows for the rapid pyramidalization and the short excited-state lifetime.

Stimulus-Responsive Supramolecular Aggregate Assembly of Auramine O Templated by Sulfated Cyclodextrin.

Self-aggregation of organic molecules is rarely seen with macrocyclic hosts like β-cyclodextrin, as they preferentially involve the formation of inclusion complexes with the guest molecule. In this contribution, we report the self-aggregation of a guest molecule induced by negatively charged sulfated β-cyclodextrin (SCD) to yield highly emissive aggregates of a recently projected amyloid marker dye, Auramine O (AuO). The SCD templated AuO aggregates display very different photophysics when compared to its reported behavior in a wide range of various chemical and biological environment but show a remarkable similarity with the recently reported photophysical behavior of AuO in human insulin fibrillar media, thus providing important insights into the molecular form of AuO responsible for its amyloid sensing ability. The self-assembled AuO aggregates formed in the presence of SCD display a significantly long excited-state lifetime, suggesting the retardation of the torsional relaxation of dye in the aggregated state, which otherwise leads to a very short excited-state lifetime for the monomeric form of the dye in the isolated form. Detailed time-resolved emission spectra (TRES) measurements show a dynamic Stokes shift suggesting excitonic migration within the AuO aggregates. The supramolecular aggregate assembly displays remarkable sensitivity to important external stimuli like temperature or ionic strength of the medium, pitching for its possible application in designing stimuli-responsive sensing schemes for important analytes.

Fluorine-free blue-emitting cationic iridium complexes with oxadiazole-type cyclometalating ligands and their use in light-emitting electrochemical cells

Fluorine-free, blue-emitting cationic iridium complexes, namely [Ir(Tipr-dphoxd)2(bpy)]PF6 (1) and [Ir(Tipr-dphoxd)2(pzpy)]PF6 (2), which use 2-phenyl-5-(2,4,6-triisopropylphenyl)-1,3,4-oxadiazole (Tipr-dphoxd) as the cyclometalating ligand (CΛN) and 2,2’-bipyridine (bpy) or 2-(1H-pyrazol-1-yl)pyridine (pzpy) as the ancillary ligands (NΛN), have been synthesized and fully characterized. In solution, complex 1 emits efficient blue-green light centered at 505 nm with notable charge-transfer (CT) (Ir/CΛN → NΛN) character; while in the lightly doped film, it emits efficient sky-blue light peaked at 474 and 502 nm with much CΛN-centered 3π-π∗ character. In both solution and films, complex 2 always emits sky-blue light peaked around 468 nm and 500 nm with dominant CΛN-centered 3π-π∗ character. The sky-blue light emission from complexes 1 and 2 is among the bluest reported for fluorine-free, blue-emitting cationic iridium complexes. Theoretical calculations reveal that complex 1 bears energetically close-lying CT and CΛN-centered 3π-π∗ states, and its emission can arise from either or both of them; while for complex 2, its emission always comes from the lowest-lying CΛN-centered 3π-π∗ state. Because the CT state contributes considerably to the emission of complex 1, complex 1 exhibits more rapid radiative decays, higher luminescent efficiencies and shorter excited-state lifetimes than complex 2 in both the solution and lightly doped film. Complexes 1 and 2 have been tested as emitting materials in solid-state light-emitting electrochemical cells (LECs), with red-shifted electroluminescence compared to the photoluminescence. The green-yellow LEC based on complex 1 and the blue-green LEC based on complex 2 show peak current efficiencies of 12.4 and 5.0 cd A−1, respectively.

Rational Design and Characterization of Heteroleptic Phosphorescent Complexes for Highly Efficient Deep-Red Organic Light-Emitting Devices.

Two new deep-red iridium(III) complexes, (fpiq)2Ir(dipba) (fIr1) and (f2piq)2Ir(dipba) (dfIr2), comprising two cyclometaling ligands of fluorophenyl-isoquinoline derivatives (fpiq and f2piq) and a N-heterocyclic carbene (NHC)-based ancillary ligand of N,N'-diisopropylbenzamidinate (dipba) are designed, synthesized, and characterized. Given the unique four-membered Ir-N-C-N backbone built by the metal center and the ancillary ligand, both phosphors achieve significant improvement for their comprehensive optoelectronic characteristics. Density function theory (DFT) calculations and electrochemical measurements support the genuine pure red phosphorescent emission of fIr1 and dfIr2 based on their clearly distinct electron density distributions of the HOMO/LUMO orbitals compared with other red-emitting Ir(III) derivatives. Both new phosphors show deep-red emission with λmax values in the region of 650-660 nm with high PLQYs and short excited-state lifetimes. The phosphorescent organic light emitting diodes (PhOLEDs) based on fIr1 and dfIr2 realize deep-red EL with the stable CIEx,y coordinates of (0.70, 0.30) and (0.69, 0.31), the peak EQE/PE values of 15.4%/9.3 lm W-1 and 16.7%/10.4 lm W-1, respectively, which maintain such high levels as 10.6%/3.5 lm W-1 and 10.8%/3.6 lm W-1 at the practical luminance of 1000 cd m-2. They are the highest EL values reported for the OLEDs with such deep-red CIE coordinates.

Radical and Diradical Formation in Naphthalene Diimides through Simple Chemical Oxidation.

We successfully synthesized, for the first time, a naphthalene diimide (NDI) radical and diradical by simple chemical oxidation using lead(IV) dioxide. The formation of the (di)radical is confirmed by UV/Vis/NIR absorption, 1 H NMR, and electron paramagnetic resonance (EPR) measurements. In particular, temperature-dependent EPR, SQUID (superconducting quantum interference device) measurements, and quantum calculations demonstrated that the generated NDI diradical has a singlet diradical character of y=0.69 in the ground state. Subsequent characterization of this (di)radical revealed its stabilities, large two-photon absorption cross-section, and short excited-state lifetime.

Role of Electron-Driven Proton-Transfer Processes in the Ultrafast Deactivation of Photoexcited Anionic 8-oxoGuanine-Adenine and 8-oxoGuanine-Cytosine Base Pairs.

It has been reported that 8-oxo-7,8-dihydro-guanosine (8-oxo-G), which is the main product of oxidative damage of DNA, can repair cyclobutane pyrimidine dimer (CPD) lesions when incorporated into DNA or RNA strands in proximity to such lesions. It has therefore been suggested that the 8-oxo-G nucleoside may have been a primordial precursor of present-day flavins in DNA or RNA repair. Because the electron transfer leading to the splitting of a thymine-thymine pair in a CPD lesion occurs in the photoexcited state, a reasonably long excited-state lifetime of 8-oxo-G is required. The neutral (protonated) form of 8-oxo-G exhibits a very short (sub-picosecond) intrinsic excited-state lifetime which is unfavorable for repair. It has therefore been argued that the anionic (deprotonated) form of 8-oxo-G, which exhibits a much longer excited-state lifetime, is more likely to be a suitable cofactor for DNA repair. Herein, we have investigated the exited-state quenching mechanisms in the hydrogen-bonded complexes of deprotonated 8-oxo-G− with adenine (A) and cytosine (C) using ab initio wave-function-based electronic-structure calculations. The calculated reaction paths and potential-energy profiles reveal the existence of barrierless electron-driven inter-base proton-transfer reactions which lead to low-lying S1/S0 conical intersections. The latter can promote ultrafast excited-state deactivation of the anionic base pairs. While the isolated deprotonated 8-oxo-G− nucleoside may have been an efficient primordial repair cofactor, the excited states of the 8-oxo-G−-A and 8-oxo-G−-C base pairs are likely too short-lived to be efficient electron-transfer repair agents.

Pyridylpyrazole N^N ligands combined with sulfonyl-functionalised cyclometalating ligands for blue-emitting iridium(iii) complexes and solution-processable PhOLEDs.

A series of blue iridium(iii) complexes (12-15) comprising sulfonyl-functionalised phenylpyridyl cyclometalating ligands and pyridylpyrazole N^N ligands are reported, with an X-ray crystal structure obtained for 12. The complexes are highly emissive with photoluminescence quantum yields of 0.52-0.70 in dichloromethane solutions: two of the complexes (12 and 14) show emissions at λ 457 nm which is considerably blue-shifted compared to the archetypal blue emitter FIrpic (λmax 468 nm). The short excited state lifetimes (1.8-3.3 μs) and spectral profiles are consistent with phosphorescence from a mixture of ligand-centred and MLCT excited states. Density functional (DFT) and time dependent DFT (TD-DFT) calculations are in agreement with the electrochemical properties and the blue phosphorescence of the complexes. The additional mesityl substituent on the pyridylpyrazole ligand of 12 and 13 enhances the solubility of the complexes facilitating thin film formation by solution processing. Phosphorescent organic light-emitting diodes (PhOLEDs) have been fabricated using 12 or 13 in a solution-processed single-emitting layer using either poly(vinylcarbazole) (PVK) or 1,3-bis(N-carbazolyl)benzene (mCP) as host. The most blue-shifted electroluminescence (λ 460 nm, CIEx,y 0.15, 0.21) is obtained for an OLED containing complex 12 and mCP, with a brightness of 5400 cd m-2 at 10 V which is high for PhOLEDs with similar blue CIE coordinates using a solution-processed emitter layer.

Triboluminescence and Metal Phosphor for Organic Light-Emitting Diodes: Functional Pt(II) Complexes with Both 2-Pyridylimidazol-2-ylidene and Bipyrazolate Chelates.

We report the utilization of both pyrid-2-yl-imidazolylidene and dianionic bipz chelates as constituents in syntheses of a new series of charge-neutral Pt(II) complexes 1-4, among which complex 4 revealed remarkable triboluminescence, i.e., generation of photoemission upon grinding or cracking of the solid sample. The triboluminescence is found to be sensitive to the subtle changes of the associated substituents of pyrid-2-yl-imidazolylidene chelate, as verified by the disappearance of the triboluminescence for complexes 1-3. Alternatively, the well-ordered solid packing of 3, as indicated by the grazing incidence X-ray scattering experiment, serves as an ideal emitter for the fabrication of highly efficient OLEDs, rendering high external quantum efficienciy (25.9%) and luminesce efficiency (90 cd A-1) at the practical brightness of 100 cd m-2. The rather low roll-off in efficiency (24.4%, 85 cd A-1 at high brightness of 1000 cd m-2) is attributed to the short excited-state lifetime of 3 (∼800 ns) in the solid state, which in turn is associated with the MMLCT transition character.

On the Molecular Form of Amyloid Marker, Auramine O, in Human Insulin Fibrils.

Designing extrinsic fluorescence sensors for amyloid fibrils is a very active and important area of research. Recently, an ultrafast molecule rotor dye, Auramine O (AuO), has been projected as a fluorescent amyloid marker. It has been claimed that AuO scores better than the most extensively utilized gold-standard amyloid probe, Thioflavin-T (ThT). This advantage arises from the fact that AuO, in addition to its usual emission band (∼500 nm), also displays a large red-shifted emission band (∼560 nm), exclusively in the presence of human insulin fibril medium and not in the native protein or buffer media. On the contrary, for ThT, the emission maximum (∼490 nm) largely remains unchanged while going from protein to fibril. This otherwise unknown large red-shifted emission band of AuO, observed in the presence of human insulin fibrils, was tentatively attributed to a species formed upon fast proton dissociation from excited AuO. It was proposed that because of the long excited-state lifetime (∼1.8 ns) of AuO upon association with human insulin fibrils, this fast proton dissociation from excited AuO could be observed, which is otherwise not observed in buffer or native protein media, owing to its very short excited-state lifetime (∼1 ps). Herein, we show that despite the long excited-state lifetime of AuO in other fibrillar media (human serum albumin and lysozyme), the new red-shifted emission band at 560 nm is not observed, thus possibly suggesting a different origin of the red-shifted emission band of AuO in human insulin fibril medium. We convincingly show that this red-shifted band of AuO (∼560 nm) could be observed under conditions that promote dye aggregation, such as a premicellar concentration of surfactants and polyelectrolytes. These AuO aggregates display strong emission wavelength dependence of transient decay traces, similar to that for AuO in human insulin fibril medium. Detailed time-resolved emission spectral (TRES) measurements suggest that the AuO/premicellar surfactant and AuO/human insulin fibril system share similar features, such as a dynamic red-shift in TRES and an isoemissive point in the time-resolved area-normalized emission spectra, suggesting that the characteristic red-shifted emission band of AuO in human insulin fibril medium may arise from AuO aggregates.

Exploitation of a spontaneous spatial charge separation effect in plasmonic polyhedral α-Fe2O3 nanocrystal photoelectrodes for hydrogen production

Abstract High specific surface area bipyramidal and pseudocubic polyhedral α-Fe 2 O 3 nanocrystals were fabricated for hydrogen reduction in a water-splitting process. Two different facets of polyhedral α-Fe 2 O 3 nanocrystals were synthesized via a versatile hydrothermal route. Typically, photocatalytic activity of hematite is limited by its relatively poor absorptivity, very short excited-state lifetime, and short hole diffusion length. To address these issues, a pseudocubic polyhedral α-Fe 2 O 3 photoelectrode was fabricated that achieved a spontaneous charge spatial separation during the water-splitting process. Additionally, Sn from the FTO substrate was diffused and doped into polyhedral α-Fe 2 O 3 during the sintering process to serve as an electron donor and increase the carrier density. To further exploit the surface plasmon resonance (SPR) properties and enhance the water-splitting efficiency of polyhedral α-Fe 2 O 3 photoelectrodes, Au nanoparticles were decorated on the surface of Fe 2 O 3 . This increased incident light absorption and suppressed charge recombination by the strong field generated from collective oscillations of surface electrons in the Au nanoparticles. Subsequently, this Au/pseudocubic Fe 2 O 3 photoelectrode demonstrated a 9.05 μmol/min hydrogen production rate.

Ideal Molecular Design of Blue Thermally Activated Delayed Fluorescent Emitter for High Efficiency, Small Singlet-Triplet Energy Splitting, Low Efficiency Roll-Off, and Long Lifetime.

Highly efficient thermally activated delayed fluorescent (TADF) emitters, 5-(2-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-5H-benzofuro[3,2-c]carbazole (oBFCzTrz), 5-(3-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-5H-benzofuro[3,2-c]carbazole (mBFCzTrz), and 5-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-5H-benzofuro[3,2-c]carbazole (pBFCzTrz), were synthesized to study the effects of ortho-, meta-, and para- linkages between donor and acceptor moieties. oBFCzTrz having ortho- linked donor and acceptor moieties showed smaller singlet-triplet energy gap, shorter excited state lifetime, and higher photoluminescence quantum yield than mBFCzTrz and pBFCzTrz which are interconnected by meta- and para- positions. The TADF device using oBFCzTrz as a blue emitter exhibited high external quantum efficiency over 20%, little efficiency roll-off, and long device lifetime.

Shining Light on Copper: Unique Opportunities for Visible-Light-Catalyzed Atom Transfer Radical Addition Reactions and Related Processes.

Visible-light photoredox catalysis offers exciting opportunities to achieve challenging carbon-carbon bond formations under mild and ecologically benign conditions. Desired features of photoredox catalysts are photostability, long excited-state lifetimes, strong absorption in the visible region, and high reduction or oxidation potentials to achieve electron transfer to substrates, thus generating radicals that can undergo synthetic organic transformations. These requirements are met in a convincing way by Ru(II)(phenanthroline)3- and Ir(III)(phenylpyridine)3-type complexes and, as a low-cost alternative, by organic dyes that offer a metal-free catalyst but suffer in general from lower photostability. Cu(I)(phenanthroline)2 complexes have been recognized for more than 30 years as photoresponsive compounds with highly negative Cu(I)* → Cu(II) oxidation potentials, but nevertheless, they have not been widely considered as suitable photoredox catalysts, mainly because their excited lifetimes are shorter by a factor of 5 to 10 compared with Ru(II) and Ir(III) complexes, their absorption in the visible region is weak, and their low Cu(II) → Cu(I) reduction potentials might impede the closure of a catalytic cycle for a given process. Contrasting again with Ru(II)L3 and Ir(III)L3 complexes, Cu(I)L2 assemblies undergo more rapid ligand exchange in solution, thus potentially reducing the concentration of the photoactive species. Focusing on atom transfer radical addition (ATRA) reactions and related processes, we highlight recent developments that show the utility of Cu(I)(phenanthroline)2 complexes as photoredox catalysts, demonstrating that despite their short excited-state lifetimes and weak absorption such complexes are efficient at low catalyst loadings. Moreover, some of the inherent disadvantages stated above can even be turned to advantages: (1) the low Cu(II) → Cu(I) reduction potential might efficiently promote reactions via a radical chain pathway, and (2) the tendency for ligand exchange in Cu(I)L2 assemblies allows the efficient synthesis of heteroleptic Cu(I)LL' complexes to tune the steric and electronic properties and also might coordinate and thus activate substrates in the course of a reaction in addition to electron transfer. Moreover, new photoredox cycles have also been discovered beyond the visible-light-induced Cu(I)* → Cu(II) electron transfer that is arguably best known: examples of the Cu(II)* → Cu(I) and Cu(I)* → Cu(0) transitions have been realized, greatly broadening the potential for copper-based photoredox-catalyzed transformations. Finally, a number of organic transformations that are unique to Cu(I) photoredox catalysts have been discovered.