Charge-transfer Triplet Research Articles

Understanding the Nonradiative Charge Recombination in Organic Photovoltaics: From Molecule to Device

AbstractOrganic photovoltaics (OPVs) have made significant strides with efficiencies now exceeding 20%, positioning them as potential competitors to inorganic solar technologies. One of the most critical challenges toward this goal is the severe open‐circuit voltage (Voc) loss caused by the nonradiative charge recombination (NRCR). Herein, this review comprehensively summarizes the NRCR mechanisms and suppression techniques of OPVs across various scales from molecule to device. Specifically, the origins of NRCR in a single molecule are first summarized, and molecular design principles toward high photoluminescence quantum yield are reviewed following the Marcus theory. Next, the effect of aggregation on NRCR is reviewed, as well as the molecular and processing strategies to modulate the film packing for NRCR suppression. Furthermore, the progresses in the avoidance of nonradiative loss pathways mediated by charge transfer states and triplet states in donor:acceptor bulk heterojunctions are tracked. Besides, the interfacial optimization and device structure design to maximize the electroluminescent quantum efficiency are presented. Finally, several potential pathways toward curtailing NRCR for high‐performance OPVs are outlined. Therefore, this review shows an insightful perspective to understand and mitigate the NRCR at multi‐scales, and is poised to provide a clear roadmap for the next breakthrough of OPVs.

Understanding Spin-Orbit-Coupling-Induced Reverse Intersystem Crossing in DMAC-TRZ-Doped Organic Light-Emitting Diodes via Magnetic-Field-Effect Measurement.

An efficient reverse intersystem crossing (RISC) process in thermally activated delayed fluorescence (TADF) material is a common way to obtain high-performance organic light-emitting diodes (OLEDs), but the physical mechanism for the spin flipping of the RISC remains vague. Here, using magneto-electroluminescence (MEL) as an effective tool, we found that the RISC (CT3 → CT1) from a triplet charge transfer (CT3) to the singlet charge transfer (CT1) state is decided by spin-orbit coupling (SOC) in metal-free OLEDs based on a typical TADF emitter DMAC-TRZ. By fitting and analyzing the current and concentration-dependent MEL data, it is found that the characteristic magnetic field of the SOC-induced RISC process is approximately 65-85 mT, which is obviously larger than that (several mT) of the hyperfine-interaction-induced RISC process. Simultaneously, the dissociation effect of the electric field on the CT3 state causes the SOC-induced RISC process to decrease with increasing bias current. The different formation methods of excited states lead to the nonmonotonic change of SOC-induced RISC process with the increase of dopant concentration. Furthermore, considering the orbital polarization of dipoles, the SOC mechanism is further verified by the measurement of magneto-photoluminescence to be the responsible for achieving the spin flipping in TADF molecules. Therefore, this work clarifies the underlying dynamic mechanism of the RISC process in TADF-OLEDs.

Configuration Interaction in Frontier Molecular Orbital Basis for Screening the Spin-Correlated, Spatially Separated Triplet Pair State 1(T···T) Formation.

In the theoretical screening of Singlet Fission rates in molecular aggregates, often the frontier molecular orbital model for dimers is employed. However, the dimer approach fails to account for recent experimental findings that suggest singlet fission progresses through a further intermediate state featuring two spatially separated, spin-correlated triplets, specifically a 1(T···T) state. We address this limitation by generalizing the often used frontier molecular orbital model for singlet fission by incorporation of both separated Charge Transfer (C···T) and 1(T···T) states as well as mixed triplet-charge transfer states, delivering analytic expressions for the diabatic matrix elements. Applying the methodology to the perylene diimide trimer, we examine the packing dependence of competing formation pathways of the 1(T···T) state by evaluation of diabatic matrix elements.

The Role of Terminal Fluorination on Energy Inversion in Organic Solar Cells.

Suppressing the non-radiative energy loss(ΔE3) mediated by triplet charge transfer stateis crucial for high-performance organic solar cells (OSCs). Here, we decode the energy inversionthroughmulti-scale theoretical simulation, which inhibits the non-emissivetriplet (T1) state formation. However, itis mystified by the system dependence.We first demonstrate a direct relationship of "the probability of Face-on orientation(PFace-on) is proportional to the probability of energy inversion (PEI)", which is related to the function of terminal fluorination. Through Pearson's correlation coefficientand machine learning model, the useful stacking structural parameters were obtained to clarify the effect of π-bridge group on the function of terminal fluorination. Basedon the molecular descriptorsestablished,we explain that the fluorination effect is beneficial to Face-onorientationand thus energy inversiondue tothe enhancedintermolecular coupling.Butthe π-bridge inhibits thiscoupling with the interfacialstacking configuration appearingmore "TT_IC".This work provides a directional standard for promotingenergy inversionto reduceΔE3for the high-performance OSCs.

Proton transfer induced persistent triplet charge transfer phosphorescence in molecule-doped polymer systems

The intermolecular interactions between small molecules and the matrix in a highly polar environment could easily lead to non-radiative transitions, which significantly hampers the development of high-performance persistent room-temperature phosphorescence (pRTP) materials in such condition. Herein, we introduce a novel strategy that integrates consecutive proton transfer and photo-induced charge transfer to facilitate triplet charge transfer emission in highly polar polymer matrix. In particular, doping aza-arene guests including 1,10-phenanthroline (1,10-Phen), 2,9-dimethyl-1,10-phenanthroline (DM-Phen), and 7,8-benzoquinoline (7,8-BQ) into polyacrylic acid (PAA) resulted in superior pRTP properties compared to other polymer hosts. Spectroscopic investigations and theoretical calculations elucidate that this is attributed to the proton transfer and triplet charge transfer characteristics between the host and guest. Moreover, due to the rigid environment and space confinement provided by PAA, pRTP with the lifetime up to 841 ms is achieved. Capitalizing on the excellent water solubility of the doped PAA films, the inkjet printings have been executed, and showcasing the promising the potential applications of these pRTP materials in the field of information encryption.

(Invited) Blue Organic Light-Emitting Diode Using Dynamic Exciton for Low Driving Voltage

Among the three primary colors, blue emission in organic light-emitting diodes (OLEDs) are highly important but very difficult to develop. OLEDs have already been commercialized; however, blue OLEDs have the problem of requiring a high applied voltage due to the high-energy of blue emission. Herein, an ultralow voltage turn-on at 1.47 V for blue emission with a peak wavelength at 462 nm (2.68 eV) is demonstrated in an OLED device. This OLED reaches 100 cd/m2, which is equivalent to the luminance of a typical commercial display, at 1.97 V. Blue emission from the OLED is achieved by the selective excitation of the low-energy triplet states at a low applied voltage by using the charge transfer (CT) state as a precursor and the triplet-triplet annihilation, which forms one emissive singlet from two triplet excitons. We found that the essential component for efficient blue emission is a smaller energy difference between the CT state and triplet exciton, accelerating the energy transfer between the two states and achieving the optimal performance by avoiding direct decay from the CT state to the ground state. Our study demonstrates that the developed OLED allows for a much longer operation lifetime than that from a typical blue phosphorescent OLED because the blue emission originates from a stable low-energy triplet exciton that avoids degrading the constituent materials.

Management of Triplet States in Modified Mononuclear Ruthenium(II) Complexes for Enhanced Photocatalysis.

Controlling the interplay between relaxation and charge/energy transfer processes in the excited states of photocatalysts is crucial for the performance of artificial photosynthesis. Metal-to-ligand charge-transfer triplet states (3MLCT*) of ruthenium(II) complexes are broadly implemented for photocatalysis, but an effective means of managing the triplets for enhanced photocatalysis has been lacking. Herein, We proposed a strategy to considerably prolong the triplet excited-state lifetime by decorating a ruthenium(II) phosphine complex (RuP-1) with pendent polyaromatic hydrocarbons (PAHs). Systematic studies demonstrate that in RuP-4 decorated with anthracene, sub-picosecond electron transfer from anthracene to 3MLCT* leads to a charge-separated state that can mediate the formation of the intra-ligand triplet state (3IL) of anthracene, resulting in an exceptionally long excited-state up to several milliseconds. This triplet management strategy enables impressive photocatalytic reduction of CO2 to CO with a turnover number (TON) of 404, an optimized quantum yield of 43 % and 100 % selectivity, which is the highest reported performance for mononuclear photocatalysts without additional photosensitizers. RuP-4 also catalyzes photochemical hydrogen generation under argon. This work opens up an avenue for regulating the excited-state charge/energy flow for the development of long-lived 3IL multi-functional mononuclear photocatalysts to boost artificial photosynthesis.

Pd and Pt Complexes of Benzo-Fused Dipyrrins: Synthesis, Structure, Electrochemical, and Optical Properties.

Benzo-fused dipyrrins are π-extended analogs of conventional dipyrrins, which exhibit bathochromically shifted absorption and possess the synthetic capability to bind various metal ions. We aimed to investigate the synthetic potential of benzo-fused dipyrrins in the complexation with transition metals. Two new complexes with Pd2+ and Pt2+ were synthesized and characterized. X-ray crystallography reveals that both complexes exhibit a zigzag geometry with square planar coordination of the central metal. The Pd2+ complex possesses a very weak fluorescence at 665 nm, while the Pt2+ complex is completely nonemissive. Transient absorption spectroscopy confirmed triplet excited state formation for both complexes; however, they are short-lived and no phosphorescence was observed even at 77K. DFT calculations support the experimental observation, revealing the existence of the low-lying ligand-metal charge-transfer (LMCT) triplet state acting as an energy sink.

Tuning Excited State Character in Iridium(III) Photosensitizers and Its Influence on TTA-UC.

A series of mixed ligand, photoluminescent organometallic Ir(III) complexes have been synthesized to incorporate substituted 2-phenyl-1H-naphtho[2,3-d]imidazole cyclometalating ligands. The structures of three example complexes were categorically confirmed using X-ray crystallography each sharing very similar structural traits including evidence of interligand hydrogen bond contacts that account for the shielding effects observed in the 1H NMR spectra. The structural iterations of the cyclometalated ligand provide tuning of the principal electronic transitions that determine the visible absorption and emission properties of the complexes: emission can be tuned in the visible region between 550 and 610 nm and with triplet lifetimes up to 10 μs. The nature of the emitting state varies across the series of complexes, with different admixtures of ligand-centered and metal-to-ligand charge transfer triplet levels evident. Finally, the use of the complexes as photosensitizers in triplet-triplet annihilation energy upconversion (TTA-UC) was investigated in the solution state. The study showed that the complexes possessing the longest triplet lifetimes showed good viability as photosensitizers in TTA-UC. Therefore, the use of an electron-withdrawing group on the 2-phenyl-1H-naphtho[2,3-d]imidazole ligand framework can be used to rationally promote TTA-UC using this class of complex.

Ultrafast Formation of a Delocalized Triplet-Excited State in an Epigenetically Modified DNA Duplex under Direct UV Excitation.

Epigenetic modifications impart important functionality to nucleic acids during gene expression but may increase the risk of photoinduced gene mutations. Thus, it is crucial to understand how these modifications affect the photostability of duplex DNA. In this work, the ultrafast formation (<20 ps) of a delocalized triplet charge transfer (CT) state spreading over two stacked neighboring nucleobases after direct UV excitation is demonstrated in a DNA duplex, d(G5fC)9•d(G5fC)9, made of alternating guanine (G) and 5-formylcytosine (5fC) nucleobases. The triplet yield is estimated to be 8 ± 3%, and the lifetime of the triplet CT state is 256 ± 22 ns, indicating that epigenetic modifications dramatically alter the excited state dynamics of duplex DNA and may enhance triplet state-induced photochemistry.

Low-symmetry A3B-type 6H-1,4-diazepinoporphyrazines with anti-Kasha effect as promising photosensitizers.

A series of tribenzo[g,l,q]-6H-1,4-diazepino[2,3-b]porphyrazines has been synthesized. A temperature-dependent steric effect was applied in the mixed Linstead macrocyclization of phthalonitrile and 5,7-bis(2'-arylethenyl)-6-propyl-6H-1,4-diazepine-2,3-dicarbonitrile to achieve high yield of low-symmetry A3B-type Mg(II) tribenzo[g,l,q]-6H-1,4-diazepino[2,3-b]porphyrazinate. The analysis of photophysical and photochemical properties of the obtained complexes showed the anti-Kasha effect: the ultrafast spin changes successfully compete with the IC. TD-DFT calculations showed that the presence of 1,4-diazepine heterocycle in the porphyrazine structure leads to the formation of additional charge-transfer triplet state T2. We propose, it could participate in the pumping of T1x state alongside with T1y state (these states are degenerate in D4h symmetry) and, therefore, increase singlet oxygen (1Δg) generation. Stable micellar nanoparticles have been obtained based on the tribenzo[g,l,q]-6H-1,4-diazepino[2,3-b]porphyrazine Mg(II) and Zn(II) complexes using polyvinylpyrrolidone. The nanoparticles effectively interact with model biological structures (FBS and brain homogenate), leading to disaggregation of the macrocycles. They also exhibit pronounced phototoxic effects in MCF-7 cells upon red light irradiation. We propose that enhancement in PDT activity could be explained by their increased resistance to aggregation due to the presence of n-propyl substituent directly attached to the C6 position of the 1,4-diazepine moiety. The demonstrated results show the promising potential of tribenzo-6H-1,4-diazepinoporphyrazines as heavy atom-free photosensitizers.

Achieving efficient and stable blue thermally activated delayed fluorescence organic light-emitting diodes based on four-coordinate fluoroboron emitters by simple substitution molecular engineering.

Achieving both high efficiency and high stability in blue thermally activated delayed fluorescence organic light-emitting diodes (TADF-OLEDs) is challenging for practical displays and lighting. Here, we have successfully developed a series of sky-blue to pure-blue emitting donor-acceptor (D-A) type TADF materials featuring a four-coordinated boron with 2,2'-(pyridine-2,6-diyl)diphenolate (dppy) ligands, i.e.1-8. Synergistic engineering of substituents on the phenyl bridge as well as the electronic properties and the attached positions of heteroatom N-donors not only enables fine-tuning of the emission colors, but also modulates the nature and energies of their triplet excited states that are important for the reverse intersystem crossing (RISC). Particularly for the compound with two methyl substituents on the phenyl bridge (compound 8), RISC is significantly facilitated through the vibronic coupling of the energetically close-lying triplet charge transfer (3CT) and the triplet local excited (3LE) states, when compared to analogue 7. Efficient sky-blue to pure-blue OLEDs with electroluminescence peaks (λ EL) at 460-492 nm have been obtained, in which ca. five-fold higher external quantum efficiencies (EQEs) of 18.9% have been demonstrated by 8 than that by 7. Moreover, ca. thirty times longer device operational half-lifetimes (LT50) of 9113 hours for 8 than that for 7 as well as satisfactory LT50 reaching 26 643 hours for 6 at an initial luminance of 100 cd m-2 have also been demonstrated. To the best of our knowledge, these results represent one of the best high-performance blue OLEDs based on tetracoordinated boron TADF emitters. Moreover, the design strategy presented here has provided an attractive strategy for enhancing the device performance of blue TADF-OLEDs.

Excited-state dynamics of C3-symmetric heptazine-based thermally activated delayed-fluorescence emitters.

Heptazine-based materials have recently emerged as a promising motif for thermally activated delayed fluorescence, as their near-zero or negative singlet-triplet energy gaps enable extremely fast reverse intersystem crossing (rISC) rates. Another method for achieving a high rate of rISC is through the use of highly symmetric emitters, which benefit from energy-level degeneracies and a high density of states. Here, we investigate the effect of combining these two design strategies on the excited-state dynamics of C3-symmetric emitters containing heptazine cores. We find that in two of the four emitters studied, the S1 state has a high degree of locally excited (LE) character with density on the heptazine moiety, preventing excited-state localization and a loss of symmetry in the energy-minimized S1 geometry. Surprisingly, these symmetric molecules still suffer from a loss of density of triplet states below the S1 state. Overall, we find that maintaining C3 symmetry will not necessarily maintain density of states, but that heptazine-based materials with LE S1 states still benefit from maximized rISC rates via increased spin-orbit coupling with low-lying charge-transfer triplet states and exhibit advantageous photophysical properties, such as near-unity photoluminescence quantum yields and high colour purity.

Uncovering the substituted-position effect on excited-state evolution of benzophenone-phenothiazine dyads.

Photofunctional materials based on donor-acceptor molecules have drawn intense attention due to their unique optical properties. Importantly, Systematic investigation of substitution effects on excited-state charge transfer dynamics of donor-acceptor molecules is a powerful approach for identifying application-relevant design principles. Here, by coupling phenothiazine (PTZ) at the ortho-, meta-, and para-positions of the benzene ring of benzophenone (BP), three regioisomeric BP-PTZ dyads were designed to understand the relationship between substituted positions and excited-state evolution channels. Ultrafast transient absorption is used to detect and trace the transient species and related evolution channels of BP-PTZ dyads at excited state. In a non-polar solvent, BP-o-PTZ undergoes the through-space charge transfer process to produce a singlet charge-transfer (1CT) state, which subsequently proceeds the intersystem crossing process and transforms into a triplet charge-transfer (3CT) state; BP-m-PTZ experiences intramolecular charge transfer (ICT) process to generate the 1CT state, which subsequently transforms into the 3CT state by the intersystem crossing (ISC) and finally converts into the local-excited triplet (3LE) state; as for BP-p-PTZ, only 3LE states can be detected after the ISC process from the 1CT state. On the other hand, the twisted ICT states are generated via twisted motion between the donor and acceptor for all BP-PTZ dyads or planarization of the PTZ unit in high polar solvents. The excited-state theoretical calculations unveil that the features of ICT and intramolecular interaction between the three dyads play a decisive role in determining the through-bond charge transfer and through-space charge transfer processes. Also, these results demonstrate that the excited-state evolution channels of PTZ derivatives could be modified by tuning the substituted positions of the donor-acceptor dyads. This study provides a deep perspective for the substitute-position effect on donor-acceptor-type PTZ derivatives.

Color-Tunable Dual-Mode Organic Afterglow for White-Light Emission and Information Encryption Based on Carbazole Doping.

Dual-mode emission materials, combining phosphorescence and delayed fluorescence, offer promising opportunities for white-light afterglow. However, the delayed fluorescence lifetime is usually significantly shorter than that of phosphorescence, limiting the duration of white-light emission. In this study, a carbazole-based host-guest system that can be activated by both ultraviolet (UV) and visible light is reported to achieve balanced phosphorescence and delayed fluorescence, resulting in a long-lived white-light afterglow. Our study demonstrated the critical role of a charge transfer state in the afterglow mechanism, where the charge separation and recombination process directly determined the lifetime of afterglow. Simultaneously, an efficient reversed intersystem crossing process was obtained between the singlet and triplet charge transfer states, which further boosted the delayed fluorescence properties of the doping system. As a result, delayed fluorescence lifetime was successfully prolonged to approach that of phosphorescence. This work presents a delayed fluorescence lifetime improvement strategy via doping method to realize durable white-light afterglow.

Color‐Tunable Dual‐Mode Organic Afterglow for White‐Light Emission and Information Encryption Based on Carbazole Doping

AbstractDual‐mode emission materials, combining phosphorescence and delayed fluorescence, offer promising opportunities for white‐light afterglow. However, the delayed fluorescence lifetime is usually significantly shorter than that of phosphorescence, limiting the duration of white‐light emission. In this study, a carbazole‐based host–guest system that can be activated by both ultraviolet (UV) and visible light is reported to achieve balanced phosphorescence and delayed fluorescence, resulting in a long‐lived white‐light afterglow. Our study demonstrated the critical role of a charge transfer state in the afterglow mechanism, where the charge separation and recombination process directly determined the lifetime of afterglow. Simultaneously, an efficient reversed intersystem crossing process was obtained between the singlet and triplet charge transfer states, which facilitating the delayed fluorescence properties of host–guest system. As a result, delayed fluorescence lifetime was successfully prolonged to approach that of phosphorescence. This work presents a delayed fluorescence lifetime improvement strategy via doping method to realize durable white‐light afterglow.

Unraveling Triplet Formation Mechanisms in Acenothiophene Chromophores.

The evolution of molecular platforms for singlet fission (SF) chromophores has fueled the quest for new compounds capable of generating triplets quantitatively at fast time scales. As the exploration of molecular motifs for SF has diversified, a key challenge has emerged in identifying when the criteria for SF have been satisfied. Here, we show how covalently bound molecular dimers uniquely provide a set of characteristic optical markers that can be used to distinguish triplet pair formation from processes that generate an individual triplet. These markers are contained within (i) triplet charge-transfer excited state absorption features, (ii) kinetic signatures of triplet-triplet annihilation processes, and (iii) the modulation of triplet formation rates using bridging moieties between chromophores. Our assignments are verified by time-resolved electron paramagnetic resonance (EPR) measurements, which directly identify triplet pairs by their electron spin and polarization patterns. We apply these diagnostic criteria to dimers of acenothiophene derivatives in solution that were recently reported to undergo efficient intermolecular SF in condensed media. While the electronic structure of these heteroatom-containing chromophores can be broadly tuned, the effect of their enhanced spin-orbit coupling and low-energy nonbonding orbitals on their SF dynamics has not been fully determined. We find that SF is fast and efficient in tetracenothiophene but that anthradithiophene exhibits fast intersystem crossing due to modifications of the singlet and triplet excited state energies upon functionalization of the heterocycle. We conclude that it is not sufficient to assign SF based on comparisons of the triplet formation kinetics between monomer and multichromophore systems.

Ultrafast Spin Crossover Photochemical Mechanism in [FeII(2,2'-bipyridine)3]2+] Revealed by Quantum Dynamics.

Photoexcitation of [FeII(2,2'-bipyridine)3]2+ induces a subpicosecond spin crossover transformation from a low-spin singlet to a high-spin quintet state. The mechanism involves metal-centered (MC) and metal-ligand charge transfer (MLCT) triplet intermediates, but their individual contributions to this efficient intersystem crossing have been object of debate. Employing quantum wavepacket dynamics, we show that MC triplets are catalyzing the transfer to the high-spin state. This photochemical pathway is made possible thanks to bipyridine stretching vibrations, facilitating the conversion between the MLCT bands to such MC triplets. We show that the lifetime of the MLCT states can be increased to tens of picoseconds by breaking the conjugation between pyridine units, which increases the energetic gap between MLCT and MC states. This opens the route for the design of new chelating ligands inducing long-lived MLCT states in iron complexes.

Tracking the Early Stage of Triplet-Induced Photodamage in a DNA Dimer and Oligomer Containing 5-Methylcytosine.

Methylation at the C5 position of cytosine, a naturally occurring epigenetic modification on DNA, shows a high correlation with mutational hotspots in disease such as skin cancer. Due to its essential biological relevance, numerous studies were devoted to confirming that the methylated sites favor the formation of the cyclobutane pyrimidine dimer (CPD), a well-known UV-induced lesion. However, photophysical and photochemical properties of dinucleotides and polynucleotides containing 5-methylcytosine (5mC) remain elusive. Herein, a charge transfer (CT) triplet state, generated via intersystem crossing (ISC) from a CT singlet state that enhanced after methylation on cytosine, is directly observed by using femtosecond transient absorption (TA) and time-resolved mid-infrared (TRIR) spectroscopy together with quantum chemical calculations for the first time in the T5mC dimer. Such an ISC process is quenched due to limitations of the ground-state geometries in 5mC-containing single-strand oligomer d(T5mC)9. This mechanistic information is important for understanding the early stage of triplet state-induced CPD formation in 5mC containing DNA.

Charge Separation from Triplet Excited States in Nonfullerene Acceptor‐Based Organic Solar Cells

Suppressing charge recombination is pivotal for further improving the power conversion efficiency of organic solar cells (OSCs). The bimolecular recombination of free charge carriers leads to the formation of both singlet and triplet charge transfer (CT) states at a ratio of 1:3 when considering simple spin statistics, meaning that charge recombination via the triplet excited state is the main deactivation channel for OSCs. Although the formation of local triplet excitons through back charge transfer from triplet CT states is thought to be a terminal loss process, it is shown that charge separation from triplet excitons can occur in nonfullerene acceptor (NFA)‐based OSCs. To reveal the triplet exciton dynamics, a triplet sensitizer PtTPBP is doped into an OSC consisting of PM6 and Y6 as an electron donor and acceptor, respectively. Upon photoexcitation of PtTPBP, triplet excitons are formed, which subsequently transfer their energy to Y6, resulting in Y6 triplet excitons formation. Based on transient absorption and external quantum efficiency measurements, clear experimental evidence of charge photogeneration from Y6 triplet excitons at the PM6:Y6 interface is provided. This study highlights the importance of minimizing the energy difference between the singlet and triplet excited states of NFAs to suppress charge recombination.