Charge-transfer Triplet State Research Articles

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.

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 A3 B-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 A3 B-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 (1 Δg ) 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.

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.

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.

Charge Transfer Excited State Promoted Multiple Resonance Delayed Fluorescence Emitter for High-Performance Narrowband Electroluminescence.

Multiple resonance thermally activated delayed fluorescence (MR-TADF) emitters are promising candidates for narrowband organic light-emitting diodes, but their electroluminescent performance is typically hindered by the slow reverse intersystem crossing rate (kRISC). Herein, we present an effective strategy to introduce a multichannel reverse intersystem crossing (RISC) pathway with large spin-orbit coupling by orthogonally linking an electron-donating unit to the MR framework. Through delicate manipulation of the excited-state energy levels, an additional intersegmental charge transfer triplet state could be "silently" induced without perturbing the MR character of the lowest excited singlet state. The proof-of-concept emitter CzBN3 not only affords 23-fold increase of kRISC compared with its prototypical MR skeleton but also realizes close-to-unity photoluminescence quantum yield, large radiative rate constant, and very narrow emission spectrum. These merits enable high maximum external quantum efficiency (EQEmax) of up to 37.1% and alleviated efficiency roll-off in the sensitizer-free device (EQE1000 = 30.4%), and a further boost of efficiency (EQEmax/1000 = 42.3/34.1%) is realized in the hyperfluorescent device. The state-of-the-art electroluminescent performance validates the superiority of our molecular design strategy.

Spin-Orbit versus Hyperfine Coupling-Mediated Intersystem Crossing in a Radical Pair.

While spin-orbit coupling (SOC) is typically the dominant interaction that couples singlet and triplet states within individual chromophores, hyperfine coupling (HFC) becomes important in multichromophoric systems, particularly in relation to the radical pair mechanism. Here, we use TD-DFT to calculate the spin-orbit coupling and hyperfine coupling between the first singlet and triplet charge transfer states of the radical pair 2Pyrene- and 2N,N-dimethylaniline+. We show that, as the intermolecular donor-acceptor distance grows, SOC decays to zero (as one would expect) because singlet and triplet states are characterized by identical orbitals in space, while the HFC remains comparatively constant. The switching region occurs around 4 Å, beyond which HFC dominates over SOC as far as defining the rate of intersystem crossing (ISC).

Donor, Acceptor, and Molecular Charge Transfer Emission All in One Molecule.

The molecular photophysics in the thermally activated delayed fluorescence (TADF) spiro-acridine-anthracenone compound, ACRSA, is dominated by the rigid orthogonal spirocarbon bridging bond between the donor and acceptor. This critically decouples the donor and acceptor units, yielding photophysics, which includes (dual) phosphorescence and the molecular charge transfer (CT) states giving rise to TADF, that are dependent upon the excitation wavelength. The molecular singlet CT state can be directly excited, and we propose that supposed "spiro-conjugation" between acridine and anthracenone is more accurately an example of intramolecular through-space charge transfer. In addition, we show that the lowest local and CT triplet states are highly dependent upon spontaneous polarization of the environment, leading to energy reorganization of the triplet states, with the CT triplet becoming lowest in energy, profoundly affecting phosphorescence and TADF, as evident by a (thermally controlled) competition between reverse intersystem crossing and reverse internal conversion, i.e., dual delayed fluorescence (DF) mechanisms.

Enhancing reverse intersystem crossing rate of red thermally activated delayed fluorescence emitters by simultaneously involving locally excited triplet states from donor and acceptor segments

Accelerating reverse intersystem crossing (RISC) rates of red thermally activated delayed fluorescence (TADF) emitters is crucial yet remains ongoing challenges. Herein, we disclose an ingenious molecular design to dramatically boost such an up-conversion process by leveraging effective participation of localized excited triplet states from both donor and acceptor segments (3LED and 3LEA). Inspiringly, energy level alignment with 3LEA-3LED close to the charge-transfer singlet and triplet (1CT and 3CT) states are obtained in the case of DBBPZ-IDPZ, by simultaneously employing polycyclic large-conjugation donor and acceptor units. Due to the joint contribution of 3LED and 3LEA, DBBPZ-IDPZ shows 15.7 folds accelerated RISC rate and delivers a higher photoluminescence quantum yield of 98.1 % relative to the control molecule (36.7 %). Eventually, the red OLED based on DBBPZ-IDPZ presents a significantly improved external quantum efficiency of 26.0 % with an emission maximum at 628 nm, which is approximately triple that of the control device (8.9 %). This is the first example of a delicate management of 3LED and 3LEA to simultaneously participate in RISC process for developing highly efficient red TADF emitters. Our work provides new insights into the molecular design tactic of high-performance red TADF OLEDs.

Room-Temperature Phosphorescence from Pd(II) and Pt(II) Complexes as Supramolecular Luminophores: The Role of Self-Assembly, Metal-Metal Interactions, Spin-Orbit Coupling, and Ligand-Field Splitting.

The synthesis as well as the structural and photophysical characterization of two isoleptic bis-cyclometalated Pt(II) and Pd(II) complexes, namely [PtL] and [PdL], bearing a tailored dianionic tetradentate ligand (L2-) are reported. The isostructural character and intermolecular interactions of [PtL] and [PdL] were assessed by NMR spectroscopy and X-ray diffraction analysis. Both complexes show fully ligand-controlled aggregation, demonstrating that a judicious molecular design can tune the photophysical properties. In fact, by introduction of fluorine atoms on defined positions and methoxy groups on complementary sites, metal-metal interactions can be forced by a head-to-tail stacking. Hence, [PtL] shows luminescence from metal-perturbed ligand-centered or from metal-metal-to-ligand charge-transfer triplet states in diluted solutions, in frozen glasses and in crystals, with high photoluminescence quantum yields and long lifetimes in the microsecond range. At room temperature (RT) in concentrated fluid solutions, the palladium analogue [PdL] surprisingly emits luminescence from aggregated species involving supramolecular interactions. Time-resolved photoluminescence and transient absorption spectroscopies demonstrated that ultrafast intersystem crossing occurs for both metals, which outruns any competitive relaxation pathway from the photoexcited singlet state. Furthermore, we demonstrate that the radiationless deactivation can be suppressed in frozen glassy matrices at 77 K and by intermolecular interactions in fluid solutions at RT. In both cases and as indicated by density functional theory calculations, the lowest emissive state acts as an energy trap from which the thermal population of dissociative states with formal occupation of an antibonding Pd-centered 4dx2-y2 orbital is suppressed. This occurs as the energy gap between the emissive and the dark states surpasses kT.

Turn on of room temperature phosphorescence of donor-acceptor-donor type compounds via transformation of excited states by rigid hosts for oxygen sensing

A series of compounds with the different donor moieties (phenothiazine, phenoxazine and acridine) and 1,4-difluorobenzene unit as the acceptor were developed for optical sensors of oxygen. Among three new compounds, oxygen sensing ability of one compound 10,10'-(2,5-difluoro-1,4-phenylene)bis(10 H-phenothiazine) (2PS-2FPh) exhibiting strong room-temperature phosphorescence in rigid matrix Zeonex shows high sensitivity to low concentration of oxygen (<0.1%). To understand the reason of the high oxygen sensitivity of 2PS-2FPh, the effect of a rigid medium on conformational distortion was studied using a set of experimental and theoretical approaches. Doping 2PS-2FPh into Zeonex results in strong room-temperature phosphorescence in a vacuum with singlet-triplet splitting (ΔEST) widened to 0.47 eV as compared to 0.17 eV observed for the solution in 2-methyltetrahydrofuran. Molecular dynamic simulation with 2PS-2FPh doped in Zeonex indicates the increased molecular heterogeneity. Concomitantly, the hybrid local and charge transfer triplet state (3HLCT) becomes more local allowing the room temperature phosphorescence to be observed from locally excited triplet states. These results showed that conformational disorder and rigidity of the medium can significantly widen ΔEST of donor-acceptor-donor type molecules transforming 3HLCT into 3LE and thus facilitating the room-temperature phosphorescence and high oxygen sensitivity of 2PS-2FPh.

Systematic investigation via controlling the energy gap of the local and charge-transfer triplet state for enabling high efficiency thermally activated delayed fluorescence emitters

Herein, we report a molecular design strategy to achieve high EQE despite having relatively large ΔEST by controlling the number and nature of intermediate states present between S1 and T1 along with f value.

Efficient Hot Electron Capture in CuPc/MoSe2 Heterostructure Assisted by Intersystem Crossing.

Efficient hot electron extraction is a promising approach to develop photovoltaic devices that exceed the Shockley-Queisser limit. However, experimental evidence of hot electron harvesting employing an organic-inorganic interface is still elusive. Here, we reveal the hot electron dynamics at a CuPc/MoSe2 interface using steady-state spectroscopy and transient absorption spectroscopy. A hot electron transfer efficiency of greater than 78% from MoSe2 to CuPc is observed, comparable to that achieved in quantum dot hybrid systems. The mechanism is proposed as follows: the photogenerated hot electrons in MoSe2 transfer to CuPc and form singlet charge transfer states, which subsequently transform into triplet charge transfer states assisted by the rapid intersystem crossing, inhibiting back-donation of electrons and facilitating exciton dissociation into CuPc polarons with a nanosecond lifetime. Our results demonstrate that the intersystem crossing of the hybrid electronic state at organic-inorganic interfaces may serve as a scheme to enable efficient hot electron extraction in photovoltaic devices.

Thermally activated delayed fluorescence materials based on acridin-9(10H)-one acceptor for organic light-emitting diodes

It is essential to manage the alignment of the locally excited (3LE) and charge-transfer triplet (3CT) and singlet (1CT) states of thermally activated delayed fluorescence (TADF) materials to accelerate reverse intersystem crossing (RISC) process. A series of donor-acceptor (D-A) type TADF emitters were developed by attaching acridine (DMAC) or phenoxazine (PXZ) donor at 3-site of acridin-9(10H)-one (i.e. acridone, AD) acceptor. Regardless of the donor, the lowest triplet excited states (T1) are always dominated by the 3LEA state with certain contribution from the 3CT state. By introducing methyl (Me) or trifluoromethyl (CF3) at 6-site of AD acceptor, the energy levels of 1CT, 3CT and 3LE states were tuned in different ways. When the donor is DMAC, incorporating Me on AD ring (3-DMAC-6-Me-AD) destructed the emission performance, while CF3 (3-DMAC-6-CF3-AD) strongly stimulated the TADF by pulling down the 1CT state level and reducing the energy splitting ΔEST to almost zero. In the case of strong donor PXZ, decorating acceptor with Me (3-PXZ-6-Me-AD) did not bring extinct effect on the TADF property. In particular, 3-DMAC-6-CF3-AD and 3-PXZ-6-Me-AD exhibited high rate constants of RISC (kRISC) of 10−6 and of radiation (kR) of 107 S−1, respectively. They achieved the maximum external quantum efficiency (EQEmax) of 21.6% and 23.3% in the doped OLEDs, and realized EQEmax of 14.7% and 17.6% with low efficiency roll-offs in non-doped devices.

Efficient Charge Dissociation of Triplet Excitons in Bulk Heterojunction Solar Cells

Understanding the sources of energy loss in bulk heterojunction (BHJ) solar cells is of outstanding importance for increasing power conversion energy. Herein, we employ a full quantum mechanics approach to determine the rates of charge transfer processes at the acceptor–donor interface for a prototypical BHJ blend. It is shown that charge recombination of a singlet charge transfer (CT) state at the acceptor/donor (A/D) interface is very fast, being the most effective process which prevents efficient charge migration toward the electrodes. Triplet CT states can also undergo charge recombination by fast decay to a low lying triplet state of the donor and/or of the acceptor, but the backward process is also fast enough to allow efficient charge dissociation. Slowing down the fast decay process of singlet CT states appears to be hardly practicable, since it would reflect either on a reduced spectral absorption window or on a slow rate of photoinduced electron transfer, so that an alternative and possibly more viable route for increasing organic solar cell efficiency could be that of favoring the formation of triplet CT states over singlet ones.

Anion-π-Induced Room Temperature Phosphorescence from Emissive Charge-Transfer States.

The burgeoning noncovalent interactions between π-acidic aromatic surfaces and anions have been recently shown to have unique functional relevance in anion transport, ion sensing, and organocatalysis. Despite its potential to instigate charge-transfer (CT) states, modulation of the emission features by toggling between the excited states using anion-π interactions is not yet explored. On the other hand, excited states with CT characteristics play an important role in the ambient triplet harvesting of organic chromophores. In this context, herein we propose an anion-π-based molecular design for the introduction of emissive singlet and triplet CT excited states, thereby expanding the functional scope of these weak supramolecular interactions. In the present study, we investigate the anion-π-induced emission from the singlet (1CT) and triplet (3CT) CT states of a dibromo dicationic pyromellitic diimide derivative. Remarkably, we accomplish dual room temperature phosphorescence emission from the anion-π-mediated 3CT state along with the locally excited triplet state (3LE) in solution phase using an organic-inorganic supramolecular scaffolding strategy. Comprehensive steady-state and time-resolved spectroscopy along with theoretical calculations provide detailed insights into the excited-state manifolds of phosphor. We envisage that the present study will expedite new molecular designs based on weak intermolecular interactions for the excited-state engineering of organic chromophores to facilitate ambient triplet harvesting and CT emission.