Lowest Excited Singlet State Research Articles

Vibrational effects on generalized oscillator strengths of ammonia

Ab initio calculations of generalized oscillator strengths (GOSs) are carried out to investigate the influence of nuclear dynamics on electron-impact electronic excitations in ammonia. Momentum-transfer-dependent GOSs or GOS profiles are calculated for transitions to the three lowest excited singlet-states at the equation-of-motion coupled-cluster singles and doubles level, and comparison is made with available experimental data. In the calculations, the effects of molecular vibration are explicitly included beyond the Condon approximation. The result reveals that the out-of-plane bending vibration leads to the shift of the minimum position of GOS profile for each vibronic level. Furthermore, isotope effects on the transition to the and states are examined by performing the calculation for NH3 and its isotopomers.

Molecular Engineering of Efficient Singlet Oxygen Generators with Near‐Infrared AIE Features for Mitochondrial Targeted Photodynamic Therapy

AbstractAggregation‐caused fluorescence quenching with insufficient production of reactive oxygen species (ROS) has limited the application of photosensitizers (PSs) in fluorescence‐imaging‐guided photodynamic therapy (PDT). Aggregation‐induced emission PSs (AIE‐PSs) exhibit enhanced fluorescence intensity and a high efficiency of ROS generation in the aggregation state, which provides an opportunity to solve the above problems. Herein, a series of AIE‐PSs are successfully designed and synthesized by adjusting the D–A intensity through molecular engineering. The photophysical properties and theoretical calculations prove that the synergistic effect of 3,4‐ethylenedioxythiophene and quinolinium increases the intramolecular charge transfer effect (ICT) of the whole molecule and promotes the intersystem crossing (ISC) from the lowest excited singlet state (S1) to the lowest triplet state (T1). Among these AIE‐PSs, the optimal AIE‐PS (TPA‐DT‐Qy) exhibits the highest generation yield of 1O2 (5.3‐fold of Rose Bengal). Further PDT experiments show that the TPA‐DT‐Qy has a highly efficient photodynamic ablation of breast cancer cells (MCF‐7 and MDA‐MB‐231) under white light irradiation. Moreover, the photodynamic antibacterial study indicates that TPA‐DT‐Qy has the discrimination and excellent photodynamic inactivation of S. aureus. This work provides a feasible strategy for the molecular engineering of novel AIE‐PSs to improve the development of fluorescence‐imaging‐guided PDT.

Up- and Down-Converting Photons in Molecular Carbon Materials

The Shockley-Queisser limit places an upper bound on solar conversion efficiency for a single p-n junction solar cell at slightly more than 30%. To surpass this limit, multi-exciton generation is being explored in inorganic semiconductors, while singlet fission (SF) is being investigated in arrays of conjugated organic molecules. In an optimal SF process, the lowest singlet excited state of one molecule (S1) that is positioned next to a second molecule in its ground state (S0) is down-converted into two triplet excited states (T1) each residing on one of the two adjacent molecules. The two triplet states initially form a correlated pair state 1(T1T1), which then evolves into two separated triplet states (T1 + T1). As such, the energetic requirement for SF is E(S1) ≥ 2 ´ E(T1).We have set our focus in recent years on intramolecular SF in molecular carbon materials and their studies in solution rather than on intermolecular SF investigations in crystalline films.Implicit in intramolecular SF is a resonant, direct excitation of the SF material. In dimers of molecular carbon materials linked by a myriad of molecular spacers, SF takes place with quantum yields of up to 200%. In addition, all key intermediates in the SF process, including the formation and decay of a quintet state that precedes formation of the pentacene triplet excitons, have been identified. This approach is, however, limited to the part of the solar spectrum, where, for example, the dimers of carbon materials feature a significant absorption cross-section. To employ the remaining part of the solar spectrum necessitates non-resonant, indirect excitation of the SF materials via either up- or down-conversion. For example, the up-conversion approach is realized with singlet excited states , which are accessed by two-photon absorptions (TPA). TPA is then followed in the second step of the sequence by an intramolecular SF – similar to what is seen upon resonant, direct excitation. Quite different is the down-conversion approach, which is based on an intramolecular Förster resonance energy transfer (FRET) and thereby the (photo)activation of the SF material. FRET requires the use of a complementary absorbing chromophore and enables funneling its excited state energy unidirectionally to the SF performing pentacene dimer. Again, SF completes the reaction sequence.

The effect of the electron-donor ability on the OLED efficiency of twisted donor-acceptor type emitters

Two twisted donor-acceptor (D-A) chemical structures, CCDMB and PCDMB, were developed as a new class of thermally activated delayed fluorescence (TADF) emitters for organic light-emitting diodes (OLEDs). Two emitters consist of 3-substituted carbazole as a first donor and trivalent boron as an electron acceptor in common, and carbazole and phenoxazine as second donors with different electron donor ability. While PCDMB with a strong phenoxazine donor decreased the lowest singlet excited state (S 1 ) level and thus showed a small singlet-triplet energy difference (ΔE ST ) value of 0.13 eV, resulting in effective reverse intersystem crossing (RISC), however, CCDMB with a weak donor showed a large ΔE ST value of 0.21 eV. Efficient triplet harvesting of PCDMB was confirmed by a delayed component in transient PL decay curves of 25 wt% PCDMB-doped bis[2-(diphenylphosphino)phenyl] ether oxide (DPEPO) films. OLED devices with a CCDMB emitter showed deep-blue emission with Commission Internationale de l’Éclairage (CIE) of (0.16, 0.12) but a low maximum EQE of 5.5%, indicative of insufficient triplet harvesting. PCDMB-based devices showed green emission with CIE of (0.21, 0.45) and a high maximum EQE of 22.3%. Our study revealed the effect of the electron donor ability of structurally similar emitters on ΔE ST values, triplet harvesting, and device efficiency. • We synthesized two thermally activated delayed fluorescence (TADF) emitters with difference only in electron-donor ability. • PCDMB has a smaller ΔE ST value than CCDMB owing to the stronger electron donating ability of phenoxazine than carbazole. • PCDMB -based devices showed a higher EQE compared to CCDMB -based devices. • This study revealed the effect of the electron donor ability of emitters on ΔE ST , triplet harvesting, and device efficiency.

Enhanced singlet oxygen production over a photocatalytic stable metal organic framework composed of porphyrin and Ag

Porphyrin is an important photosensitizer for singlet oxygen (1O2) formation. However, porphyrin derivatives still display low efficiency and difficult recovery. In this work, an Ag-based MOFs (AgTPyP; TPyP, 5,10,15,20-tetra(4-pyridyl) porphyrin) is synthesized. Compared with ligand TPyP, AgTPyP shows nearly 100% conversion and selectivity in the photocatalytic selective oxidation of sulfides to sulfoxides and obvious photodynamic therapeutic effect under visible light. Combined characterizations suggest that 1O2 is the only active oxygen species, which is due to the large exciton binding energy and narrow energy gap between the lowest excited singlet state (S1) and the lowest triplet state (T1) induced by the presence of Ag+ ion. The selectivity and conversion efficiency of five-cycle experiments do not decrease, indicating the excellent stability of AgTPyP. This work provides an alternative approach to simultaneously enhance the exciton effect of porphyrin and stabilize Ag-based photocatalysts.

Molecular Vibration Accelerates Charge Transfer Emission in a Highly Twisted Blue Thermally Activated Delayed Fluorescence Material.

In the development of new organic light-emitting diodes, thermally activated delayed fluorescence (TADF) materials have drawn interest because of their ability to upconvert electrically generated triplet excitons into singlets. Efficient TADF requires a well-balanced large transition dipole moment (μ) between the lowest excited singlet state (S1) and the ground state (S0) and a small energy splitting (ΔEST) between S1 and the lowest triplet state (T1). However, a number of highly twisted donor-acceptor-type TADF molecules have been reported to exhibit high performance in OLEDs, although these molecules may sacrifice μ in exchange for a very small ΔEST. Here, we theoretically investigate the origin of efficient emission from a perpendicularly twisted blue emitter, MA-TA. In this system, the μ value almost vanishes in the static approximation; however, vibrational contributions increase μ considerably. Hence, we show that the dynamics of excitons have a critical role in such TADF systems.

Controlling Emissive Properties by Intramolecular Hydrogen Bonds: Alkyl and Aryl meso-Substituted Porphycenes.

Porphycene, a porphyrin isomer, is an efficient fluorophore. However, four-fold meso substitution with alkyl groups decreases the fluorescence quantum yield by orders of magnitude. For aryl substituents, this effect is small. To explain this difference, we have synthesized and studied a mixed aryl-alkyl-substituted compound, 9,20-diphenyl-10,19-dimethylporphycene, as well as the 9,20-diphenyl and 9,20-dimethyl derivatives. Analysis of the structural, spectroscopic, and photophysical data of the six porphycenes, combined with quantum chemical calculations, shows a clear correlation between the strength of the intramolecular NH⋅⋅⋅N hydrogen bonds and the efficiency of the radiationless depopulation of the lowest-excited singlet state. This result led us to propose a model in which the delocalization of the inner protons in the cavity of the macrocycle is responsible for the nonradiative deactivation channel. The applicability of the model is confirmed by the literature data for other alkyl- or aryl-substituted porphycenes. The finding of a correlation between structural and emissive characteristics enables a rational design of porphycenes with desired photophysical properties.

Unraveling the Important Role of High-Lying Triplet-Lowest Excited Singlet Transitions in Achieving Highly Efficient Deep-Blue AIE-Based OLEDs.

Aggregation-induced emission (AIE) materials are attractive for achieving highly efficient nondoped organic light-emitting diodes (OLEDs) owing to their strong luminescence in the solid state. However, the electroluminescence efficiency of most AIE-based OLEDs remains low owing to the waste of triplet excitons. Here, using theoretical calculations, photophysical dynamics, and magnetoluminescence measurements, the spin conversion process is demonstrated between the high-lying triplet state (Tn ) and the lowest excited singlet state (S1 ) in AIE materials. Moreover, the relative positions of Tn (n<4) and S1 are shown to have a significant impact on the spin-conversion efficiency, thus influencing the harvesting of triplet excitons and the device efficiency. Finally, by selecting an upconversion material with an appropriate energy level for further utilizing the triplet excitons, a deep-blue fluorescent OLED with CIE coordinates of (0.15, 0.08), amaximum external quantum efficiency of 10.2%, low efficiency roll-off, and a high brightness of 16817cdm-2 is developed. This is one of the most efficient deep-blue OLEDs based on AIE materials reported so far. These findings also provide new insights into the design of more efficient AIE molecules and corresponding OLEDs by managing high-lying triplet excitons.

Light-harvesting photocatalysis for H2 evolution by methylacridone-bridged periodic mesoporous organosilica

Photocatalytic H2 production from water is one approach to solving global energy problems. Much attention has been paid to the promising applications of solid-state molecular photocatalysts because of their ease of handling, recyclability, and designable catalytic properties. We report an efficient photocatalytic H2 evolution system using methylacridone (MeAcd)-bridged periodic mesoporous organosilica (MeAcd-PMO), which has visible light absorption and electron-donating properties. MeAcd-PMO loaded with Pt nanoparticles efficiently generated H2 under visible light irradiation with a high apparent quantum yield (8.6% at 400 nm). Transient absorption measurements revealed a unique light-harvesting mechanism involving the excitation energy migration of the lowest singlet excited state on the MeAcd-PMO framework, which has not been observed in conventional solid-state molecular photocatalysts. MeAcd-PMO exhibiting this energy migration behavior is expected to be useful as a host material for constructing efficient photocatalytic systems which mimic natural photosynthesis

Excited state structure of isolated 2-cyanoindole and the binary 2-cyanoindole-(H2O)1 cluster from a combined Franck-Condon and rotational constants fit

Laser induced fluorescence and fluorescence emission spectra of 2-cyanoindole and the binary 2-cyanoindole-(H2O)1 cluster in a molecular beam were analyzed using a combination of Franck-Condon (FC) fits of the emission spectra and ab initio calculations. The structural changes upon electronic excitation to the lowest excited singlet state have been elucidated by a combined rotational constants/vibronic intensities fit. The experimentally determined geometry changes and the ab initio calculations point to an Lb-like excited state for the monomer. FC fits of emission spectra of the binary water cluster and quantum chemical calculations show that the emission of the water cluster also takes place from an Lb-like excited state structure. However, for 2-cyanoindole-(H2O)1 the second highest occupied molecular orbital mixes strongly with the highest one. Therefore, the difference between La and Lb characteristics are smaller than for other substituted indoles, including the other positional conformers of the n-cyanoindoles.

Phototransformations of 2-aminonicotinic acid resolved with matrix isolation infrared spectroscopy and ab initio calculations

The structure and phototransformations of 2-aminonicotinic acid (ANA) induced by tunable UV laser were studied in low-temperature argon matrices with the use of Fourier-transform infrared (FTIR) spectroscopy. After the deposition of the argon matrix, two amino isomers (ANA-1 and ANA-2) were detected, with populations of 96 % and 4 %, respectively. Electronic structure calculations confirm these to be the two most stable isomers of ANA. The irradiation of the amino isomers gives rise to an intricate system of photoinduced reactions, which involves amino-imino phototautomerism, and rotations of the carboxylic group. Following irradiation at 350 nm, a rotation of the carboxylic group of ANA-1 was detected leading to increase of the ANA-2 form. In turn, at 310 nm two new imino forms (INA-1 and INA-2) of 2-aminonicotinic acid were generated indicating that UV-induced hydrogen atom transfer between ANA and INA tautomeric forms took place. The corresponding reverse photorotamerizations were also detected at 390 nm, resulting in the recovery of the initial ANA-1 and ANA-2 isomers. On the basis of electronic structure calculations, we hypothesize that ANA-INA phototautomerization takes place in the vibrationally hot electronic ground state. Its occurrence appears to be contingent on the molecule being able to access a conical intersection between the lowest singlet excited state and the ground state.

Slow photoionization via higher excited states of N,N-dimethylaniline in ethanol solution probed by femtosecond transient absorption spectroscopy under two-pulse two-photon excitation.

Photoionization dynamics of N,N-dimethylaniline (DMA) from highly electronically excited states in ethanol solution was investigated by means of femtosecond two-pulse two-photon excitation transient absorption (2PE-TA) spectroscopy. The first pump pulse prepares the lowest singlet excited state (S1 state) of DMA, and the second one excites the S1 state into higher excited states. In the case with the second pulse at 500 nm, the ionization took place via a rapid channel (<100 fs) and a slow one with the time constant of ∼10 ps. The excitation wavelength effect of the second pulse indicated that a specific electronic state produced directly from higher excited states was responsible for the slow ionization. By integrating these results with the time evolution of the transient absorption spectra of the solvated electron in neat ethanol detected by the simultaneous two-photon excitation, it was revealed that the slow ionization of DMA in ethanol was regulated by the formation of the anionic species just before the completion of the solvation of the electron, leading to the solvated electron in the relaxed state. From these results, it was strongly suggested that the capture of the electron of the Rydberg-like state by the solvent or solvent cluster regulates the appearance of the cation radical.

The role of the synthetic chromophore of GFP in generating polymorphism-dependent on/off photoluminescence

Polymorphism, i.e. the ability of a solid material to exist in more than one crystal form, gives unique opportunity to study the correlation between the solid-state molecular arrangement and the photoluminescence properties, and allows optimizing the performances of materials with reduced synthesis effort. Compound 1, which results from the combination of 2-phenylbenzoxazole (PBO) and benzylideneimidazolinone (BDI), was known to be weakly fluorescent in solution and in the solid state. Time-dependent density functional theory (TD-DFT) and spin-flip TD-DFT calculations were first used to revisit the behaviour of the isolated molecule in solution. It was shown that the relaxation pathway in the lowest singlet excited state leads to a thermally-accessible conical intersection (CI), which favours non-radiative deactivation. Then, crystallizing molecule 1 under precise experimental conditions allowed four distinct polymorphs and one solvate to be obtained. Remarkably, forms 1a and 1b exhibited strong photoluminescence (PL), while polymorphs 1c-e were virtually not emissive. Recrystallization allowed passing reversibly from one crystalline species to another. Mechanical stimuli, exhibition to solvent vapours and heating were not very effective, but a non-trivial effect of PL enhancement upon freezing was observed for 1a and 1b. The non-emissive polymorphs were characterized by the presence of a centrosymmetric dimer structured by hydrogen bonds, in which paired molecules vibrate with the same mode and are in turn subjected to maximum deformation, according to DFT calculations. Based on the whole set of results, possible deactivation mechanisms were proposed to account for the opposite PL behaviour of the two types of crystalline species. It is shown that the emergence of polymorphism and of a rare on/off PL mechanism may be attributed to the presence of the BDI unit in the molecule.

The primary photo-dissociation dynamics of lactate in aqueous solution: decarboxylation prevents dehydroxylation.

We study the primary photolysis dynamics of aqueous lactate induced by photo-excitation at λ = 200 nm. Our calculations indicate that both decarboxylation and dehydroxylation are energetically possible, but decarboxylation is favoured dynamically. UV pump - IR probe transient absorption spectroscopy shows that the photolysis is dominated by decarboxylation, whereas dehydroxylation is not observed. Analysis of the transient IR spectrum suggests that photo-dissociation of lactate primarily produces CO2 and CH3CHOH- through the lowest singlet excited state of lactate, which has a lifetime of τ = 11 ps. UV pump - VIS probe transient absorption spectroscopy of electrons from the dissociating lactate anion indicates that the anionic electron from the CO2˙- fragment is transferred to the CH3CHOH˙ counter radical during the decarboxylation process, and CO2˙- is consequently only observed as a minor photo-product. The photo-dissociation quantum yield after the full decay of the excited state is Φ(100ps) = 38 ± 5%.

Face-to-Face Orientation of Quasiplanar Donor and Acceptor Enables Highly Efficient Intramolecular Exciplex Fluorescence.

Intramolecular through-space charge-transfer (TSCT) excited states have been exploited for developing thermally activated delayed fluorescence (TADF) emitters, but the tuning of excited state dynamics by conformational engineering remains sparse. Designed here is a series of TSCT emitters with precisely controlled alignment of the donor and acceptor segments. With increasing intramolecular π-π interactions, the radiative decay rate of the lowest singlet excited state (S1 ) progressively increased together with a suppression of nonradiative decay, leading to significantly enhanced photoluminescence quantum yields of up to 0.99 in doped thin films. A high-efficiency electroluminescence device, with a maximum external quantum efficiency (EQE) of 23.96 %, was achieved and maintains >20 % at a brightness of 1000 cd m-2 . This work sheds light on the importance of conformation control for achieving high-efficiency intramolecular exciplex emitters.

High-Efficiency, Non-doped, Pure-Blue Fluorescent Organic Light-Emitting Diodes via Molecular Tuning Regulation of Hot Exciton Excited States.

Tremendous efforts have been made on researching triplet-triplet annihilation (TTA) and thermally activated delayed fluorescence (TADF) materials for realizing high-efficiency blue organic light-emitting diodes (OLEDs) through utilizing triplet exciton conversion to the lowest singlet excited state (S1) from the lowest triplet excited state (T1). However, hot exciton conversion from the upper triplet energy level state (Tn, n > 1) to the lowest singlet excited state (S1) is an increasingly promising method for realizing pure-blue non-doped OLEDs with performances comparable to those of TTA and TADF materials. Herein, two pure-blue fluorescent emitters of donor (D)-π-acceptor (A) type, PIAnCz and PIAnPO, were designed and synthesized. The excited-state characteristics of PIAnCz and PIAnPO, confirmed by theoretical calculations and photophysical experiments, demonstrated these materials' hot exciton properties. Based on PIAnCz and PIAnPO as emission layer materials, the fabricated non-doped devices exhibited pure-blue emission with Commission Internationale de l'Eclairage (CIE) coordinates of (0.16, 0.12) and (0.16, 0.15), maximum luminescences of 10,484 and 15,485 cd m-2, and maximum external quantum efficiencies (EQEs) of 10.9 and 8.3%. Besides, at a luminescence of 1000 cd m-2, the EQEs of PIAnPO-based devices can still be high at 7.7%, and the negligible efficiency roll-off was 6.0%. The device performance of both materials demonstrates their outstanding potential for commercial application.

Face‐to‐Face Orientation of Quasiplanar Donor and Acceptor Enables Highly Efficient Intramolecular Exciplex Fluorescence

AbstractIntramolecular through‐space charge‐transfer (TSCT) excited states have been exploited for developing thermally activated delayed fluorescence (TADF) emitters, but the tuning of excited state dynamics by conformational engineering remains sparse. Designed here is a series of TSCT emitters with precisely controlled alignment of the donor and acceptor segments. With increasing intramolecular π–π interactions, the radiative decay rate of the lowest singlet excited state (S1) progressively increased together with a suppression of nonradiative decay, leading to significantly enhanced photoluminescence quantum yields of up to 0.99 in doped thin films. A high‐efficiency electroluminescence device, with a maximum external quantum efficiency (EQE) of 23.96 %, was achieved and maintains &gt;20 % at a brightness of 1000 cd m−2. This work sheds light on the importance of conformation control for achieving high‐efficiency intramolecular exciplex emitters.

New Generation UV-A Filters: Understanding Their Photodynamics on a Human Skin Mimic.

The sparsity of efficient commercial ultraviolet-A (UV-A) filters is a major challenge toward developing effective broadband sunscreens with minimal human- and eco-toxicity. To combat this, we have designed a new class of Meldrum-based phenolic UV-A filters. We explore the ultrafast photodynamics of coumaryl Meldrum, CMe, and sinapyl Meldrum (SMe), both in an industry-standard emollient and on a synthetic skin mimic, using femtosecond transient electronic and vibrational absorption spectroscopies and computational simulations. Upon photoexcitation to the lowest excited singlet state (S1), these Meldrum-based phenolics undergo fast and efficient nonradiative decay to repopulate the electronic ground state (S0). We propose an initial ultrafast twisted intramolecular charge-transfer mechanism as these systems evolve out of the Franck-Condon region toward an S1/S0 conical intersection, followed by internal conversion to S0 and subsequent vibrational cooling. Importantly, we correlate these findings to their long-term photostability upon irradiation with a solar simulator and conclude that these molecules surpass the basic requirements of an industry-standard UV filter.

Prediction of the Existence of LiCH: A Carbene-like Organometallic Molecule.

Carbenes comprise a well-known class of organometallic compounds each consisting of a neutral, divalent carbon and two unshared electrons. Carbenes can have singlet or triplet ground states, each giving rise to a distinct reactivity. Methylene (CH2), the parent hydride, is well-known to be bent in its triplet ground state. Here, we predict the existence of LiCH, a carbene-like organometallic molecule. Computationally, we treat the electronic structure with parametric and variational two-electron reduced density matrix (2-RDM) methods, which are capable of capturing multireference correlation typically associated with the singlet state of a diradical. Similar to methylene, LiCH is a triplet ground state with a predicted 15.8 kcal/mol singlet-triplet gap. However, unlike methylene, LiCH is linear in both the triplet state and the lowest excited singlet state. Furthermore, the singlet state is found to exhibit strong electron correlation as a diradical. In comparison to dissociation channels Li + CH and Li+ + CH-, the LiCH was found to be stable by approximately 77 kcal/mol.

Investigating the interaction of mitoxantrone with anionic surfactants by spectrofluorimetry and its application for the feasible analysis of pharmaceutical preparation and biological fluids.

The behaviour of mitoxantrone (MTX), an anthracenedione antineoplastic agent, in different types of organized medium was explored using molecular spectrofluorometry. The original fluorescence and quantum yield of MTX were augmented by about five-fold in the aqueous buffered solution (Britton-Robinson, pH 3.0) by the addition of sodium dodecyl sulfate. Enhancement in the fluorescence intensity did not come from the boost in the ultraviolet (UV) light absorbance of the drug in the presence of micelles but due to shielding of the lowest excited singlet state of the drug from a radiationless process inside the cavity of the micelle. Accordingly, a versatile, sensitive, and feasible spectrofluorimetric method was constructed and evaluated for MTX determination. Fluorescence measurements were performed at 675 nm (λex 610 nm). A linear relationship was shown between fluorescence intensity and drug concentration within the range 0.01-2.0 μg ml-1 of MTX with a correlation coefficient of 0.9999 and a detection limit of 2 ng ml-1 . The developed method was effectively used for analysis of MTX in biological samples and dosage forms. In addition, the method was expanded to study the stability of MTX exposed to different drastic degradations and the kinetic parameters of the degradation were calculated.