Conformational, Host, and Vibrational Effects Giving Rise to Dynamic TADF Behavior in the Through-Space Charge Transfer, Triptycene Bridged Acridine-Triazine Donor Acceptor TADF Molecule TpAT-tFFO.

We present a joint experimental and theoretical study of the through-space charge transfer (CT) TADF molecule TpAT-tFFO. The measured fluorescence has a singular Gaussian line shape but two decay components, coming from two distinct molecular CT conformers, energetically only 20 meV apart. We determined the intersystem crossing rate (1 × 107 s-1) to be 1 order of magnitude faster than radiative decay, and prompt emission (PF) is therefore quenched within 30 ns, leaving delayed fluorescence (DF) observable from 30 ns onward as the measured reverse intersystem crossing (rISC) rate is >1 × 106 s-1, yielding a DF/PF ratio >98%. Time-resolved emission spectra measured between 30 ns and 900 ms in films show no change in the spectral band shape, but between 50 and 400 ms, we observe a ca. 65 meV red shift of the emission, ascribed to the DF to phosphorescence transition, with the phosphorescence (lifetime >1 s) emanating from the lowest 3CT state. A host-independent thermal activation energy of 16 meV is found, indicating that small-amplitude vibrational motions (∼140 cm-1) of the donor with respect to the acceptor dominate rISC. TpAT-tFFO photophysics is dynamic, and these vibrational motions drive the molecule between maximal rISC rate and high radiative decay configurations so that the molecule can be thought to be "self-optimizing" for the best TADF performance.

Vibronic effects accelerate the intersystem crossing processes of the through-space charge transfer states in the triptycene bridged acridine-triazine donor-acceptor molecule TpAT-tFFO.

Quantum chemical studies employing combined density functional and multireference configuration interaction methods suggest five excited electronic states to be involved in the prompt and delayed fluorescence emission of TpAT-tFFO. Three of them, a pair of singlet and triplet charge transfer (CT) states (S1 and T1) and a locally excited (LE) triplet state (T3), can be associated with the (Me → N) conformer, the other two CT-type states (S2 and T2) form the lowest excited singlet and triplet states of the (Me → Ph) conformer. The two conformers, which differ in essence by the shearing angle of the face-to-face aligned donor and acceptor moieties, are easily interconverted in the electronic ground state whereas the reorganization energy is substantial in the excited singlet state, thus explaining the two experimentally observed time constants of prompt fluorescence emission. Forward and reverse intersystem crossing between the singlet and triplet CT states is mediated by vibronic spin-orbit interactions involving the LE T3 state. Low-frequency vibrational modes altering the distance and alignment of the donor and acceptor π-systems tune the S1 and T3 states (likewise S2 and T3) into and out of resonance. The enhancement of intersystem crossing due to the interplay of vibronic and spin-orbit coupling is considered a general feature of organic through-space charge-transfer thermally activated delayed fluorescence emitters.

Intersystem crossing processes in the 2CzPN emitter: a DFT/MRCI study including vibrational spin-orbit interactions.

Multireference quantum chemical calculations were performed in order to investigate the (reverse) intersystem crossing ((R)ISC) mechanisms of 4,5-di(9H-carbazol-9-yl)-phthalonitrile (2CzPN). A combination of density funcional theory (DFT) and multireference configuration interaction methods (MRCI) was used. The excellent agreement of the computed absorption spectrum with available experimental absorption spectra lends confidence to the chosen computational protocol. Vertically, two triplet excited states (T1 and T2) are found below the S1 state. At the excited state minima, the calculated adiabatic energies locate only the T1 state below the S1 state. The enhanced charge transfer (CT) character of the geometrically relaxed excited states causes their mutual (direct) spin-orbit coupling (SOC) interaction to be low. Contributions of vibronic SOC to the (R)ISC probability, evaluated by a Herzberg-Teller-like procedure for a temperature of 300 K, are small but not negligible. For ISC, the S1→ T1 channel is the fastest (8 × 106 s-1), while the S1→ T2 channel is found to be thermally activated (9 × 104 s-1) and less efficient when proceeding from the adiabatic S1 state. Our calculations also reveal, however, a barrierless S1→ T2 ISC pathway near the Franck-Condon region. RISC is found to essentially proceed via the T1→ S1 channel, with a rate constant of (3 × 104 s-1) if our adiabatic singlet-triplet energy gap in vacuum (ΔEST = 0.12 eV) is employed. Shifting the potentials to match two experimentally reported singlet-triplet energy gaps in toluene (ΔEST = 0.21 and 0.31 eV, respectively) leads to a drastic reduction of the computed rate constant by up to 4 orders of magnitude. The T2 state is not expected to play a major role in mediating triplet-singlet transitions in 2CzPN unless it is directly populated by hot excitons. No indication for a strong vibronic coupling of the T2 and T1 potentials is found, which could help overcome the negative exponential dependence of the RISC rate constant on the magnitude of the energy gap.

DFT/MRCI assessment of the excited-state interplay in a coumarin-schiff Mg2+ fluorescent sensor.

Fluorescent sensors with selectivity and sensitivity to metal ions are an active field in supramolecular chemistry for biochemical, analytical, and environmental problems. Mg2+ is one of the most abundant divalent ions in the cell, and it plays a critical role in many biological processes. Coumarin-based sensors are widely used as desirable fluorophore and binding moieties showing a remarkable sensitivity and fluorometric enhancement for Mg2+ . In this work, density functional theory/multireference configuration interaction (DFT/MRCI) calculations were performed in order to understand the sensing behavior of the organic fluorescent sensor 7-hydroxy-4-methyl-8-((2-(pyridin-2-yl)hydrazono)methyl)-2H-chromen-2-one (PyHC) in ethanol to solvated Mg2+ ions. The computed optical properties reproduce well-reported experimental data. Our results suggest that after photoexcitation of the free PyHC, a photo-induced electron transfer (PET) mechanism may compete with the fluorescence decay to the ground state. In contrast, this PET channel is no longer available in the complex with Mg2+ making the emissive decay more efficient. © 2019 Wiley Periodicals, Inc.

Efecto de la estructura molecular sobre el espectro electrónico vertical de la oxoglaucina / Effects of the molecular structure on the electronic vertical spectrum of oxoglaucine / Efeito da estrutura molecular no espectro eletrônico vertical da oxoglaucina

Resumen Se investigó el espectro electrónico vertical de la oxoglaucina utilizando el método de interacción de configuraciones multi-referencial (DFT/MRCI). Se analizó el efecto de los grupos metilo y metoxilo sobre la geometría y la distribución energética de los estados excitados de baja energía utilizando la base TZVP. Se encontró que, en los mínimos del estado fundamental de oxoglaucina, oxoglaucina sin grupos metilo y sin grupos metoxilo, los estados excitados singulete de más baja energía son de tipo nП* (S1) y ПП* (S2) y que los estados triplete ПП* (T1) y nП* (T2) son energéticamente accesibles desde S1 Desde el punto de vista energético, se encontró que el canal más eficiente para el entrecruzamiento de sistemas singulete-triplete es 1(nП*)3(ПП*). Aunque la presencia de los grupos metilo y metoxilo distorsionan la planaridad del sistema de anillos, su efecto en la distribución (vertical) de los estados singulete y triplete de más baja energía es mínimo.

Abstract The vertical electronic spectrum of oxoglaucine by means of a multireference configuration interaction method (DFT/ MRCI) was studied. The effect of both methyl and methoxy groups on the geometries and energetic distribution of the low-lying excited states was analyzed. The results show that, by means of the TZVP basis set, at the ground state minima of oxoglaucine, oxoglaucine without methyl and methoxy groups, the lowest excited singlet states are of nП* (S1) and ПП* (S2) type. Triplet states of ПП* (T1) and nП* (T2) type are energetically accessible from S1. From the energetic point of view, it can be proposed that the channel for an efficient intersystem crossing 1(nП*)3(ПП*) is plausible. Although the presence of the methyl and methoxyl groups distort the planarity of the rings system, the effect in the vertical distribution of the lowest lying singlet and triplet states can be considered as negligible.

Resumo O espectro eletrônico vertical da oxoglaucina foi investigado por meio do método de interação de configurações multi referência (DFT/MRCI). Foi estudado o efeito dos grupos metilo e metoxi nas geometrias e na distribuição energética dos estados excitados de baixa energia utilizando a base TZVP. Os resultados mostram que nos mínimos do estado fundamental da oxoglaucina, a oxoglaucina sem grupos metilo e metoxi os estados excitados singletos de mais baixa energia são de tipo nП* (S1) y ПП* (S2) e que os estados tripletos do tipo ПП* (T1) e nП* (T2) são energeticamente acessíveis a partir de S1 Do ponto de vista energético, pode-se propor que o canal para um cruzamento intersistema eficiente 1(nП*)3(ПП*) seja plausível. Embora a presença dos grupos metilo e metoxilo distorçam a planaridade do sistema de anéis, o efeito na distribuição vertical dos estados de singleto e de tripleto de mais baixa energía pode ser considerado negligenciável.

Exploring the relevance of thiophene rings as bridge unit in acceptor-bridge-donor dyes on self-aggregation and performance in DSSCs.

The possibility of dye charge recombination in DSSCs remains a challenge for the field. This consists of: (a) back-transfer from the TiO2 to the oxidized dye and (b) intermolecular electron transfer between dyes. The latter is attributed to dye aggregation due to dimeric conformations. This leads to poor electron injection which decreases the photocurrent conversion efficiency. Most organic sensitizers are characterized by an Acceptor-Bridge-Donor (A-Bridge-D) arrangement that is commonly employed to provide charge separation and, therefore, lowering the unwanted back-transfer. Here, we address the intermolecular electron transfer by studying the dimerization and photovoltaic performance of a group of A-Bridge-D structured dyes. Specifically, eight famous sulfur containing π-bridges were analyzed (A and D remained fixed). Through quantum mechanical and molecular dynamics approaches, it was found that the formation of weakly stabilized dimers is allowed. The dyes with covalently bonded and fused thiophene rings as Bridges, 6d and 7d as well as 8d with a fluorene, would present high aggregation and, therefore, high probability of recombination processes. Conversely, using TiO2 cluster and surface models, delineated the shortest bridges to improve the adsorption energy and the stability of the system. Finally, the elongation of the bridge up to 2 and 3 units and their photovoltaic parameters were studied. These results showed that all the sensitizers are able to provide similar photocurrent outcomes, regardless of whether the bridge is elongated. © 2017 Wiley Periodicals, Inc.

Effects of the acceptor unit in dyes with acceptor-bridge-donor architecture on the electron photo-injection mechanism and aggregation in DSSCs.

Dye-sensitized solar cells (DSSCs) are devices that convert light to electrical energy. Nowadays, researchers have focused on the understanding of the performance of dyes in solar cells. In this way, new efficient dyes have been obtained which can act as efficient light-harvesting compounds where the combination and the balance of acceptor(A)-bridge-donor(D) architectures confer suitable attributes and properties to the dye. Herein, we have carried out a DFT study on the optical and electronic properties of eight different A motifs and their influence on the electron photo-injection (PI) mechanisms through type I (indirect) or type II (direct) pathways in A-bridge-D dyes in DSSCs. The models consisted of thiophene as a bridge and triphenylamine as a D anchored to a TiO2 anatase cluster. All geometry optimizations were calculated using the B3LYP, CAM-B3LYP and BHandHLYP functionals combined with the 6-31G(d,p) basis set for C, H, N, O and S and the LANL2DZ pseudopotential for Ti atoms. Most of the A dyes display optoelectronic properties consistent with a type-I (indirect) mechanism except for the A5 dye where the results suggest a type-II (direct) PI pathway. In addition, molecular dynamics (MD) simulations have been carried out in order to describe the formation of dye dimers and analyze the stability of the aggregates due to intermolecular interactions. The observed trends indicate that dyes with A2 and A5 anchoring groups have less tendency to dimerize due to weaker intermolecular interactions resulting in less stable dimer complexes. Specifically, we found that the A motif influences the PI by a dye and the dimerization profiles.

The Nature of the Donor Motif in Acceptor-Bridge-Donor Dyes as an Influence in the Electron Photo-Injection Mechanism in DSSCs.

The combination and balance of acceptor(A)-bridge-donor(D) architecture of molecules confer suitable attributes and/or properties to act as efficient light-harvesting and sensitizers in dye sensitized solar cells (DSSCs). An important process in a DSSC performance is the electron photoinjection (PI) mechanism which can take place either via type I (indirect), that consists in injecting from the excited state of the dye to the semiconductor, or type II (direct), where the PI is from the ground state of the dye to the semiconductor upon photoexcitation. Here, we present a computational study about the role of the donor motif in the PI mechanisms displayed from a family of 11 A-bridge-D structured dyes to a (TiO2)15 anatase cluster. To this end, different donor motifs (D1-D11) were evaluated while the A and bridge motifs remained the same. All the computations were carried out within the DFT framework, using the B3LYP, PW91, PBE, M06L and CAM-B3LYP functionals. The 6-31G(d) basis set was employed for nonmetallic atoms and the LANL2DZ pseudopotential for Ti atoms. The solvation effects were incorporated using the polarized continuum model (PCM) for acetonitrile. As benchmark systems, alizarin and naphthalenediol dyes were analyzed, as they are known to undergo Type I and Type II PI pathways in DSSCs, respectively. Donors in the studied family of dyes could influence to drive Type I or II PI since it was found that D2 could show some Type II PI route, showing a new absorption band, although with CAM-B3LYP this shows a very low oscillator strength, while the remaining dyes behave according to Type I photoinjectors. Finally, the photovoltaic parameters that govern the light absorption process were evaluated, as the use of these criteria could be applied to predict the efficiency of the studied dyes in DSSCs devices.

Internal heavy atom effects in phenothiazinium dyes: enhancement of intersystem crossing via vibronic spin-orbit coupling.

The effect of substituting the intra-cyclic sulphur of thionine by oxygen (oxonine) and selenium (selenine) on the intersystem crossing (ISC) efficiency has been studied using high level quantum mechanical methods. The ISC rate constants are considerably increased when going from O towards Se while the fluorescence rate constants remain unchanged. For the three dyes, all accessible ISC channels are driven by vibronic spin-orbit coupling (SOC) between ππ* states. The interplay between the ground and low-lying excited states has been investigated in order to determine the dominant relaxation pathways. In oxonine the relaxation to the ground state after photoexcitation in water proceeds essentially via fluorescence from the S1(πHπL*) bright state (kF = 2.10 × 10(8) s(-1)), in agreement with the high experimental fluorescence quantum yield. In aqueous solution of thionine, the ISC rate constant (kISC ∼ 1 × 10(9) s(-1)) is one order of magnitude higher than fluorescence (kF = 1.66 × 10(8) s(-1)) which is consistent with its high triplet quantum yield observed in water (ϕT = 0.53). Due to a stronger vibronic SOC in selenine, the ISC rate is very high (kISC ∼ 10(10) s(-1)) and much faster than fluorescence (kF = 1.59 × 10(8) s(-1)). This suggests selenine-based dyes as very efficient triplet photosensitizers.

Proton transfer from 1,4-pentadiene to superoxide radical anion: a QTAIM analysis / Transferencia de protón desde el 1,4-pentadieno al radical anión superóxido: un análisis QTAIM / Transferência de prótons de 1,4-pentadieno o ânion radical superóxido: uma análise QTAI

We studied the bis-allylic proton transfer reaction from 1,4-pentadiene to superoxide radical anion (O2.-). Minima and transition state geometries, as well as thermochemical parameters were computed at the B3LYP/6-311+G(3df,2p) level of theory. The electronic wave functions of reactants, intermediates, and products were analyzed within the framework of the Quantum Theory of Atoms in Molecules. The results show the formation of strongly hydrogen bonded complexes between the 1,4-pentadien- 3-yl anion and the hydroperoxyl radical as the reaction products. These product complexes (PCs) are more stable than the isolated reactants and much more stable than the isolated products. This reaction occurs via pre-reactive complexes which are more stable than the PCs and the transition states. This is in agreement with the fact that the net proton transfer reaction that leads to free products is an endothermic and nonspontaneous process.

Nosotros estudiamos la reacción de transferencia de protón bis-alílico del 1,4-pentadieno al radical anión superóxido (O2.--). Las geometrías de los mínimos y de los estados de transición, así como también los parámetros termoquímicos se calcularon usando el nivel de teoría B3LYP/6-311+G(3df,2p). Las funciones de onda electrónicas de los reactantes, intermedios y productos se analizaron dentro del marco de la teoría cuántica de átomos en moléculas. Nuestros resultados muestran la formación de complejos estabilizados por enlaces de hidrógeno entre el anión 1,4pentadien-3-ilo y el radical hidroperoxilo como productos de reacción. Estos complejos producto (PCs) son más estables que los reactantes aislados y mucho más estables que los productos aislados. Esta reacción ocurre vía la formación de complejos pre-reactivos, los cuales son más estables que los PCs y los estados de transición. Estos resultados están de acuerdo con el hecho de que la reacción global de transferencia de protón que conduce a la formación de los productos libres es un proceso endotérmico y no espontáneo.

Estudou-se a reação de transferência do próton bis-alílico do 1,4-pentadieno ao radical ânion superóxido (O2.-). As geometrias dos mínimos e dos estados de transição, bem como os parâmetros termoquímicos foram calculadas utilizando o nível de teoria B3LYP/6-311+G(3df, 2p). As funçÃμes de onda eletrònica dos reagentes, intermediários e produtos foram analisadas no âmbito da teoria quântica de átomos em moléculas. Os resultados obtidos demonstram a formação de complexos estabilizados por ligaçÃμes de hidrogênio entre o ânion 1,4-pentadieno- 3-ilo e o radical hidroperoxilo como produtos de reação. Estes complexos formados como produtos (PCs) são mais estáveis do que os reagentes isolados e muito mais estáveis do que os produtos isolados. Esta reação ocorre por meio de complexos pré-reativos mais estáveis do que os PCs e os estados de transição. Estes resultados estão de acordo com o fato da reação global de transferência de próton que conduz à formação dos produtos livres, é um processo endotérmico e não espontâneo.

A theoretical study of thionine: spin-orbit coupling and intersystem crossing.

A study of the possible intersystem crossing (ISC) mechanisms (S→T) in thionine (3,7-diamino-phenothiazin-5-ium), which is conducive to the efficient population of the triplet manifold, is presented. The radiationless deactivation channels {S(1),S(2)(π → π*) → T(1),T(2)(π → π*)} have been examined. Since the direct ISC does not explain the high triplet quantum yield in this system, attention has been centered on the vibronic spin-orbit coupling between the low-lying singlet and triplet (π → π*) states of interest. An efficient population transfer from the S(1)(π(H) → π(L)*) state to the T(2)(π(H-1) → π(L)*) state via this channel is confirmed. The calculated ISC rate constant for this channel is k(ISC) ≈ 3.35 × 10(8) s(-1), which can compete with the radiative depopulation of the S(1)(π(H) → π(L)*) state via fluorescence (k(F) ≈ 1.66 × 10(8) s(-1)) in a vacuum. The S(1)(π(H) → π(L)*) → T(1)(π(H) → π(L)*) and {S(2)(π(H-1) → π(L)*) → T(1),T(2)(π → π*)} ISC channels have been estimated to be less efficient (k(ISC) ≈ 10(5)-10(6) s(-1)). Based on the computed ISC rate constants and excited-state solvent shifts, it is suggested that the efficient triplet quantum yield of thionine in water is primarily due to the S(1)(π(H) → π(L)*) → T(2)(π(H-1) → π(L)*) channel with a computed rate constant of the order of 10(8)-10(9) s(-1) which is in accord with the experimental finding (k(ISC) = 2.8 × 10(9) s(-1)).

A quantum chemical investigation of the electronic structure of thionine.

We have examined the electronic and molecular structure of 3,7-diaminophenothiazin-5-ium dye (thionine) in the electronic ground state and in the lowest excited states. The electronic structure was calculated using a combination of density functional theory and multi-reference configuration interaction (DFT/MRCI). Equilibrium geometries were optimized employing (time-dependent) density functional theory (B3LYP functional) combined with the TZVP basis set. Solvent effects were estimated using the COSMO model and micro-hydration with up to five explicit water molecules. Our calculated electronic energies are in good agreement with experimental data. We find the lowest excited singlet and triplet states at the ground state geometry to be of π→π* (S(1), S(2), T(1), T(2)) and n→π* (S(3), T(3)) character. This order changes when the molecular structure in the electronically excited states is relaxed. Geometry relaxation has almost no effect on the energy of the S(1) and T(1) states (~0.02 eV). The relaxation effects on the energies of S(2) and T(2) are moderate (0.14-0.20 eV). The very small emission energy results in a very low fluorescence rate. While we were not able to locate the energetic minimum of the S(3) state, we found a non-planar minimum for the T(3) state with an energy which is very close to the energy of the S(1) minimum in the gas phase (0.04 eV above). When hydration effects are taken into account, the n→π* states S(3) and T(3) are strongly blueshifted (0.33 and 0.46 eV), while the π→π* states are only slightly affected (<0.06 eV).