Excited Singlet Research Articles

Steric Hindrance Governs the Photoinduced Structural Planarization of Cycloparaphenylene Materials.

Cycloparaphenylenes ([n]CPPs) and their derivatives are known for the unique size-dependent photophysical properties, which are largely attributed to the structural planarization-associated exciton localization, attracting substantial research attention. In this work, we show that the steric hindrance between neighboring structural units plays a key role in governing the photoinduced global/local structural planarization and electron-hole distribution features of [n]CPP materials, due to the tunable strength of H···H repulsion between neighboring units via structural modification or C-H distance variation as revealed by density functional theory (DFT) and time-dependent DFT calculations. According to our results, steric hindrance controls the manner and also the extent of excited-state structural planarization, where a weak (strong) steric hindrance favors (hinders) structural planarization upon relaxation in the first excited singlet (S1) state as compared to the ground (S0)-state structure. Depending on the molecular structures, steric hindrance leads to fully delocalized, partially separated, or more localized electron-hole distributions. For example, via H···H repulsion release by manually shortening the C-H distance or by chemical substitution of C-H with N atoms, the modified [10]CPP structures show fully planarized configurations (each dihedral angle can be less than 2°) and entirely delocalized electron-hole distribution upon photorelaxation. This work provides insights into the structural origin of the unusual photophysical properties of [n]CPPs and shows the promise of steric hindrance tuning in accessing diverse excited-state features in [n]CPP materials.

Highly Emissive Luminogens in Both Solution and Aggregate States

Highly Emissive Luminogens in Both Solution and Aggregate States

Performance of Boltzmann and crossover single-emitter luminescent thermometers and their recommended operation modes

Despite its simplicity, Boltzmann thermometry suffers from limited relative sensitivities since the energy gap must be in the order of magnitude of the thermal energy used for probing. Luminescent crossover thermometry is a potential alternative to exploit high energy gaps at low temperatures based on a generally high non-radiative coupling rate via a configurational crossover. The number of possible candidates for this type of temperature-dependent luminescence is quite versatile. Inorganic systems include transition metal ions with split d orbitals in a ligand field such as Cr3+ and Mn4+, mercury-like s2 ions such as Bi3+ or Pb2+ with coupled 3P0 and 3P1 levels, or lanthanoid ions with low energetic 4fn-15d1 levels next to their 4fn spin-orbit levels. In the case of organic emitters, thermally activated delayed fluorescence (TADF) has become increasingly popular, which relies on thermal coupling between an excited singlet and triplet state via intersystem crossing. While the general concept of two thermally excited radiatively emitting states is valid for each of the mentioned types of luminophores and thus, thermometry is in principle possible, the mechanistic details of the thermal coupling can be different. In this tutorial and theoretically motivated article, I will shortly review the relevant aspects of the excited state kinetics of two thermally coupled levels and demonstrate how the ratio between radiative and intrinsic non-radiative coupling rates affects the possible readout choices for optical signals used as a measure for temperature and how this influences the relative sensitivity of ratiometric and lifetime-based thermometers. An overview over various possible inorganic and organic emitters with the potential for thermometry will be given and based on these kinetic considerations, it will be assessed what readout mode is most recommendable for a given luminescent center.

Selective Excitation of Carotenoids of the Allochromatium vinosum Light-Harvesting LH2 Complexes Leads to Oxidation of Bacteriochlorophyll.

The mechanism of bacteriochlorophyll photooxidation in light-harvesting complexes of a number of purple photosynthetic bacteria when the complexes are excited into the carotenoid absorption bands remains unclear for many years. Here, using narrow-band laser illumination we measured action spectrum of this process for the spectral ranges of carotenoid and bacteriochlorophyll. It is shown that bacteriochlorophyll excitation results in almost no photooxidation of these molecules, while carotenoid excitation leads to oxidation with quantum yield of about 0,0003. Low value of the yield enabled an assumption that the studied process is initiated by the triplet states of the main carotenoids of the complexes with the number of conjugated double-bond chain length of N = 11. Interaction of these states with oxygen facilitates formation, though with low efficiency, of the excited singlet oxygen, which oxidizes bacteriochlorophylls. The carotenoid triplet states are formed in the process of the earlier studied singlet-triplet fission. The obtained results point at the necessity of reconsidering the functions of carotenoids in the light-harvesting complexes of purple bacteria.

Comparative Absorption Dynamics of the Singlet Excited States of Chlorophylls a and d.

Transient absorption dynamics of chlorophylls a and d dissolved in tetrahydrofuran was measured by the broadband femtosecond laser pump-probe spectroscopy in a spectral range from 400 to 870 nm. The absorption spectra of the excited S1 singlet states of chlorophylls a and d were recorded, and the dynamics of the of the Qy band shift of the stimulated emission (Stokes shift of fluorescence) was determined in a time range from 60 fs to 4 ps. The kinetics of the intramolecular conversion Qx→Qy (electronic transition S2→S1) was measured; the characteristic relaxation time was 54 ± 3 and 45 ± 9 fs for chlorophylls a and d, respectively.

Coexistence of ground and excited state intramolecular proton transfer of the Schiff base 2-((E)-(naphthalene-3-ylimino)-methyl)phenol: A combined experimental and computational study

Prototropic behavior of the Schiff base compound, 2-((E)-(naphthalene-3-ylimino)-methyl)phenol (NMP), has been explored by photophysical/photochemical means and reported in this paper in a comprehensive manner. The study, comprising of absorption, steady state and time resolved fluorescence, 1H NMR, together with quantum chemical calculations, establishes that the probe undergoes intramolecular proton transfer (IPT) process in both ground and lowest excited singlet state. Resolution of the low energy absorption band of NMP in different solvents and observation of two different emission bands upon excitation at two different positions within the low energy absorption band indicates co-existence of two distinct species of the probe in the ground state, implying occurrence of the ground state intramolecular proton transfer (GSIPT) process. Large downshifted 1H NMR peak at δ > 13 ppm suggests a strong intramolecular H-bonding in the ground state and goes in favor of the GSIPT process. Excited state intramolecular proton transfer (ESIPT) of the probe has been confirmed by the existence of the large Stokes shifted emission band of the tautomer along with the normal (enol) emission band, especially in non-polar media. Observation of the single isoemissive point in the time resolved area normalized emission spectra (TRANES) in nonpolar solvents confirms existence of two distinct species (enol (N*) and tautomer (T*)) in the photoexcited state. Quantum chemical calculations using DFT/TDDFT have been performed to provide support to the experimental findings.

On the origin of photodynamic activity of hypericin and its iodine-containing derivatives.

The main photophysical properties, useful for establishing whether hypericin in anionic form and some of its derivatives containing heavy atoms such as iodine, can be proposed for their use in photodynamic therapy, were determined using density functional based computations. The results showed that in the anionic form and in the iodinated derivatives, the absorption wavelength undergoes a bathochromic shift, the singlet-triplet energy gap assumes values ​that allow to excite the oxygen molecule from its ground to the excited singlet state, and that the spin-orbit couplings between singlet and triplet states significantly increase.

Radiationless deactivation pathways versus H-atom elimination from the N-H bond photodissociation in PhNH2-(Py)n (n = 1,2) complexes.

Minimum energy structures of the ground and lowest excited states of aniline (PhNH2) solvated by pyridine (Py) show that the clusters formed are stabilized by hydrogen bonds in which only one or both hydrogen atoms of the NH2 group take part. Two different N-H bonds photodissociation in PhNH2-(Py)n (n = 1,2) complexes, free and hydrogen bonded have been studied by analyzing excited state potential energy surfaces. In the first one, only N-H bonds engaged in hydrogen bonding in these complexes are considered. RICC2 calculations of potential energy (PE) profiles indicate that all photochemical reaction paths along N-H stretching occur mainly via the proton-coupled electron transfer (PCET) mechanism. The repulsive charge transfer 1ππ*(CT) state dominates the PE profiles, leading to low-lying 1ππ*(CT)/S0 conical intersections and thus provide channels for ultrafast radiationless deactivation of the electronic excitation or stabilization to biradical complexes. The second photoreaction consists of a direct dissociation along the free N-H bond of the NH2 group. It has been shown that this process is played by excited singlet states of 1πσ* character having repulsive potential energy profiles with respect to the stretching of N-H bond, which dissociates over an exit barrier about 0.5eV giving rise to the formation of a 1πσ*/S0 conical intersection. This may cause an internal conversion to the ground state or may lead to H-atom elimination. This photophysical process is the same in both planar and T-shaped conformers of the PhNH2-Py monomer complex. Our findings reveal that there is no single dominating path in the photodissociation of N-H bonds in PhNH2-(Py)n complexes, but rather a variety of paths involving H-atom elimination and several quenching mechanisms.

Benchmarking time-dependent density functional theory for singlet excited states of thermally activated delayed fluorescence chromophores

Thermally activated delayed fluorescence (TADF) is the internal conversion of triplet excitons into singlet excitons via reverse intersystem crossing. TADF can significantly enhance the efficiency of organic light-emitting diodes (OLEDs). In order for a chromophore to display TADF the energy difference between its lowest singlet and lowest triplet states, ${S}_{1}$ and ${T}_{1}$, should be as small as possible. This requirement is facilitated by spatial separation between the frontier orbitals. Computer simulations based on time-dependent density functional theory (TDDFT) have been used extensively to predict the excited state properties of TADF chromophores. However, the accuracy of TDDFT largely depends on the choice of exchange-correlation functional. Here, we present a benchmark study of the performance of TDDFT based on different classes of hybrid functionals for 16 TADF chromophores consisting of different donor and acceptor moieties. We find that only the range-separated double hybrid functionals, $\ensuremath{\omega}\mathrm{B}2\mathrm{PLYP}$ and $\ensuremath{\omega}\mathrm{B}2\mathrm{GP}\text{\ensuremath{-}}\mathrm{PLYP}$, provide qualitatively correct predictions of the relative singlet excitation energies of different molecules, the spectral composition of excited states, and the energy ordering of intramolecular charge-transfer versus valence excited states. Therefore, we recommend using these functionals to assess prospective TADF chromophores. Nevertheless, further development is needed to improve the quantitative performance of TDDFT. These findings are important for our ability to computationally screen and design candidate TADF chromophores and advance the development of highly efficient OLEDs.

Organic molecules with inverted singlet-triplet gaps.

According to Hund’s multiplicity rule, the energy of the lowest excited triplet state (T1) is always lower than that of the lowest excited singlet state (S1) in organic molecules, resulting in a positive singlet-triplet energy gap (ΔE ST). Therefore, the up-converted reverse intersystem crossing (RISC) from T1 to S1 is an endothermic process, which may lead to the quenching of long-lived triplet excitons in electroluminescence, and subsequently the reduction of device efficiency. Interestingly, organic molecules with inverted singlet-triplet (INVEST) gaps in violation of Hund’s multiplicity rule have recently come into the limelight. The unique feature has attracted extensive attention in the fields of organic optoelectronics and photocatalysis over the past few years. For an INVEST molecule possessing a higher T1 with respect to S1, namely a negative ΔE ST, the down-converted RISC from T1 to S1 does not require thermal activation, which is possibly conducive to solving the problems of fast efficiency roll-off and short lifetime of organic light-emitting devices. By virtue of this property, INVEST molecules are recently regarded as a new generation of organic light-emitting materials. In this review, we briefly summarized the significant progress of INVEST molecules in both theoretical calculations and experimental studies, and put forward suggestions and expectations for future research.

Regulating Singlet-Triplet Energy Gaps through Substituent-Driven Modulation of the Exchange and Coulomb Interactions.

Control of the singlet-triplet energy gap (ΔEST) is central to realizing productive energy conversion reactions, photochemical reaction trajectories, and emergent applications that exploit molecular spin physics. Despite this, no systematic methods have been defined to tune ΔEST in simple molecular frameworks, let alone by an approach that also holds chromophore size and electronic structural parameters (such as the HOMO-LUMO gap) constant. Using a combination of molecular design, photophysical and potentiometric experiments, and quantum chemical analyses, we show that the degree of electron-electron repulsion in excited singlet and triplet states may be finely controlled through the substitution pattern of a simple porphyrin absorber, enabling regulation of relative electronically excited singlet and triplet state energies by the designed restriction of the electron-electron Coulomb (J) and exchange (K) interaction magnitudes. This approach modulates the ΔEST magnitude by controlling the densities of state in the occupied and virtual molecular orbital manifolds, natural transition orbital polarization, and the relative contributions of one electron transitions involving select natural transition orbital pairs. This road map, which regulates electron density overlaps in the occupied and virtual states that define the singlet and triplet wave functions of these chromophores, enables new approaches to preserve excitation energy despite intersystem crossing.

An optimally tuned range-separated hybrid starting point for ab initio GW plus Bethe-Salpeter equation calculations of molecules.

The ab initio GW plus Bethe-Salpeter equation (GW-BSE, where G is the one particle Green's function and W is the screened Coulomb interaction) approach has emerged as a leading method for predicting excitations in both solids and molecules with a predictive power contingent upon several factors. Among these factors are the (1) generalized Kohn-Sham eigensystem used to construct the GW self-energy and to solve the BSE and (2) the efficacy and suitability of the Tamm-Dancoff approximation. Here, we present a detailed benchmark study of low-lying singlet excitations from a generalized Kohn-Sham (gKS) starting point based on an optimally tuned range-separated hybrid (OTRSH) functional. We show that the use of this gKS starting point with one-shot G0W0 and G0W0-BSE leads to the lowest mean absolute errors (MAEs) and mean signed errors (MSEs), with respect to high-accuracy reference values, demonstrated in the literature thus far for the ionization potentials of the GW100 benchmark set and for low-lying neutral excitations of Thiel's set molecules in the gas phase, without the need for self-consistency. The MSEs and MAEs of one-shot G0W0-BSE@OTRSH excitation energies are comparable to or lower than those obtained with other functional starting points after self-consistency. Additionally, we compare these results with linear-response time-dependent density functional theory (TDDFT) calculations and find GW-BSE to be superior to TDDFT when calculations are based on the same exchange-correlation functional. This work demonstrates tuned range-separated hybrids used in combination with GW and GW-BSE can greatly suppress starting point dependence for molecules, leading to accuracy similar to that for higher-order wavefunction-based theories for molecules without the need for costlier iterations to self-consistency.

Enhanced Red Persistent Room-Temperature Phosphorescence Induced by Orthogonal Structure Disruption during Electronic Relaxation.

Bright, persistent, room-temperature phosphorescence (RTP) at long wavelengths is crucial for high-resolution imaging in the absence of in vivo autofluorescence. However, efficient long-wavelength RTP is difficult. Here, enhanced red RTP based on a unique mechanism was observed from deuterated dibenzo[g.p]chrysenes substituted with a phenoxazine. The yield was 16%, with an average lifetime of 1.8 s. An orthogonal dihedral angle between the dibenzo[g.p]chrysene and the phenoxazine in the lowest excited singlet state allowed a forbidden fluorescence to increase triplet generation. When the dihedral angle changed, disengagement of the forbidden fluorescence from the excited singlet state occurred, and the lowest triplet excited state had a facilitated phosphorescence rate without increasing its nonradiative transition rate. The facilitated phosphorescence rate as well as the large triplet yield led to the enhanced red RTP.

Ab initio investigation of substituent effects on the excited electronic states of flavylium cation analogues of anthocyanin pigments

Anthocyanins are particularly noteworthy natural plant pigments that are responsible for the red, purple and blue color of many fruits, vegetables and flowers. Dietary anthocyanins have potential health benefits, but their color loss due to pH-dependent chemistry limits widespread application as coloring agents in food and consumer products. The ability to predict the color of synthetic analogues of natural pigments is crucial to the rational development of new bioinspired dyes and pigments with tailored colors and properties with lower potential for environmental and health concerns than those of conventional synthetic pigments. In this work, we employ the ab initio second-order algebraic diagrammatic construction, ADC(2), level of theory to calculate the absorption spectra of 26 synthetic flavylium cation analogues of anthocyanins. In general, the theoretical absorption spectra compare favorably with the corresponding experimental absorption spectra in an aqueous solution. The excitation energies, the charge transfer character, and the oscillator strengths of the electronic transitions provide insight into the effects of the type and position of the substituents on the excited singlet and triplet states of these compounds. The results show that the theoretical framework employed constitutes a reliable tool for obtaining a deeper understanding of the properties of bioinspired synthetic analogues of anthocyanin pigments.

All-Visible (>500 nm)-Light-Induced Diarylethene Photochromism Based on Multiplicity Conversion via Intramolecular Energy Transfer.

Photoswitching molecules that reversibly switch upon visible-light irradiation are some of the most attractive targets for biological and imaging applications. In this study, we found a diarylethene (DAE) derivative having a covalently attached perylenebisimide (PBI) unit (DAE-PBI dyad) underwent an unexpected cyclization reaction upon irradiation with green (500-550 nm) light, where the DAE unit has no absorbance. The photoreactivity was enhanced in solvents containing heavy atoms and in the presence of oxygen. As inferred from the solvent dependence and the calculated excited-state energies of DAE and PBI units, it was suggested that the probable mechanism for this unique visible-light-induced cyclization reaction is multiplicity conversion based on intramolecular energy transfer from the excited singlet state of the PBI unit to the triplet state of DAE units (i.e., DAE-1[PBI]* → 3[DAE]*-PBI). Such a unique photoreaction mechanism with the assistance of oxygen will pave the way for new molecular design for the development of visible-light switching molecules.

The Role of Thermally Activated Quenching and Energy Migration in Luminescence of Silver Clusters in Glasses

Luminescent silver clusters stabilized in different hosts represent an important class of new luminophores with numerous potential applications. Nevertheless, there are a lot of open questions regarding the emission mechanism of the clusters. In this work, luminescence properties of silver clusters in a silica-based glass were analyzed by using steady-state and time-resolved spectroscopy. The obtained results suggest that deactivation of cluster excited states does not include cluster interaction and should be treated on a single cluster scale. The temperature change of cluster emission confirms the mechanism of temperature-activated intersystem crossing as a way of excited singlet state deactivation. The study of fluorescence temperature dependence reveals the features attributed to the existence of two different temperature-activated quenching processes.

Dielectric control of reverse intersystem crossing in thermally activated delayed fluorescence emitters.

Thermally activated delayed fluorescence enables organic semiconductors with charge transfer-type excitons to convert dark triplet states into bright singlets via reverse intersystem crossing. However, thus far, the contribution from the dielectric environment has received insufficient attention. Here we study the role of the dielectric environment in a range of thermally activated delayed fluorescence materials with varying changes in dipole moment upon optical excitation. In dipolar emitters, we observe how environmental reorganization after excitation triggers the full charge transfer exciton formation, minimizing the singlet-triplet energy gap, with the emergence of two (reactant-inactive) modes acting as a vibrational fingerprint of the charge transfer product. In contrast, the dielectric environment plays a smaller role in less dipolar materials. The analysis of energy-time trajectories and their free-energy functions reveals that the dielectric environment substantially reduces the activation energy for reverse intersystem crossing in dipolar thermally activated delayed fluorescence emitters, increasing the reverse intersystem crossing rate by three orders of magnitude versus the isolated molecule.

Vibrational deactivation in O(3P) + N2 collisions: from an old problem towards its solution

In a recent communication [2021 Phys. Chem. Chem. Phys. 23 15475–79] we showed that the correct modelling of vibrational quenching events in O + N2(v) collisions, a fundamental process in air plasmas, requires the detailed representation of intermediate and asymptotic regions of the interaction and the inclusion of several types of processes as vibration to translation (V–T) and vibro-electronic (V–E) energy transfer. For the first time from the publication of experimental results in the 70’s, we obtained theoretical results in agreement with experiments, even at room temperature. In the present work we extend the approach to better describe non-adiabatic V–E deactivation and include the evaluation of the role of the higher excited singlet N2O surface, characterized by new high quality ab initio calculations, to that of the triplet Π and Σ ones. Within this framework, we calculate V–T, V–E and the corresponding total vibrational relaxation rate coefficients for initial vibrational N2(v) quantum numbers up to v = 10 in a wide temperature range (200–10 000 K). These data are of uttermost importance for the modelling of air plasmas, of earth’s and planetary atmospheres and for the design and construction of aircrafts and air-breathing propulsion systems for very low earth orbit (VLEO) satellites.

Investigation of pyrene vs Anthracene-based oxime esters: Role of the excited states on their polymerization initiating abilities

In this work, two series of oxime esters (OXE) based on Pyrene and Anthracene chromophores were synthesized and tested as Type Ⅰ photoinitiators for the Free Radical Photopolymerization (FRP) under visible irradiation at 405 nm using light-emitting diodes (LED) as the irradiation source. Interestingly, the two series of oxime esters only differ by the nature of the substituents connected to the oxime ester functional group. As a result of this modification, the reactivity of the different compounds only depends on the rate of decomposition from their excited states as well as the reactivity of the generated radicals. Remarkably, the 18 Pyrene-based OXEs (Py-OXE) and 17 Anthracene-based OXEs (An-OXE) were never synthesized prior to this work. Pyrene and Anthracene chromophores were selected as benchmark structures reacting mainly from their first excited singlet (S1) and triplet (T1) states, respectively. This will allow to shed some light on the excited states responsible of the cleavage of OXEs. Therefore, originality of this study relies in the difference of the initiation ability between these two families. In fact, the results showed that Py-OXEs (0.5 % w) are the most efficient systems compared to An-OXEs, so that higher final conversions and rates of polymerization could be obtained with Py-OXEs. Influence of the OXE structure (both chromophores and leaving groups) is discussed on the basis of different characterization techniques such as: UV–visible absorption spectroscopy, photolysis experiments, real-time Fourier transform infrared spectroscopy (RT-FTIR), stationary fluorescence and time-resolved fluorescence. Therefore, a global mechanism is finally provided based on the complementary characterization data. Oxime ester derivatives were also tested under air for the generation of 3D patterns (by Direct Laser Write DLW) using a diode laser at 405 nm.

Understanding of Photo-Induced Reversible Rearrangement from Borepin to Borirane.

The first reversible photoisomerization between a borepin and a borirane was reported in the photo-induced reactions of B(npy)Ar2 (npy=2-(naphthalen-1-yl) pyridine, Ar=phenyl or electron rich aryl; S. Wang, et al. Angew. Chem. Int. Ed. 2019, 58, 6683-6687). In this work, the detailed mechanisms of the unprecedented reversible photoisomerization between the borepin (compound a) and the borirane (compound b) of B(npy)Ph2 in the first excited singlet (S1 ) state and the ground (S0 ) state were studied by carrying out calculations with the complete active space self-consistent field (CASSCF) and its second-order perturbation (CASPT2) methods combined with time-dependent density functional theory (TD-DFT). The calculation results show that photoexcitation of a-S0 at 365 nm and b-S0 at 450 nm populate their S1 state with evident charge transfer characteristics. The photoisomerization is triggered in the S1 state and ends in the S0 state, at which the intersection points in a (S1 /S0 )x intersection seam participate in and promote phenyl migration and ring-closure processes. Furthermore, we reveal that the not large energy difference (less than 0.6 eV) and similar conjugation properties of π electrons between a-S0 and b-S0 are responsible for their unique photo-reversible reactivity, compared with those of the isomers of the thermally reversible compound B(ppy)Mes2 . Our results contribute to an understanding of the excited-state reactivity of organoboron compounds and will be useful to support the design of new boron-based photo-responsive materials.