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Solar cells convert light energy directly into electricity using semiconductor materials. The ternary system, composed of poly(3-hexylthiophene) (P3HT), fullerene (C60), and phenyl-C61-butyric-acid-methyl-ester (PCBM), expressed as P3HT-C60-PCBM, is one of the most efficient organic solar cells. In the present study, the structures and electronic states of P3HT-C60-PCBM have been investigated by means of the density functional theory (DFT) method to shed light on the mechanism of charge separation in semiconductor materials. The thiophene hexamer was used as a model of P3HT. Five geometrical conformers were obtained as the C60-PCBM binary complexes. In the ternary system, P3HT wrapped around C60 in the stable structure of P3HT-C60-PCBM. The intermolecular distances for P3HT-(C60-PCBM) and (P3HT-C60)-PCBM were 3.255 and 2.885 Å, respectively. The binding energies of P3HT + (C60-PCBM) and (P3HT-C60) + PCBM were 27.2 and 19.1 kcal/mol, respectively. The charge transfer bands were found at the low-lying excited states of P3HT-C60-PCBM. These bands strongly correlated with the carrier separation and electron transfer in solar cells. The electronic states at the ground and excited states of P3HT-C60-PCBM were discussed on the basis of the calculated results. [ABSTRACT FROM AUTHOR]

The dependence of photochemistry on excitation wavelength is not a recently observed phenomenon; nonetheless, it has, surprisingly enough, been largely ignored in the field. The reasons for this situation are not fully understood but might be related to a provisional extension of Kasha’s rule to photochemistry, or perhaps to a difficulty to justify the kind of short time-scales implied in such photochemistry, that challenges the usually held view giving predominance to fast internal conversion and vibrational relaxation. Regardless of the reasons, it is still a matter of fact that a complete and satisfactory interpretation for experimentally proven wavelength-dependent photochemistry is not yet available and the community endeavor to build a holistic understanding and a comprehensive view of the phenomenon. The present review is a non-exhaustive overview of the published data in the field, reporting on some of the most prominent features, issues, and interpretations.

The dependence of photochemistry on excitation wavelength is not a recently observed phenomenon; nonetheless, it has, surprisingly enough, been largely ignored in the field. The reasons for this situation are not fully understood but might be related to a provisional extension of Kasha's rule to photochemistry, or perhaps to a difficulty to justify the kind of short time-scales implied in such photochemistry, that challenges the usually held view giving predominance to fast internal conversion and vibrational relaxation. Regardless of the reasons, it is still a matter of fact that a complete and satisfactory interpretation for experimentally proven wavelength-dependent photochemistry is not yet available and the community endeavor to build a holistic understanding and a comprehensive view of the phenomenon. The present review is a non-exhaustive overview of the published data in the field, reporting on some of the most prominent features, issues, and interpretations. [ABSTRACT FROM AUTHOR]

The dynamics of excited‐state charge transfer (CT) in second near‐infrared (NIR‐II) fluorophore proceeds with conformational change that govern fluorescence. Unveiling the relationship between CT dynamics and conformational change in excited state is of great fundamental significance in optimizing NIR‐II fluorescence but remains less explored. This study unveils the pivotal role of large conformational change in promoting the evolution of emissive CT state into nonemissive CT state in NIR‐II fluorophore (CA‐BBT). Spectroscopic and computational results reveal that large conformational rotation/twisting leads to a pronounced increase in the CT character in excited state. This heightened CT character in CA‐BBT enables a rapid evolution of emissive CT state into a nonemissive CT state within 1.4 ps, as observed by ultrafast spectroscopy. Subsequently, this nonemissive state dominates nonradiative decay, resulting in very low NIR‐II fluorescence. Preventing such detrimental evolution by constraining the conformational dynamics of CA‐BBT results in a 10‐fold enhancement of NIR‐II fluorescence, enabling high‐resolution dynamic visualization of vasculature within osteosarcoma. This study offers a profound understanding of the relationship between CT dynamics and conformational changes in NIR‐II fluorescence, presenting innovative perspectives to develop bright NIR‐II fluorophores. [ABSTRACT FROM AUTHOR]

A series of reported Pt(II) carbene complexes possibly have the ability to serve as the new generation of blue emitters in luminescent devices because of their narrow emission spectra, high photoluminescence quantum yields (PLQYs), and rigid molecular skeleton. However, the combination of all carbene ligands with different multidentate structures will affect the overall planarity and horizontal dipole ratio to varying degrees, but the specific extent of this effect has not previously been analyzed in detail. In this work, density functional computation is used to study a class of platinum tetracarbene bidentate complexes with similar absorption and emission band characteristics, which is the main reason for the remarkable difference in quantum efficiency due to subtle differences in electronic states caused by different ligands. From the calculation results, the major reason, which results in significantly decrease in quantum efficiency for [Pt(cyim)2]2+, is that [Pt(cyim)2]2+ can reach the non-radiative deactivation metal-centered d-d excited state through an easier pathway compared with [Pt(meim)2]2+. The result, based on changes in the dihedral angle between ligands, can achieve the goal of improving and designing materials by adjusting the degree of the dihedral angle. (meim: bis(1,1′-dimethyl-3,3′-methylene-diimidazoline-2,2′-diylidene); cyim: bis(1,1′-dicyclohexyl-3,3′-methylene-diimidazoline-2,2′-diylidene). [ABSTRACT FROM AUTHOR]

The dynamics of excited‐state charge transfer (CT) in second near‐infrared (NIR‐II) fluorophore proceeds with conformational change that govern fluorescence. Unveiling the relationship between CT dynamics and conformational change in excited state is of great fundamental significance in optimizing NIR‐II fluorescence but remains less explored. This study unveils the pivotal role of large conformational change in promoting the evolution of emissive CT state into nonemissive CT state in NIR‐II fluorophore (CA‐BBT). Spectroscopic and computational results reveal that large conformational rotation/twisting leads to a pronounced increase in the CT character in excited state. This heightened CT character in CA‐BBT enables a rapid evolution of emissive CT state into a nonemissive CT state within 1.4 ps, as observed by ultrafast spectroscopy. Subsequently, this nonemissive state dominates nonradiative decay, resulting in very low NIR‐II fluorescence. Preventing such detrimental evolution by constraining the conformational dynamics of CA‐BBT results in a 10‐fold enhancement of NIR‐II fluorescence, enabling high‐resolution dynamic visualization of vasculature within osteosarcoma. This study offers a profound understanding of the relationship between CT dynamics and conformational changes in NIR‐II fluorescence, presenting innovative perspectives to develop bright NIR‐II fluorophores.

Solar cells convert light energy directly into electricity using semiconductor materials. The ternary system, composed of poly(3-hexylthiophene) (P3HT), fullerene (C60), and phenyl-C61-butyric-acid-methyl-ester (PCBM), expressed as P3HT-C60-PCBM, is one of the most efficient organic solar cells. In the present study, the structures and electronic states of P3HT-C60-PCBM have been investigated by means of the density functional theory (DFT) method to shed light on the mechanism of charge separation in semiconductor materials. The thiophene hexamer was used as a model of P3HT. Five geometrical conformers were obtained as the C60-PCBM binary complexes. In the ternary system, P3HT wrapped around C60 in the stable structure of P3HT-C60-PCBM. The intermolecular distances for P3HT-(C60-PCBM) and (P3HT-C60)-PCBM were 3.255 and 2.885 Å, respectively. The binding energies of P3HT + (C60-PCBM) and (P3HT-C60) + PCBM were 27.2 and 19.1 kcal/mol, respectively. The charge transfer bands were found at the low-lying excited states of P3HT-C60-PCBM. These bands strongly correlated with the carrier separation and electron transfer in solar cells. The electronic states at the ground and excited states of P3HT-C60-PCBM were discussed on the basis of the calculated results.

Photoionization of neutral chlorine atom is investigated in this paper in the framework of the screening constant per unit nuclear charge (SCUNC) method. Resonance energies, quantum defects and effective charges of the 3s23p4 (3P2,1,0)ns and 3s23p4 (3P1,0)nd Rydberg series originating from both the 2P03/2 ground state and the 2P01/2 excited state of chlorine atom are reported. The present study believed to be the first theoretical investigation is compared with the recent experimental measurements (Yang et al., Astrophys. J. 810:132, 2015). Good agreements are obtained between theory and experiments. New SCUNC data are tabulated as useful references for interpreting astrophysical spectra from neutral atomic chlorine. [ABSTRACT FROM AUTHOR]

Photo-controlled self-assembly of nanoparticles (NPs) is an advanced and promising approach to address a series of material issues from the molecular level to the nano/micro scale, owing to the fact that light stimulus is typically precise and rapid, and can provide contactless spatial and temporal control. The traditional photo-controlled assembly of NPs is based on photochemical processes through NPs modified by photo-responsive molecules, which are realized through the change in chemical structure under irradiation. Moreover, photoexcitation-induced assembly of NPs is another promising physical strategy, and such a strategy aims to employ molecular conformational change in the excited state (rather than the chemical structure) to drive molecular motion and assembly. The exploration and control of NP assembly through such a photo-controlled strategy can open a new paradigm for scientists to deal with "bottom-up" behaviors and develop unprecedented optoelectronic functional materials. [ABSTRACT FROM AUTHOR]

A series of reported Pt(II) carbene complexes possibly have the ability to serve as the new generation of blue emitters in luminescent devices because of their narrow emission spectra, high photoluminescence quantum yields (PLQYs), and rigid molecular skeleton. However, the combination of all carbene ligands with different multidentate structures will affect the overall planarity and horizontal dipole ratio to varying degrees, but the specific extent of this effect has not previously been analyzed in detail. In this work, density functional computation is used to study a class of platinum tetracarbene bidentate complexes with similar absorption and emission band characteristics, which is the main reason for the remarkable difference in quantum efficiency due to subtle differences in electronic states caused by different ligands. From the calculation results, the major reason, which results in significantly decrease in quantum efficiency for [Pt(cyim)2]2+, is that [Pt(cyim)2]2+ can reach the non-radiative deactivation metal-centered d-d excited state through an easier pathway compared with [Pt(meim)2]2+. The result, based on changes in the dihedral angle between ligands, can achieve the goal of improving and designing materials by adjusting the degree of the dihedral angle. (meim: bis(1,1′-dimethyl-3,3′-methylene-diimidazoline-2,2′-diylidene); cyim: bis(1,1′-dicyclohexyl-3,3′-methylene-diimidazoline-2,2′-diylidene).

Abstract The temperature-dependent dual fluorescence from a single component of small organic molecules (SOMs) is very promising for the ratiometric fluorescence sensing of temperature. However, the SOM-based fluorophores typically only show one fluorescence band. It is thus very challenging to achieve dual fluorescence, let alone the dual fluorescence that is sensitive to temperature. Herein, in this short review, we briefly summarized the examples of SOM-based fluorophores exhibiting temperature-dependent dual fluorescence. Through representative examples, we mainly focus on the illustration of the intrinsic mechanisms of this unusual phenomenon, which may take place because of the presence of two local minimum conformations that are thermally equilibrated in the ground state, the presence of two local minimum conformations in the first excited state as the result of significant structural relaxation upon excitation, or the presence of thermal equilibrium between the first and second excited states. Hopefully, the discussions in this short review will provide some important guidelines for the further rational molecular design of such fluorophores.

It is shown that, like conventional diffuse fluorescence spectra and despite the complex mechanism of formation of the excited state, the chemi- and bioluminescence spectra contain information about the frequency of the purely electronic (vertical) transition of the emitting molecules and the degree of homogeneity of their ensemble. As in the case of ordinary diffuse luminescence spectra, the frequency of a purely electronic transition is shifted significantly toward the short wavelength side (by almost the half-width of the band) from the maximum of the fluorescence spectrum. An example of the separation of the overlapping spectra using the position of a short-wavelength purely electronic transition is presented. [ABSTRACT FROM AUTHOR]

The two-state lasing phenomenon, which manifests itself in simultaneous laser emission through several optical transitions of quantum dots, is studied in microdisk diode lasers with different cavity diameters. The active region represents a multiply stacked array of self-organized InAs/InGaAs/GaAs quantum dots emitting in the wavelength range of 1.1–1.3 µm. Two-state lasing, which involves the ground-state and the first excited-state optical transitions, is observed in microdisks with cavity diameters of 20 to 28 µm, whereas two-state lasing via the first and the second excited-state optical transitions is observed in 9 µm microdisks. The threshold currents for one-state and two-state lasing are investigated as functions of the microdisk diameter. Optical loss in the microdisk lasers is evaluated by comparing the two-state lasing behavior of the microdisks with that of edge-emitting stripe lasers made of the same epitaxial wafer. [ABSTRACT FROM AUTHOR]

In this review, we have attempted to briefly summarize the influence of an external electric field on an assembly of tautomeric molecules and to what experimentally observable effects this interaction can lead to. We have focused more extensively on the influence of an oriented external electric field (OEEF) on excited-state intramolecular proton transfer (ESIPT) from the studies available to date. The possibilities provided by OEEF for regulating several processes and studying physicochemical processes in tautomers have turned this direction into an attractive area of research due to its numerous applications. [ABSTRACT FROM AUTHOR]

With the deterioration of global greenhouse effect, CF3SO2F (Trifluoromethanesulphonyl Fluoride) has become an alternative gas of SF6 (Sulfur Hexafluoride). CF3SO2F has environmentally friendly characteristics and has been widely concerned. In this work, we analyze the electric field level of the discharge in the dielectric and the direction of the molecular intrinsic dipole moment. Density functional theory and time-dependent density functional theory are used to calculate the molecular structure parameters, dipole moments, frontier orbitals and excited states at corresponding electric field levels and in specific electric field directions. C2F5SO2F and C3F7SO2F, as by-products in the synthesis of CF3SO2F, are studied together to explore the influence of carbon atom number on molecular properties. As a result, the molecular structure is clearly changed under the electric field and the molecular bond length strongly depended on the electric field. The HOMO is mainly contributed by O atoms and the LUMO is mainly contributed by S atoms. When the electric field in the -z-axis direction is applied, the HOMO energies of molecules increase, especially for C3F7SO2F molecule, and the ionization potentials decrease correspondingly. The reduced ionization potentials facilitate the liberation of electrons from the molecule and the propagation of the streamer. The excitation energy of each molecule in each excited state decreases with the increasing of electric field intensity. Therefore, during the preparation process of CF3SO2F molecule, C3F7SO2F molecule should be reduced or removed as much as possible to prevent the reduction of dielectric insulation strength. The finding of this study is beneficial to the performance improvement and application research of CF3SO2F.

The search for new materials is constantly ongoing. Recently, a two-dimensional carbon allotrope , γ-graphyne, has been synthesized with a unified crystalline structure . Because of its low LUMO and excellent electron mobility, it appears to be a promising electron acceptor for photovoltaic applications. Here we report an analysis of the electronic properties of model van der Waals complexes of γ-graphyne with several partners of different electronic nature. We show that photoinduced electron transfer from electron-donating partners to γ-graphyne is favorable and occurs on nano to picosecond time scale. In contrast, electron transfer from γ-graphyne to strong electron acceptors is unlikely. Our results open perspectives for the future application of γ-graphyne in photovoltaic devices.

The excited state dipole moments of two positional isomers of dicyanobenzene have been determined from thermochromic shifts of the absorption and fluorescence emission spectra in ethyl acetate solution and compared to the results of ab initio calculations. We found that the dipole moments of excited states from thermochromic shifts closely resemble the ab initio values of the isolated molecule, while the results of conductor-like screening model (COSMO) using the same wave function model as for the isolated molecule, shows considerable deviations from the experimental values. It is shown that the dipole moments of the two cyano groups add up vectorially for both the ground and excited states. • Dipole moment of two isomeric dicyanobenzenes determined in ground and excited state from thermochromic shifts. • Comparison of the experimental results to SCS-CC2 calculations shows a very good agreement for excited state dipole moments of the isolated molecules. • Bad agreement with the results of COSMO calculations. • Additivity rule for dipole increments works in the ground state and the excited state. [Display omitted] [ABSTRACT FROM AUTHOR]

Photo-controlled self-assembly of nanoparticles (NPs) is an advanced and promising approach to address a series of material issues from the molecular level to the nano/micro scale, owing to the fact that light stimulus is typically precise and rapid, and can provide contactless spatial and temporal control. The traditional photo-controlled assembly of NPs is based on photochemical processes through NPs modified by photo-responsive molecules, which are realized through the change in chemical structure under irradiation. Moreover, photoexcitation-induced assembly of NPs is another promising physical strategy, and such a strategy aims to employ molecular conformational change in the excited state (rather than the chemical structure) to drive molecular motion and assembly. The exploration and control of NP assembly through such a photo-controlled strategy can open a new paradigm for scientists to deal with “bottom-up” behaviors and develop unprecedented optoelectronic functional materials.

Synthetic ester can replace the mineral oil traditionally used in transformers to avoid the environmental problems caused by oil leakage. However, the fast discharge phenomenon in a high electric field in transformers using synthetic ester seems to indicate its insulation property is inferior to that of mineral oil. In this paper, typical molecular models of synthetic ester, including F2, F4, F6, F8, and F10, are constructed. We studied the effect of electric fields on the molecular properties of the five molecules by density functional theory and time-dependent density functional theory. According to the electric field intensity required for discharge initiation and propagation in insulating oil, the electric field intensity applied in this study varied from 108 to 109 V/m. The results showed that the molecular bond lengths are obviously dependent on the electric field. The ionization potential (IP) of the F8 and F10 molecules decreases sharply under electric field intensities of 3.1 × 109 and 4.0 × 109 V/m. It can be inferred that the IP reduction of the long carbon chain molecules, such as F8 and F10, is the reason for the formation of fast discharge in the case of synthesis ester. Calculations for excited states show that the introduction of an electric field makes the electron transition more active. The results obtained by this work improve our understanding of the discharge mechanism in synthetic ester dielectrics and provide theoretical support for improvement in the performance of synthetic ester insulating oil.

Ultrafast optical spectroscopy delivers unparalleled insights into the dynamic response of photoactive materials, including semiconducting, photonic and phase-change materials. The most applied experimental tool – transient absorption spectroscopy – derives signals from induced changes in the intensity of transmitted light, assumed to relate to the imaginary part of the refractive index. However, the entire complex refractive index of materials changes in the excited state; changes in the real part of the refractive index can have significant effects on transient absorption spectra and the function of optical devices. In this review, we introduce an emerging ultrafast spectroscopy method – frequency domain interferometry. This simple adaptation of transient absorption spectroscopy provides a model-independent means of spectrally resolving photoinduced changes in a materials refractive index. After introducing the theory and implementation of the method, we describe several case studies, including the optical response of metal-halide perovskites and phase modulators, and surface displacement of phase-change materials. Finally, we describe recent and future improvements that can enhance the time-resolution and signal sensitivity of this technique. The advances and applications highlighted in this review demonstrate the potential of the method to become a standard part of the ultrafast spectroscopy toolbox for characterising optoelectronic and photonic materials and devices.

The complex quantum-chemical and spectral studies of the anionic polymethine dyes with the simplest symmetrical terminal groups and with different-length polymethine chain are performed. As shown, these dyes produce the specific molecular orbitals (МО) positioned nearly the energy gap and located only within the terminal groups. By investigation of the absorption spectra, it is established that the typical highly intensive longwave- length spectral band is observed and bathochromically shifted upon lengthening of the open conjugated chain; this polymethine band is concerned with the electron transition between the frontier levels of the opposite symmetry. In the contrast, the local MO takes part in so-called quasi- local electron transitions involving also the frontier orbital. The local transitions have small dipole moments, and hence, they do not practically appear in the absorption spectra; however, the local transitions cause the appearance of the non-deep minima in the spectra of the fluorescenceexcitation anisotropy. [ABSTRACT FROM AUTHOR]

The two-state lasing phenomenon, which manifests itself in simultaneous laser emission through several optical transitions of quantum dots, is studied in microdisk diode lasers with different cavity diameters. The active region represents a multiply stacked array of self-organized InAs/InGaAs/GaAs quantum dots emitting in the wavelength range of 1.1–1.3 µm. Two-state lasing, which involves the ground-state and the first excited-state optical transitions, is observed in microdisks with cavity diameters of 20 to 28 µm, whereas two-state lasing via the first and the second excited-state optical transitions is observed in 9 µm microdisks. The threshold currents for one-state and two-state lasing are investigated as functions of the microdisk diameter. Optical loss in the microdisk lasers is evaluated by comparing the two-state lasing behavior of the microdisks with that of edge-emitting stripe lasers made of the same epitaxial wafer.

In this review, we have attempted to briefly summarize the influence of an external electric field on an assembly of tautomeric molecules and to what experimentally observable effects this interaction can lead to. We have focused more extensively on the influence of an oriented external electric field (OEEF) on excited-state intramolecular proton transfer (ESIPT) from the studies available to date. The possibilities provided by OEEF for regulating several processes and studying physicochemical processes in tautomers have turned this direction into an attractive area of research due to its numerous applications.

This work presents data coming from electronic structure calculations at the Density Functional Theory level, performed in a series of organic photovoltaic materials. The data represents the Cartesian coordinates of such molecular systems at the lowest energy geometry and at the first excited state. Data evidencing the nature of the photo-isomerization in the OPV systems was also obtained. Additionally, the highest probabilities of the molecular electronic transitions giving rise to the absorption spectra observed in excited state were also computed. These data may aid to estimate photovoltaic parameters, and to tailor materials intended to be implemented in solar cell devices. They may also be used as input to design a training set for machine learning analysis and artificial intelligence.

Intramolecular charge transfer and the associated changes in molecular structure in N,N'-dimethylpiperazine are tracked using femtosecond gas-phase X-ray scattering. The molecules are optically excited to the 3p state at 200 nm. Following rapid relaxation to the 3s state, distinct charge-localized and charge-delocalized species related by charge transfer are observed. The experiment determines the molecular structure of the two species, with the redistribution of electron density accounted for by a scattering correction factor. The initially dominant charge-localized state has a weakened carbon-carbon bond and reorients one methyl group compared with the ground state. Subsequent charge transfer to the charge-delocalized state elongates the carbon-carbon bond further, creating an extended 1.634 Å bond, and also reorients the second methyl group. At the same time, the bond lengths between the nitrogen and the ring-carbon atoms contract from an average of 1.505 to 1.465 Å. The experiment determines the overall charge transfer time constant for approaching the equilibrium between charge-localized and charge-delocalized species to 3.0 ps. [ABSTRACT FROM AUTHOR]

XMHFL is a computer program that implements the Single Center Expansion (SCE) method to describe ground, excited and ionized states of a molecular system with one or more vacancies. The simulation mechanism is based on a combination of the three-point difference Numerov scheme and Thomas algorithm. The modular architecture of the XMHFL solvers allows using the generated code as the basis for the development of new methods and algorithms, as well as increasing the possibilities of molecular characterization calculation for X-ray molecular spectroscopy research and training as an educational platform.

The sensitized photoexcitation of 2-diazocyclopentane-1,3-diones in the presence of THF leads to the insertion of the terminal N-atom of the diazo group into the α-С–Н bond of THF, producing the associated N-alkylhydrazones in yields of up to 63–71%. Further irradiation of hydrazones derived from furan-fused tricyclic diazocyclopentanediones culminates in the cycloelimination of furans to yield 2-N-(alkyl)hydrazone of cyclopentene-1,2,3-trione. By contrast, the direct photolysis of carbocyclic diazodiketones gives only Wolff rearrangement products with up to 90–97% yield.

Inspired by nature, improving photosensitization represents a vital direction for the development of artificial photosynthesis. The sensitization ability of photosensitizers (PSs) reflects in their electron-transfer ability, which highly depends on their excited-state lifetime and redox potential. Herein, for the first time, we put forward a facile strategy to improve sensitizing ability via finely tuning the excited state of Ru(II)-PSs (Ru-1 – Ru-4) for efficient CO2 reduction. Remarkably, [Ru(Phen)2(3-pyrenylPhen)]2+ (Ru-3) exhibits the best sensitizing ability among Ru-1 – Ru-4 , over 17 times higher than that of typical Ru(Phen)32+. It can efficiently sensitize a dinuclear cobalt catalyst for CO2-to-CO conversion with a maximum turnover number of 66 480. Systematic investigations demonstrate that its long-lived excited state and suitable redox driving force greatly contributed to this superior sensitizing ability. This work provides a new insight into dramatically boosting photocatalytic CO2 reduction via improving photosensitization. [ABSTRACT FROM AUTHOR]

At the intersection of spectroscopy and microscopy lie techniques that are capable of providing subnanometer imaging of excited states of individual molecules or nanoparticles. Such approaches are particularly important for imaging macromolecules or nanoparticles large enough to have a high probability of containing a defect. These inevitable defects often control properties and function despite an otherwise ideal structure. We discuss real-space imaging techniques such as using scanning tunneling microscopy tips to enhance optical measurements and electron energy-loss spectroscopy in a scanning transmission electron microscope, which is based on focused electron beams to obtain high-resolution spatial information on excited states. The outlook for these methods is bright, as they will provide critical information for the characterization and improvement of energy-switching, electron-switching, and energy-harvesting materials. [ABSTRACT FROM AUTHOR]

The way to obtain new materials was usually to introduce some groups to molecules. Correlations among the molecular structure and photoelectric properties of rubrene and its eight derivatives have been studied in this paper, and the influences of the introduction of different electron-donating and withdrawing substituents on the molecular orbital, reorganization energies, absorption spectra, and fluorescence spectra of rubrene and its derivatives were discussed. In the present work, density functional theory calculations were performed at the B3LYP/6-311G(d,p) level to optimize the structure, and TD-DFT was used to calculate the absorption and emission spectra. Quantum chemistry calculation results indicated that the maximum absorption wavelengths are redshifted due to the introduction of cyclopentadienyl or furan groups, the maximum absorption wavelengths are blueshifted while methoxy groups substituted on tetracene backbone. We also discussed the influence of substituents on the molecule structure, which suggests that introducing furan and cyclopentadienyl substituents on the tetracene backbone can increase the rigidity of rubrene and improve the fluorescence intensity. The results of reorganization show that the introduction of cyclopentadienyl or furan groups into rubrene is advantageous to the holes transportation, and the introduction of F groups contributes to the electrons transportation. This study provides an insight into the properties of rubrene and its derivatives and supplies an effective method to design new organic semiconductor materials. [ABSTRACT FROM AUTHOR]

[Display omitted] • The excited states of three blue emitters are manipulated from CT to HLCT. • Blue vacuum-evaporated HLCT OLEDs exhibit maximum EQE of 7.52%. • The solution-processed HLCT OLED shows a high maximum EQE of 6.49%. • Very low efficiency roll-offs of 0.8% and 3.7% at 100 and 1000 cd m−2 are achieved. Manipulating the excited states of organic luminescent materials can efficiently improve the utilization of both singlet and triplet excitons for developing high-performance organic light-emitting diodes (OLEDs), but the issue remains difficult due to the lack of well-controlled ways. Here, we proposed a molecular design strategy of excited state manipulation from charge-transfer (CT) to hybridized local and charge-transfer (HLCT) via adjusting the cyano position on pyridine acceptor. The meta -substituted PyAn4CN is mainly composed of a CT component, while the para -substituted PyAn5CN is endowed with a HLCT component. On further extending the conjugation of PyAn5CN by inserting a benzene unit between the pyrene and anthracene core, the HLCT character is preserved in PyPhAn5CN , accompanied by a faster radiative decay. Consequently, the vacuum-evaporated OLEDs exhibit blue electroluminescence (EL) with the emission peaks in the range of 455–460 nm and high external quantum efficiency (EQE) up to 7.52%, together with well-suppressed efficiency roll-offs of 0.8% and 3.7% at the luminance of 100 and 1000 cd m-2, respectively. More importantly, the solution-processed device shows an excellent performance with EQE of 6.49%, which is one of the best results in the solution-processed HLCT OLEDs. Our results clearly indicate that the formation of HLCT state is an efficient way to realize high-efficiency blue electrofluorescence. [ABSTRACT FROM AUTHOR]

In this paper, we study the ground and first excited states of the fractional Bose–Einstein condensates (BEC) which is modeled by fractional Gross–Pitaevskii (GP) equation. We first introduce the normalized gradient flow method and prove its energy diminishing property. Then the weighted shifted Grünwald–Letnikov difference (WSGD) method is used to discretize the Gross–Pitaevskii equation. The corresponding normalization and energy diminishing property for the semi-discrete scheme are proved. For the time discretization, we use the implicit integration factor (IIF) method which decouples the diffusion and nonlinear terms separately. Finally the numerical methods are applied to compute the ground and first excited states of fractional BEC with harmonic oscillator, harmonic-plus-optical lattice and box potential. Our numerical results show that the ground and excited states in fractional GP equation differ from those of the standard (non-fractional) GP equation. [ABSTRACT FROM AUTHOR]

The excited state dipole moment of anisole has been determined in the gas phase from electronic Stark spectroscopy and in solution using thermochromic shifts in ethyl acetate. Electronic excitation increases the anisole dipole moment in the gas phase from 1.26 D in the ground state to 2.19 D in the electronically excited singlet state, leaving the orientation of the dipole moment practically unchanged. These values are compared to solution phase dipole moments. From variation of the fluorescence emission and absorption maxima with temperature, an excited state dipole moment of 2.7 D was determined. Several solvent polarity functions have been used in combination with experimentally determined cavity volumes at the respective temperatures. Both gas phase and condensed phase experimental dipole moments are compared to the results of ab initio calculations at the CC2 level of theory, using the cc-pVTZ basis set for the isolated molecule and using the COnductor-like Screening MOdel (COSMO), implemented in Turbomole, for the solvated anisole molecule. [ABSTRACT FROM AUTHOR]

Vacuum ultraviolet (VUV) based advanced oxidation processes (AOPs) recently attracted widespread interests. However, the role of UV 185 in VUV is only considered to be generating a series of active species, while the effect of photoexcitation has long been overlooked. In this work, the role of UV 185 induced high-energy excited state for the dephosphorization of organophosphorus pesticides was studied using malathion as a model. Results showed malathion degradation was highly related to radical yield, while its dephosphorization was not. It was UV 185 rather than UV 254 or radical yield that was responsible for malathion dephosphorization by VUV/persulfate. DFT calculation results demonstrated that the polarity of P-S bond was further increased during UV 185 excitation, favoring dephosphorization while UV 254 did not. The conclusion was further supported by degradation path identification. Moreover, despite the fact that anions (Cl-, SO 4 2- and NO 3 -) considerably affected radical yield, only Cl- and NO 3 - with high molar extinction coefficient at 185 nm significantly affected dephosphorization. This study shed light on the crucial role of excited states in VUV based AOPs and provided a new idea for the development of mineralization technology of organophosphorus pesticides. [Display omitted] • High-energy excited states rather than radical yield dominated OPs mineralization. • Photoexcitation altered the regioselectivity of radical attack. • UV 185 weakened P–S bond, which decided dephosphorization, while UV 254 cannot. • Anions affected mineralization via UV 185 absorption rather than radical quenching. [ABSTRACT FROM AUTHOR]

The time-dependent density functional theory (TDDFT) method was performed to investigate the excited-state intramolecular double proton transfer (ESIDPT) reaction of calix[4]arene (C4A) and the role of the intramolecular hydrogen bonds in the ESIDPT process. The geometries of C4A in the ground state and excited states (S1, S2 and T1) were optimized. Four intramolecular hydrogen bonds formed in the C4A are strengthened or weakened in the S2 and T1 states compared to those in the ground state. Interestingly, upon excitation to the S1 state of C4A, two protons H1 and H2 transfer along the two intramolecular hydrogen bonds O1-H1···O2 and O2-H2···O3, while the other two protons do not transfer. The ESIDPT reaction breaks the primary symmetry of C4A in the ground state. The potential energy curves of proton transfer demonstrate that the ESIDPT process follows the stepwise mechanism but not the concerted mechanism. Findings indicate that intramolecular hydrogen bonding is critical to the ESIDPT reactions in intramolecular hydrogen-bonded systems.

The solute–solvent interactions of 4-nitro-1,8-naphthalimide (4NNI) as a hydrogen bond acceptor in hydrogen donating methanol (MeOH) solvent in electronic excited states were investigated by means of the time-dependent density functional theory(TDDFT). We calculated the S0 state geometry optimizations, electronic transition energies and corresponding oscillation strengths of the low-lying electronically excited states for the isolated 4NNi and hydrogen-bonded 4NNi-(MeOH)1,4 complexes using the density functional theory (DFT) and TDDFT methods. The electronic excitation energies of the hydrogen-bonded complexes are correspondingly decreased compared to that of the isolated 4NNi, which revealed that the intermolecular hydrogen bond C=O···H–O and N=O···H–O in the hydrogen-bonded 4NNi-(MeOH)1,4 are strengthened in the electronically excited state. The calculated results are consistent with the mechanism that hydrogen bond strengthening will induce a redshift of the corresponding electronic spectra, while hydrogen bond weakening will cause a blueshift. Furthermore, we believe that the deduction we used to depict the trend of the hydrogen bond changes in excited states exists in many other fuorescent dyes in solution.

Dye sensitizers can significantly affect power conversion efficiency of dye-sensitized solar cells (DSSCs). Porphyrin-based dyes are promising sensitizers due to their performances in DSSCs. Here, based upon a N-fused carbazole-zinc porphyrin-free-base porphyrin triad containing an ethynyl-linkage (coded as DTBC), the novel porphyrin dyes named DTBC-MP and DTBC-TP were designed by varying the porphyrin-free-base units in the π conjugation of DTBC in order to study the effect of porphyrin-free-base in the modification of electronic structures and related properties. The calculated results indicate that, the extension of the conjugate bridge with the porphyrin-free-base unit results in elevation of the highest occupied molecular orbital (HOMO) energies, decrease of the lowest unoccupied molecular orbital (LUMO) energies, reduction of the HOMO-LUMO gap, red-shift of the absorption bands, and enhancement of the absorbance. The free energy changes demonstrate that introducing more porphyrin-free-base units in the conjugate bridge induces a faster rate of electron injection. The transition properties and molecular orbital characters suggest that the different transition properties might lead to a different electron injection mechanism. In terms of electronic structure, absorption spectra, light harvesting capability, and free energy changes, the designed DTBC-TP is a promising candidate dye sensitizer for DSSCs.

Y6 is a new type of non-fullerene acceptor, which has led to power conversion efficiencies of single-junction polymer solar cells over 17% when combined with a careful choice of polymeric donors. However, the excited state characteristics of Y6, which is closely correlated with its opto-electronic applications, are not clear yet. In this work, we studied the excited state properties of the Y6 solution and Y6 film, by using steady-state and time-resolved spectroscopies as well as time-dependent density functional theory (TD-DFT) calculations. UV-Vis absorption and fluorescence simulation, natural transition orbitals (NTOs) and hole-electron distribution analysis of Y6 solution were performed for understanding the excitation properties of Y6 by using TD-DFT calculations. The lifetimes of the lowest singlet excited state in Y6 solution and film were estimated to be 0.98 and 0.8 ns, respectively. Combining the exciton lifetime and photoluminescence (PL) quantum yield, the intrinsic radiative decay lifetimes of Y6 in the solution and film were estimated, which were 1.3 and 10.5 ns for the Y6 solution and film, respectively. Long exciton lifetime (~0.8 ns) and intrinsic radiative decay lifetime (~10.5 ns) of Y6 film enable Y6 to be a good acceptor material for the application of polymer solar cells.

Seventeen singlet excited states of ethylene have been calculated via time-dependent density functional theory (TDDFT) with the CAM-B3LYP functional and the geometries of 11 excited states were optimized successfully. The local vibrational mode theory was employed to examine the intrinsic C=C/C–H bond strengths and their change upon excitation. The natural transition orbital (NTO) analysis was used to further analyze the C=C/C–H bond strength change in excited states versus the ground state. For the first time, three excited states including πy′ → 3s, πy′ → 3py and πy′ → 3pz were identified with stronger C=C ethylene double bonds than in the ground state.

A large number of studies have examined the origins of high-catalytic activities of nanoparticles, but very few have discussed the lifetime of high-energy electrons in nanoparticles. The lifetime is one of the factors determining electron transfer and thus catalytic activity. Much of the lifetime of electrons reported in the literature is too short for a high transfer-efficiency of photo-excited electrons from a catalyst to the attached molecules. We observed TiO2 nanoparticles using the femtosecond laser two-color pump-probe technique with photoemission electron microscopy having a 40 nm spatial resolution. A lifetime longer than 4 ps was observed together with a fast decay component of 100 fs time constant when excited by a 760 nm laser. The slow decay component was observed only when the electrons in an intermediate state pumped by the fundamental laser pulse were excited by the second harmonic pulse. The electronic structure for the asymmetry of the pump-probe signal and the origin of the two decay components are discussed based on the color center model of the oxygen vacancy.

Recent studies have shown that molecular aggregation structures in precursor solutions of organic photovoltaic (OPV) polymers have substantial influence on polymer film morphology, exciton and charge carrier transport dynamics, and hence, the resultant device performance. To distinguish photophysical impacts due to increasing π-conjugation from chain lengthening and π–π stacking from single/multi chain aggregation in solution and film, we used oligomers of a well-studied charge transfer polymer PTB7 with different lengths as models to reveal intrinsic photophysical properties of a conjugated segment in the absence of inter-segment aggregation. In comparison with previously studied photophysical properties in polymeric PTB7, we found that oligomer dynamics are dominated by a process of planarization of the conjugated backbone into a quinoidal structure that resembles the self-folded polymer and that, when its emission is isolated, this quinoidal excited state resembling the planar polymer chain exhibits substantial charge transfer character via solvent-dependent emission shifts. Furthermore, the oligomers distinctly lack the long-lived charge separated species characteristic of PTB7, suggesting that the progression from charge transfer character in isolated chains to exciton splitting in neat polymer solution is modulated by the interchain interactions enabled by self-folding.

Triplet–triplet annihilation photon upconversion (TTA-UC) is a photophysical process in which the energy of two photons are combined into a single photon of higher energy. While this strategy has been recognized in applications ranging from bioimaging to solar energy conversion, its uses in synthetic organic chemistry have not been extensively developed. Here, we present a short tutorial on the theoretical underpinnings and the design principles of TTA-UC systems. Selected applications are then discussed to highlight key features of the TTA mechanism, along with a prospective discussion of the potential of these mechanisms to enable innovation in photocatalysis, methods development, and synthetic organic chemistry.

Based on Baird́s rule, which states that the excited state of an aromatic compound is antiaromatic, a new method for hydrogenating aromatic molecules by metal-free photochemical reaction employing 1,4-cyclohexadiene (1,4-CHD) as hydrogen source is proposed. Here only light and 1,4-CHD are required to hydrogenate aromatic compounds without metal catalysts or temperature. The proposed mechanism is related to the reduction potential of the aromatic probes on their first triplet excited state. In such methodology, besides the non-aromatic character of the excited state, the aromatization of the hydrogen donor turns this reaction feasible as a hydrogen storage procedure at low cost, expanding the chemical interest for materials like graphene. [ABSTRACT FROM AUTHOR]

Variable-temperature absorption and emission data over the range 80–300 K have been collected for acetylene and buta-1,3-diyne derivatives with polycyclic aromatic hydrocarbons as substituents, in 2-methyltetrahydrofuran (2-MeTHF). The influence of temperature on fluorescence lifetime and quantum yield has also been investigated. A strong coupling between solvation dynamics and solute structure is observed. Moreover, the dependency of fluorescence quantum yield on temperature indicates that the energy of activation of conformational changes for studied compounds is comparable or slightly larger than the activation energy of viscous flow of 2-MeTHF. Moreover, contrary to diphenyl derivatives of buta-1,3-diyne for which conformation changes are barrierless, polycyclic aromatic hydrocarbons derivatives are characterized by a relatively high activation energy. [ABSTRACT FROM AUTHOR]

A systematic study on the first excited-state proton transfer (ESPT) in 2RAI-HO (R = OH, OCH, CN, NO, CHO) complexes in solution were investigated at the TD-M06-2X/6-31 + G(d, p) level. The double proton transfer occurred in an asynchronous but concerted protolysis fashion no matter which substituent R was used at C position in pyrrole ring in the 7AI-HO complex. The specific vibrational-mode of ESPT in the model systems was confirmed and contributed to promote the reaction rate by shortening the reaction path. The substituent effects of different groups on the ESPT thermodynamics and kinetics were discussed. It was obvious that the geometries, barrier height, asynchrony, and specific vibration-mode of excited-state proton transfer changed with the different substituent groups. [ABSTRACT FROM AUTHOR]

Variable-temperature absorption and emission data over the range 80–300 K have been collected for 1,2-diphenylacetylene (DPA) and its derivatives, as well as 1,4-diphenylbuta-1,3-diyne (DPB) derivatives in 2-methyltetrahydrofuran (2-MeTHF). Moreover, the influence of temperature on fluorescence lifetime and quantum yield has also been investigated. A strong coupling between solvation dynamics and solute structure is observed in the low-temperature regime as well as at intermediate temperature where the structure of the compound is evolving in the course of excited-state relaxation. Moreover, the dependency of fluorescence quantum yield on temperature indicates that the energy of activation of radiationless transition for 1,2-diphenylacetylene and its derivatives is larger than the activation energy of viscous flow of 2-MeTHF, while for 1,4-diphenylbuta-1,3-diyne derivatives the opposite is observed. This finding implies that the conformational change of DPA and its derivatives in the excited state in 2-MeTHF shows an activation energy, contrary to DPB and its derivatives for which conformational changes are barrierless. [ABSTRACT FROM AUTHOR]