Excited State Energy Research Articles

Benzanthrone sulfides: synthesis, solvatochromism characterization and analysis of experimental photophysical parameters and theoretical calculations

In this paper the synthesis, characterization, and photophysical properties of benzanthrone-derived sulfides are discussed. The compounds were synthesized using a practical method of nucleophilic aromatic substitution reactions of 3-bromobenzanthrone with thiols. Structural characterization was conducted using nuclear magnetic resonance (NMR) and Fourier-transform (FTIR) spectroscopy and high-resolution mass spectrometry, confirming the chemical structures. The crystal structure of a compound was determined via X-ray diffraction, revealing π-π stacking interactions and intermolecular hydrogen bonding. Photophysical analysis showed solvatochromic effects (emission of light upon excitation from 507 nm in benzene to 591 nm in ethanol) and attributed fluorescence to intramolecular charge transfer. Computational analysis using DFT calculations provided further insights into the compounds' properties, including excited state energies and the potential for efficient intersystem crossing. Overall, the research provides a comprehensive understanding of the newly synthesized benzanthrone alkyl and aryl sulfides as well as a phenylselanyl derivative. The photophysical properties, charge transfer characteristics observed in the compounds and the solvatochromic effects exhibited by the compounds suggest their potential to be utilized for developing sensors for detecting environmental changes, chemical substances or biological targets.

Ligand‐dependent aggregation‐enhanced photoacoustic of atomically precise metal nanocluster

AbstractAtomically precise metal nanoclusters (MNCs), as a potential type of photoacoustic (PA) contrast agent, are limited in application due to their low PA conversion efficiency (PACE). Here, with hydrophilic Au25SR18 (SR = thiolate) as model NCs, we present a result that weakly polar solvent induces aggregation, which effectively enhances PA intensity and PACE. The PA intensity and PACE are highly dependent on the degree of aggregation, while the aggregation‐enhanced PA intensity (AEPA) positively correlates to the protected ligands. Such an AEPA phenomenon indicates that aggregation actually accelerates the intramolecular motion of Au NCs, and enlarges the proportion of excited state energy dissipated through vibrational relaxation. This result conflicts with the restriction of intramolecular motion mechanism of aggregation‐induced emission. Further experiments show that the increased energy of AEPA originates from the aggregation inhibiting the intermolecular energy transfer from excited Au NCs to their surrounding medium molecules, including solvent molecule and dissolved oxygen, rather than restricting radiative relaxations. This study develops a new strategy for enhancing the PA intensity of Au NCs, and contributes to a deeper understanding of the origin of the PA signal and the excited state energy dissipation processes for MNCs.

Analysis of the Plasma Discharge Phenomena for Ceramic Metal Halide Lamps Using Finite Element Method

Metal halide (MH) lamps comes under the category of high intensity discharge (HID) lamps, which involves high intensity thermal plasma discharge phenomenon. In this paper we have considered ceramic MH lamps. The plasma discharge in MH lamps is different from other plasma discharge by the velocity of all the discharge obeys Maxwellian distribution, the excited energy states are occupied by Boltzmann distribution, and composition of the plasma can be derived from local chemical equilibrium. This paper explores the temperature distribution and the immediate effects towards the life of MH lamp. The temperature profile for the MH lamp is numerically investigated using WELSIM 2.0, considering the lamp plasma model mathematical equations. Since electrode plays a significant role, influencing MH lamp life, it becomes imperative to understand the electrodes. In this context, the energy balance equation is taken into consideration. The equations are solved using finite element method. The necessary boundary condition for solution of the equation is the plasma boundary layer for the cathode heat conduction. The analysis provides the temperature distribution inside the lamp for different electrode geometry. The solutions to the equations have been pictorially investigated for the lamp starting phenomena and E/N ratio. For HID lamps in general, the improvements in HID technology are all linked to the performance of the arc tube, and within that tube, the chemical reaction, the pressure, and the temperature of the reaction. Another feature in the case of MH lamps that is worth mentioning in the determination of lamp life is the ratio between electric field to gas number density. From the above, we can draw the following conclusion that the radiation transport mechanism of the HID lamps plays a pivotal role apart from other parameters influencing the lifetime of HID lamps. The paper envisages all these aspects using PDEs, the solutions of which provide an insightful understanding of the phenomena substantially.

Tuning photophysical properties of bicarbazole-based blue TADF emitters with AIE feature: Trade-off between small singlet-triplet energy splitting and high photoluminescence quantum yield

Three aggregation-induced emission (AIE) and thermally activated delayed fluorescence (TADF) blue light-emitting materials were successfully synthesized using bicarbazole and phenylsulfonyl/benzophenone moieties as electron donors and acceptors, respectively, via Buchwald-Hartwig reaction. Density functional theory calculations showed that the three emitters have a moderate dihedral angle between the two carbazole planes and between the donor and acceptor, resulting in a good conjugated structure and a trade-off between the small ΔEST and high photoluminescence quantum yield of the BCzDPM molecule. The cyclic voltammetry, thermogravimetric analysis and differential scanning calorimetry tests demonstrated that the three materials had high electrochemical and thermal stability. The three emitters exhibited obvious intramolecular charge transfer characteristics in solution and film. According to the fluorescence and low-temperature phosphorescence spectra, the lowest singlet and triplet excited state energy levels of BCzDPM were 2.96 eV and 2.78 eV, respectively, with a ΔEST of 0.18 eV. Using these emitters as non-doped emissive layers, efficient blue electroluminescent devices were fabricated. The maximum current efficiency and external quantum efficiency reached 15.4 cd/A and 7.05% for the BCzDPM device, respectively, and the efficiency roll-off of BCzDPM was smaller than that of PSOBCz and PBCzPM, under high current density.

Photoswitchable Molecular Units with Tunable Nonlinear Optical Activity: A Theoretical Investigation.

The first-, second-, and third-order molecular nonlinear optical properties, including two-photon absorption of a series of derivatives, involving two dithienylethene (DTE) groups connected by several molecular linkers (bis(ethylene-1,2-dithiolato)Ni- (NiBDT), naphthalene, quasilinear oligothiophene chains), are investigated by employing density functional theory (DFT). These properties can be efficiently controlled by DTE switches, in connection with light of appropriate frequency. NiBDT, as a linker, is associated with a greater contrast, in comparison to naphthalene, between the first and second hyperpolarizabilities of the "open-open" and the "closed-closed" isomers. This is explained by invoking the low-lying excited states of NiBDT. It is shown that the second hyperpolarizability can be used as an index, which follows the structural changes induced by photochromism. Assuming a Förster type transfer mechanism, the intramolecular excited-state energy transfer (EET) mechanism is studied. Two important parameters related to this are computed: the electronic coupling (VDA) between the donor and acceptor fragments as well as the overlap between the absorption and emission spectra of the donor and acceptor groups. NiBDT as a linker is associated with a low electronic coupling, VDA, value. We found that VDA is affected by molecular geometry. Our results predict that the linker strongly influences the communication between the open-closed DTE groups. The sensitivity of the molecular nonlinear optical properties could assist with identification of molecular isomers.

Probing ground and low-lying excited states for indole derived compounds of potential herbicide action

In this work, we present a study on a series of recently synthesized indole derived compounds that were previously suggested by experimental tests as having herbicidal action. Density functional theory (DFT) and its time-dependent formalism (TD-DFT) were used for investigating ground and excited state properties, respectively. The M06-2X and CAM-B3LYP exchange-correlation functionals were used with the aug-cc-pVTZ basis set. Computations were performed in the gas-phase, water, and dimethyl sulfoxide. Solvation was found to play minor effects on the ground state structure of any given compound. On the other hand, solvation was found to cause a decrease in the excitation energies of the lowest-lying excited singlet states of almost all the compounds. More importantly, all the indole derived compounds presented excitation energies that are considerably lower than those determined for their parent molecule. For instance, the lowest-lying excited singlet was probed at 3.45 eV in the case of 6-chloro-8-nitro-2,3,4,9-tetrahydro-1H-carbazole (compound 15a) versus 5.00 eV for the corresponding state of the indole molecule, both values determined at the TD-DFT/M06-2X/aug-cc-pVTZ level in water. In addition, compound 15a along with 5-chloro-2,3-dimethyl-7-nitro-1H-indole (15b) presented three excited states (each) that are likely to be accessed (having oscillator strength ≳0.1 at the TD-DFT/M06-2X/aug-cc-pVTZ level of theory in water) among the five respective lowest-lying excited singlets. The existence of various excited states accessible at lower energies may provide competing alternative paths to the chlorophyll-solar radiation interaction, in detrimental manner to the photosynthetic process; an observation in agreement to the previous experimental findings. Frontier orbital analysis suggested electron delocalization (advent from structural modifications) to be responsible for lowering the excitation energies.

In silico investigation of the photoisomerization mechanism of push-push azoderivatives

The design of novel chromophores showing specific photophysical traits is nowadays crucial for the development of new dyes and optical devices. Azobenzene derivatives comprise the largest fraction of industrial dyes and remain one of the main functions to be employed in the design of general-purpose photoactivated switches. In this context, we have investigated the optical and photoisomerization properties of two push-push azo derivatives, from a purely theoretical perspective, to outline a mechanistic landscape that can contribute to setting the grounds for the future goal of formulating dyes with specific photophysical properties. Both derivatives show downhill excited-state potential energy surfaces, hinting at fast photoactivated isomerization. In addition, both the trans and cis forms show nearly complementary absorption spectra leading to discriminatory color and differential excitation possibilities. Additionally, the reverse cis to trans ground state thermal isomerization reactions show higher energy barriers than the parent azobenzene molecule, supporting their potential use in different technological applications.

Molecular Structure Related to an S = 5/2 High-Spin S2 State Manganese Cluster of Photosystem II Investigated by Q-Band Pulse EPR Spectroscopy.

The high-spin S2 state of the photosynthetic oxygen-evolving cluster Mn4CaO5, corresponding to the g = 4.1 signal for X-band electron paramagnetic resonance (EPR), was investigated using Q-band pulsed EPR, which detected a main peak at g = 3.10 and satellite peaks at 5.25, 4.55, and 2.80. We evaluated the spin state as the zero-field splitting of D = 0.465 cm-1 and E/D = 0.245 with S = 5/2. The temperature dependence of the T1 relaxation time revealed that the excited-state energy was 28.7 cm-1 higher than that of the S = 5/2 ground state. By comparing present quantum mechanical (QM) calculation models, a closed-cubane structure with the protonation state of two oxygens, W1 (= OH-) and W2 (= H2O), was the most probable structure for the S = 5/2 state. The three-pulse electron spin-echo envelope modulation (ESEEM) detected the nuclear signal, which was assigned to nitrogen as His332 ligated to the Mn1 ion. The obtained hyperfine constant for the nitrogen signal was significantly reduced from that in the S = 1/2 low-spin state. These results indicate that the S = 5/2 spin state arises from the closed-cubane structure.

Solvent-activated fluorescent Eu(III) complex with carbazole-benzene-tripyridine D-π-A system for rapid identification of specific alcohols

In this work, we prepared a Carbazole-Benzene-Tripyridine ligand with a d-π-A structure and synthesized its corresponding Eu (III) and Tb (III) complexes. The structures of the complexes were analyzed by single crystal X-ray diffraction. It was surprisingly that the fluorescence emission intensity of the Eu(III) complex increases significantly once the solvent was added, indicating that the fluorescence could be activated by the solvent. However, it was confirmed that in the solid state, the complex molecules in the lattice were orderly arranged and there was a π-π stacking effect. The π-π stacking caused strong intermolecular interactions leading to emission quenching. The addition of solvent broke the π-π stacking and enhanced the fluorescence of the Eu(III) complex. The color of the fluorescence emission of the Eu(III) complex depended on the kind of added solvent. The results of nuclear magnetic resonance (NMR) and density functional theory (DFT) calculations suggested that the rotation of the CC single bond in the d-π-A structure ligand in different solvents caused a change in the spatial angle between the carbazole-benzene unit and the tripolypyridine unit in the ligand, thereby changing the excited state energy of the ligand. Hence, the varied emission colors of the complex reflect differences in the energy transfer efficiency of the ligand to rare earth ion. The solvent effect of the Eu(III) complex was then successfully applied in the rapid identification of specific alcohols.

Influence of Mode-Specific Excitation on the Nonadiabatic Dynamics of Methyl Nitrate (CH3ONO2).

The impact of mode-specific vibrational excitations on initial-preparation conditions was studied by examining the excited-state population decay rates in the nonadiabatic dynamics of methyl nitrate (CH3ONO2). In particular, exciting a few specific modes by adding a single quantum of energy clearly decelerated the nonadiabatic dynamics population decay rates. The underlying reason for this slower population decay was explained by analyzing the profiles of the excited-state potential energy surfaces in the Franck-Condon regions and the topology of the S1/S0 conical intersection. This study not only provides physical insights into the key mechanisms controlling nonadiabatic dynamics but also shows the possibility of controlling nonadiabatic dynamics via mode-specific vibrational excitations.

Activating Molecular Room-Temperature Phosphorescence by Manipulating Excited-State Energy Levels in Poly(vinyl alcohol) Matrix.

Poly(vinyl alcohol) (PVA) has been found as a wonderful matrix for chromophores to boost their room-temperature phosphorescence (RTP) character by forming abundant hydrogen bonding. Despite the well-utilized protective effect, the constructive role in accelerating the intersystem crossing is less investigated. Here, we focus on its role in manipulating the excited-state energy level to facilitate multiple intersystem crossing channels. Six benzoyl carbazole derivatives do not emit RTP in their solutions, powders, or crystals but exhibit significantly persistent RTP signals when embedded into the PVA matrix. Charge-transfer excited states were trapped by cofacial stacking in crystal, which blocks the intersystem crossing channels. In the PVA matrix, the allowed broad distribution of charge-transfer states covers the locally excited states, offering multiple intersystem crossing pathways via spin-vibronic orbit coupling. Consequently, efficient and persistent heavy-atom-free phosphors have been developed with the highest quantum yields of 7.7% and the longest lifetime of 2.3 s.

Electronic excitations of the charged nitrogen-vacancy center in diamond obtained using time-independent variational density functional calculations

Elucidation of the mechanism for optical spin initialization of point defects in solids in the context of quantum applications requires an accurate description of the excited electronic states involved. While variational density functional calculations have been successful in describing the ground state of a great variety of systems, doubts have been expressed in the literature regarding the ability of such calculations to describe electronic excitations of point defects. A direct orbital optimization method is used here to perform time-independent, variational density functional calculations of a prototypical defect, the negatively charged nitrogen-vacancy center in diamond. The calculations include up to 511 atoms subject to periodic boundary conditions and the excited state calculations require similar computational effort as ground state calculations. Contrary to some previous reports, the use of local and semilocal density functionals gives the correct ordering of the low-lying triplet and singlet states, namely {}^{3}A_2 < {}^{1}E < {}^{1}A_1 < {}^{3}E3A2<1E<1A1<3E. Furthermore, the more advanced meta generalized gradient approximation functionals give results that are in remarkably good agreement with high-level, many-body calculations as well as available experimental estimates, even for the excited singlet state which is often referred to as having multireference character. The lowering of the energy in the triplet excited state as the atom coordinates are optimized in accordance with analytical forces is also close to the experimental estimate and the resulting zero-phonon line triplet excitation energy is underestimated by only 0.15 eV. The approach used here is found to be a promising tool for studying electronic excitations of point defects in, for example, systems relevant for quantum technologies.

Lagrangian Z-vector approach to Bethe-Salpeter analytic gradients: Assessing approximations.

We present an implementation of excited-state analytic gradients within the Bethe-Salpeter equation formalism using an adapted Lagrangian Z-vector approach with a cost independent of the number of perturbations. We focus on excited-state electronic dipole moments associated with the derivatives of the excited-state energy with respect to an electric field. In this framework, we assess the accuracy of neglecting the screened Coulomb potential derivatives, a common approximation in the Bethe-Salpeter community, as well as the impact of replacing the GW quasiparticle energy gradients by their Kohn-Sham analogs. The pros and cons of these approaches are benchmarked using both a set of small molecules for which very accurate reference data are available and the challenging case of increasingly extended push-pull oligomer chains. The resulting approximate Bethe-Salpeter analytic gradients are shown to compare well with the most accurate time-dependent density-functional theory (TD-DFT) data, curing in particular most of the pathological cases encountered with TD-DFT when a nonoptimal exchange-correlation functional is used.

Dissecting the ingredients of optimally tuned range-separated hybrid models for reliable description of non-adiabatic couplings.

Perusing the non-radiative processes requires a reliable prediction of non-adiabatic couplings (NACs) describing the interaction of two Born-Oppenheimer surfaces. In this regard, the development of appropriate and affordable theoretical methods that accurately account for the NAC terms between different excited-states is desirable. In this work, we develop and validate several variants of the optimally tuned range-separated hybrid functionals (OT-RSHs) for investigating NACs and related properties, such as excited states energy gaps and NAC forces, within the time-dependent density functional theory framework. Particular attention is paid to the influence of the underlying density functional approximations (DFAs), the short- and long-range Hartree-Fock (HF) exchange contributions, and the range-separation parameter. Considering several radical cations and sodium-doped ammonia clusters with the available reference data for the NACs and related quantities as the working models, we have evaluated the applicability and accountability of the proposed OT-RSHs. The obtained results unveil that any combination of the ingredients in the proposed models is not proper for describing the NACs, but a particular compromise among the involved parameters is needed to achieve reliable accuracy. Scrutinizing the results of our developed methods, the OT-RSHs based on the PBEPW91, BPW91, and PBE exchange and correlation DFAs, including about 30% HF exchange at the short-range regime, appeared to be the best performers. We also find that the newly developed OT-RSHs with correct asymptotic exchange-correlation potential have superior performances as compared to their standard counterparts with the default parameters and many earlier hybrids with both fixed and interelectronic distance-dependent HF exchange. The recommended OT-RSHs in this study can hopefully be applicable as computationally efficient alternatives to the expensive wave function-based methods for the systems prone to non-adiabatic properties as well as to screen out the novel candidates prior to their challenging synthesis.

Förster Resonance Energy Transfer Sensitized Singlet Fission in BODIPY‐Pentacene Dimer Conjugates

AbstractIn the present work, the energy donor 4,4‐difluoro‐4‐bora‐3a,4a‐diaza‐s‐indacene (BODIPY) is used for the first time in combination with a pentacene dimer (Pnc2) to provide the conjugate BODIPYPnc2 that features absorption throughout a large part of the solar spectrum. Upon photoexcitation, the singlet excited state energy of BODIPY is transferred to the pentacene dimer via intramolecular Förster resonance energy transfer (FRET). Subsequently, the pentacene dimer undergoes intramolecular singlet fission. In this process, a singlet correlated triplet pair is generated from the first singlet excited state via coupling to an intermediate state. The results show that solvent polarity has an influence on the system, with the largest FRET rate (i.e., 7.46 × 1011 s−1) being obtained in the most polar solvent (namely, benzonitrile) along with the largest triplet quantum yield (i.e., 207 ± 20%).

Analyzing exciton distribution in organic light-emitting devices using near-infrared probes

Although organic light-emitting devices (OLEDs) can achieve an internal quantum efficiency (QE) of 100%, their external QE remains limited to less than 40% due to low outcoupling efficiency (OCE). The distribution of excitons is a critical factor in determining OLED OCE, as it affects the coupling to surface plasmon polaritons. Therefore, investigating exciton distribution through experiments is crucial to optimize device structure and enhance OCE. In this study, we propose a probe method to analyze exciton distribution by inserting an ultrathin (∼0.1 nm) near-infrared (NIR) material at a specific position within the emitting layer (EML). With a low excited state energy level, the NIR probe can utilize excitons generated around the specific EML position. By preparing different devices with the NIR probe at various EML positions, we can quantitatively present exciton distribution using the NIR to visible emission ratio. Our results demonstrate that this probe method is effective for investigating exciton distribution in blue, green, and red OLEDs with hole-dominant, electron-dominant, and bipolar transport EMLs, respectively.

Basis-set correction based on density-functional theory: Linear-response formalism for excited-state energies.

The basis-set correction method based on density-functional theory consists in correcting the energy calculated by a wave-function method with a given basis set by a density functional. This basis-set correction density functional incorporates the short-range electron correlation effects missing in the basis set. This results in accelerated basis convergences of ground-state energies to the complete-basis-set limit. In this work, we extend the basis-set correction method to a linear-response formalism for calculating excited-state energies. We give the general linear-response equations as well as the more specific equations for configuration-interaction wave functions. As a proof of concept, we apply this approach to the calculations of excited-state energies in a one-dimensional two-electron model system with harmonic potential and a Dirac-delta electron-electron interaction. The results obtained with full-configuration-interaction wave functions expanded in a basis of Hermite functions and a local-density-approximation basis-set correction functional show that the present approach does not help in accelerating the basis convergence of excitation energies. However, we show that it significantly accelerates basis convergences of excited-state total energies.

Bleeding the Excited State Energy to the Utmost: Single-molecule Iridium Complexes for in Vivo Dual Photodynamic and Photothermal Therapy by an Infrared Low-power Laser.

A series of cyclometalated Ir(III) complexes with morpholine and piperazine groups have been designed as dual photosensitizers and photothermal agents for more efficient antitumor phototherapy via infrared low-power laser. Their ground and excited state properties, as well as the structural effect on their photophysical and biological properties, have been investigated by spectroscopic, electrochemical, and quantum chemical theoretical calculations. They target mitochondria in human melanoma tumor cells and trigger apoptosis related to mitochondrial dysfunction upon irradiation. The Ir(III) complexes, particularly Ir6, demonstrated high phototherapy indexes to melanoma tumor cells and a manifest photothermal effect. Ir6, with minimal hepato-/nephrotoxicity in vitro, significantly inhibits the growth of melanoma tumors in vivo under 808nm laser irradiation by dual PDT and PTT and can be efficiently eliminated from the body. These results may contribute to the development of highly efficient phototherapeutic drugs for large, deeply buried solid tumors. This article is protected by copyright. All rights reserved.

Electrochemical Surface-Enhanced Raman Spectroscopy for Aniline Adsorbed on Silver Electrodes: A DFT Study of the Anharmonic Effects of Amino Wagging Vibrational Modes

Aniline molecules exhibit a special amino wagging vibrational mode on the surface of the silver electrode. The corresponding Raman peak not only displays enhanced Raman signals in surface-enhanced Raman spectroscopy (SERS) experiments but also shows a shift toward higher wavenumbers. This work adopts a molecule-metal cluster model to investigate the normal Raman spectra of aniline molecules and their SERS spectra adsorbed on silver electrode surfaces by density functional theory (DFT) approaches. Based on the harmonic approximation, the influence of anharmonic effects on the normal Raman and SERS spectra of aniline is further considered. Our calculated results of the anharmonic Raman spectra of free aniline molecules show that their anharmonic effect mainly comes from the amino wagging vibrational mode. Then, the anharmonic Raman spectra of aniline binding to neutral and positively charged silver clusters were calculated and compared with the harmonic Raman spectra. It was found that the coupling effect between vibrational modes also greatly affects the Raman spectral characteristics. Finally, the excited state energy gap and pre-resonance Raman spectra of the aniline–silver cluster complexes are studied, and it is believed that there may be a charge-transfer enhancement mechanism.

Directed Gradients in the Excited-State Energy Landscape of Poly(3-hexylthiophene) Nanofibers.

Funneling excitation energy toward lower energy excited states is a key concept in photosynthesis, which is often realized with at most two chemically different types of pigment molecules. However, current synthetic approaches to establish energy funnels, or gradients, typically rely on Förster-type energy-transfer cascades along many chemically different molecules. Here, we demonstrate an elegant concept for a gradient in the excited-state energy landscape along micrometer-long supramolecular nanofibers based on the conjugated polymer poly(3-hexylthiophene), P3HT, as the single component. Precisely aligned P3HT nanofibers within a supramolecular superstructure are prepared by solution processing involving an efficient supramolecular nucleating agent. Employing hyperspectral imaging, we find that the lowest-energy exciton band edge continuously shifts to lower energies along the nanofibers' growth direction. We attribute this directed excited-state energy gradient to defect fractionation during nanofiber growth. Our concept provides guidelines for the design of supramolecular structures with an intrinsic energy gradient for nanophotonic applications.