Local Excited State Research Articles

Chemical Bonding as a New Avenue for Controlling Excited-State Properties and Excitation Energy-Transfer Processes in Zinc Phthalocyanine-Fullerene Dyads.

Whether chemical bonding can regulate the excited-state and optoelectronic properties of donor-acceptor dyads has been largely elusive. In this work, we used electronic structure and nonadiabatic dynamics methods to explore the excited-state properties of covalently bonded zinc phthalocyanine (ZnPc)-fullerene (C60 ) dyads with a 6-6 (or 5-6) bonding configuration in which ZnPc is bonded to two carbon atoms shared by the two hexagonal rings (or a pentagonal and a hexagonal ring) in C60 . In both cases, the locally excited (LE) states on ZnPc are spectroscopically bright. However, their different chemical bonding differentiates the electronic interactions between ZnPc and C60 . In the 5-6 bonding configuration, the LE states on ZnPc are much higher in energy than the LE states on C60 . Thus, the excitation energy transfer from ZnPc to C60 is thermodynamically favorable. On the other hand, in the 6-6 bonding configuration, such a process is inhibited because the LE states on ZnPc are the lowest ones. More detailed mechanisms are elucidated from nonadiabatic dynamics simulations. In the 6-6 bonding configuration, no excitation energy transfer was observed. In contrast, in the 5-6 bonding configuration, several LE and charge-transfer (CT) excitons were shown to participate in the energy-transfer process. Further analysis reveals that the photoinduced energy transfer is mediated by a CT exciton, such that electron- and hole-transfer processes take place in a concerted but asynchronous manner in the excitation energy transfer. It is also found that high-level electronic structure methods including exciton effects are indispensable to accurately describe photoinduced energy- and electron-transfer processes. Furthermore, this work opens up new avenues for regulating the excited-state properties of molecular donor-acceptor dyads by means of chemical bonding.

Oxygen defect dominated photoluminescence emission of ScxAl1−xN grown by molecular beam epitaxy

A fundamental understanding and control of impurity incorporation and charge carrier recombination are critical for emerging ScxAl1−xN electronics, optoelectronics, and photonics. We report on the photoluminescence properties of ScxAl1−xN grown by plasma-assisted molecular beam epitaxy with varying growth temperatures and Sc contents. Bright and broad emission comprising a dominant peak at ∼3.52 eV and a weak peak at ∼2.90 eV was observed in Sc0.05Al0.95N. The origin of the ∼3.52 eV emission line is attributed to charge carrier recombination from the localized excited state of (Vcation-ON)2−/− to its ground state, whereas the second peak at ∼2.90 eV results from charge carrier recombination of isolated Vcation3−/2− to the valence band. We further show that oxygen defect-related emission can be significantly suppressed by increasing growth temperature. This work sheds light on the recombination dynamics of photoexcited carriers in ScxAl1−xN and further offers insight into how to improve the optical and electrical properties of ScxAl1−xN that are relevant for a broad range of applications.

Unusual Solvatofluorochromism of the Asymmetrically Substituted Aromatic Acetylene Derivative CNAacDMA**

AbstractThe photophysical properties of the newly synthesized unsymmetrically substituted aromatic acetylene derivative 9‐(2‐(4‐(N,N‐dimethylamino)phenyl)ethynyl)anthracene‐10‐carbonitrile (CNAacDMA) were investigated with the steady‐state and time‐resolved fluorometry. In saturated hydrocarbon solvents, only fluorescence from a locally‐excited state (LE) is recorded. In more polar solvents however, excitation of this dye leads to a charge transfer state (CT). In moderate polar solvents (ϵ=4–8) dual emission is observed as a result of competition between structural change and intramolecular charge transfer in the excited state. In polar solvents only one emission band, at shorter wavelength than CT emission, is observed, indicating a bidirectional solvatofluorochromism.

Hybridized local and charge-transfer excited state fluorophores enabling organic light-emitting diodes with record high efficiencies close to 20.

Pure organic emitters with full utilization of triplet excitons are in high demand for organic light-emitting diodes (OLEDs). Herein, through modulation of electron donors and introduction of phenyl rings as π spacers, we present three pure organic fluorophores (BCz, BTCz and BPTCz) with the hybridized local and charge-transfer (HLCT) excited state feature for OLED fabrication. Importantly, the introduction of π spacers in BPTCz not only enhances locally excited character with a fast radiative decay but also promotes intermolecular interactions to suppress non-radiative decays, contributing to a high solid-state fluorescence efficiency over 90%. Significantly, BPTCz not only endows its doped OLEDs with an external quantum efficiency (EQE) up to 19.5%, but also its non-doped OLED with a high EQE of 17.8%, and these outstanding efficiencies are the state-of-the-art performances of HLCT-based OLEDs.

Enhanced emission under proton stimuli based on a phenanthroimidazole derivative by switching the excited state type from the CT to the LE state

We report a vertical D–A molecule showing rare off–on TFA-fuming stimuli responsiveness via changing the excited state from the charge transfer (CT) state to the localized excited (LE) state.

Blue organic light-emitting diodes with hybridized local and charge-transfer excited state realizing high external quantum efficiency.

Donor–spacer–acceptor (D–π–A) materials CAPI and CCAPI, with hybridized local and charge transfer (HLCT) emissive states, have been synthesized. The twisting D–π–A architecture promotes the partial separation of HOMO and LUMO, leading to an enhanced % CT component, and the anthracene moiety in CAPI and CCAPI increases the conjugation length, leading to an enhanced % LE component. The non-doped device with CCAPIb shows the blue emission (450 nm) with maximum current efficiency (ηc), power efficiency (ηp), and external quantum efficiency (ηex) of 16.83 cd A−1, 15.32 lm W−1, and 12.0%, respectively, as well as exciton utilization efficiency (EUE) of 95% with a luminance of 32 546 cd m−2 and a roll-off efficiency of 0.53%. The new design strategy has great potential for developing high-performance blue electroluminescent materials.

Novel carbazole-based multifunctional materials with a hybridized local and charge-transfer excited state acting as deep-blue emitters and phosphorescent hosts for highly efficient organic light-emitting diodes

Two novel multifunctional HLCT materials, namely OCI and OCT, were presented and they proved to be better green hosts than CBP. Acting as a deep-blue emitter, the OCI-based device achieved an EQE of 5.06% and CIEy < 0.04 with an EL peak at 424 nm.

Impact of chemical modifications on the luminescence properties of organic neutral radical emitters

The hybridization of the charge-transfer (CT) state with both the ground state (GS) and local-excitation (LE) states is essential in order to describe accurately the radiative and non-radiative transition rates in TTM-based radicals.

GW/BSE nonadiabatic dynamics simulations on excited-state relaxation processes of zinc phthalocyanine-fullerene dyads: Roles of bridging chemical bonds

In this work, we employ electronic structure calculations and nonadiabatic dynamics simulations based on many-body Green function and Bethe-Salpeter equation (GW/BSE) methods to study excited-state properties of a zinc phthalocyanine-fullerene (ZnPc-C60) dyad with 6-6 and 5-6 configurations. In the former, the initially populated locally excited (LE) state of ZnPc is the lowest S1 state and thus, its subsequent charge separation is relatively slow. In contrast, in the latter, the S1 state is the LE state of C60 while the LE state of ZnPc is much higher in energy. There also exist several charge-transfer (CT) states between the LE states of ZnPc and C60. Thus, one can see apparent charge separation dynamics during excited-state relaxation dynamics from the LE state of ZnPc to that of C60. These points are verified in dynamics simulations. In the first 200 fs, there is a rapid excitation energy transfer from ZnPc to C60, followed by an ultrafast charge separation to form a CT intermediate state. This process is mainly driven by hole transfer from C60 to ZnPc. The present work demonstrates that different bonding patterns (i.e. 5-6 and 6-6) of the C−N linker can be used to tune excited-state properties and thereto optoelectronic properties of covalently bonded ZnPc-C60 dyads. Methodologically, it is proven that combined GW/BSE nonadiabatic dynamics method is a practical and reliable tool for exploring photoinduced dynamics of nonperiodic dyads, organometallic molecules, quantum dots, nanoclusters, etc.

Synthesis and fluorescent properties of quinoxaline derived ionic liquids

Ionic liquids (ILs) have attracted increasing attention since last few decades due to their high molecular design abilities and wide applications in different fields. In this study, four novel fluorescent isoquinolino [2,1-a]quinoxalin-5-ium ILs were designed and synthesized via a two-step process including a simple dual Schiff's base formation and a subsequent [RhCp∗Cl2]2-catalyzed oxidative C–H activation/annulation reaction. The as-synthesized ILs were extensively characterized using FT-IR, 1H-NMR, 13C-NMR, 19F-NMR, HSQC-NMR, HMBC-NMR and HR-MS. Their photophysical properties were determined by steady-state fluorescence spectroscopy. The results demonstrate that all these ILs showed dual or triple emissions, large stokes shift (90 nm) and mechanochromic behaviors. Basing on solvatochromism and titration experiments, it is thought that the emission bands of the ILs are raised from their local excited states, charge transfer states or excited state proton transfer of cations, while the substitute effect of these quinoxaline derived ILs on their stokes shifts is negligible.

Structures and Spectroscopic Properties of Large Molecules and Condensed-Phase Systems Predicted by Generalized Energy-Based Fragmentation Approach.

ConspectusThe structures and spectroscopic properties of molecules and condensed-phase systems are usually experimentally characterized by X-ray, infrared (IR), Raman, nuclear magnetic resonance (NMR), and electronic absorption/emission spectra. Quantum mechanics (QM) calculations are critical in quantitatively understanding the relationship between the structure and physicochemical properties of various chemical systems. However, it is very challenging to apply traditional QM methods to large molecules and condensed-phase systems with large unit cells due to their steep computational scaling with the system size. To overcome this difficulty, theoretical chemists have developed various linear (or low) scaling QM methods, among which energy-based fragmentation methods have achieved great success for large molecules or clusters. One of the most popular energy-based fragmentation methods is the generalized energy-based fragmentation (GEBF) approach developed by us.In this approach, the ground-state energy of a large molecule can be evaluated from the ground-state energies of a series of embedded subsystems. In this Account, we focus on the recent developments and applicability of the GEBF approach for the structures and spectroscopic properties of complicated large molecules and condensed-phase systems. With new fragmentation schemes, the GEBF approach can now describe ionic liquid clusters and metal-containing supramolecular systems accurately and can provide accurate binding energies for host-guest complexes. In addition, the GEBF approach is now available for describing the localized excited states of large systems including a chromophore. More importantly, the GEBF approach under periodic boundary conditions (PBC-GEBF) has been developed to deal with periodic molecular crystals and liquids. Then, the ground-state energy (or property) per unit cell of a periodic condensed phase system can be predicted with QM calculations on nonperiodic embedded subsystems. This feature enables accurate electron correlation calculations on molecular crystals and liquids to be feasible on ordinary workstations. The PBC-GEBF approach has been applied to predict the crystal structures, lattice energies, and spectroscopic properties of some typical molecular crystals and solutions. By combining the GEBF method and machine learning (ML) method, a GEBF-ML force field has been developed for long normal alkanes, and the IR spectra of long alkanes can be obtained from the GEBF-ML molecular dynamics (MD) simulations. The GEBF and its periodic variant are expected to play increasingly important roles in investigating real-life chemical systems of broad interests at the ab initio levels.

Singlet Fission Dynamics in Tetracene Single Crystals Probed by Polarization-Dependent Two-Dimensional Electronic Spectroscopy.

The exact mechanism of endothermic singlet fission in crystalline polyacene remains to be clarified. It has been elusive whether the excess energy of vibrational hot states and the upper branch of Davydov splitting is important for the energy compensation. Here, we probe the excited-state specified singlet fission dynamics in tetracene single crystals by polarization-dependent two-dimensional electronic spectroscopy (2DES). While a major spectral transfer with a characteristic lifetime of 86 ps is observed to be largely independent of the excitation energy due to formation of the spatially separated triplet pairs (1(T···T)), the excitation-energy dependent subpicosecond dynamics show marked differences for different states probed, implying the possible involvement of a coherently formed triplet pair state (1(TT)). Analysis of coherent vibrational modes suggests the coupling to high energy modes may offset the energy difference between singlet and triplet pair states. Moreover, the beating map of the low frequency mode indicates a vibrational hot state violating the aggregation behavior of Davydov exciton, which can be explained as a resonance of the 1(TT) state. These results suggest that the coherent vibronic mixing between local excitation and triplet pair states is essential for the singlet fission dynamics in molecule aggregates.

Interactions between isoalloxazine and o-aminobenzoate in o-aminobenzoate−d-amino acid oxidase complex. Molecular dynamics and molecular orbital studies

Molecular interactions between o-aminobenzoate (oAB) and isoallxazine (Iso) in o-aminobenzoate - d-amino acid oxidase complex (ODC) were studied by means of molecular dynamics simulation (MDS) and molecular orbital (MO) method. The distances between oAB and Iso were 0.60 in subunit A (Sub A) and 0.58 nm in Sub B. Four of six heteroatoms of Iso ring formed hydrogen bonding (HB) with nearby amino acids. The oxygen atom and nitrogen atom of oAB also formed HB with amino acids. The ODC displays a broad absorption band around 570 nm, which is considered to be charge transfer (CT) band between oAB and Iso in the complex. The ODC displayed fluorescence at around 520 nm, which is different from that of DAAO (emission peak at 535 nm) without oAB. The emission is from a local excited state (LE) without CT interaction. The CT interaction was studied by a semi-empirical molecular orbital (MO) method, using the structures of oAB and Iso obtained by MDS. The CT state did not form in the ground state of Iso, but formed in the excited state of Iso (Iso*), when the both cannot move appreciably from the MDS structures. The CT states formed both in the oAB – Iso and oAB – Iso* (excited state of Iso) systems when the both molecules can move so as to attain the minimum energy state. It is concluded that a part of oAB could move to form CT complex with Iso in the ground state, which displays the CT absorption band, but others could not move in the protein without CT interaction, which displays the fluorescence.

Tumor tissues diagnosis with PIEE lipid droplet vesicles

A molecule ANB (6-(4-aminostyryl)-1,3-dioxo-1H-benzo[de]isoquinolin -2(3 H)-yl)-n-butane), was developed for fluorescence turn-on imaging of cancer cells lipid droplets (LDs). ANB exercised synergistically aggregation induced emission enhancement and photochromism, which are essential to construct a photoinduced emission enhancement (PIEE) LDs. The fluorescent nanostructures PIEE-LDs extremely improve intracellular imaging contrast with high emission intensity, high emission contrast to cytosol and high photostability. Following these unique optical properties, we statistically analyzed the differences of LDs between cancer and normal cells as well as successfully observed LDs progression and fusion in living cells over a long period. Consequently, the xenograft tumor tissues were favorably distinguished from normal based on this superior PIEE-LDs. It is known that ANB exhibited excited-state conformational conversion between the twist intramolecular charge transfer (TICT) and local excited (LE) states. Based on the results of our studies, we found that LE state became dominate and presented aggregation induced emission enhancement when ANB stay in LDs, which allows us to build a fluorescent nanovesicle model to explain the distinct fluorescence imaging of ANB in LDs. Furthermore, photochromism occurred when irradiation that could promoted emission behavior of bright nanovesicles LDs to further level, and achieve PIEE-LDs.

Electrochemical and Solvent-Mediated Visible-to-Near-Infrared Spectroscopic Switching of Benzoselenadiazole Fluorophores.

A series of electronic push-pull, pull-pull, and push fluorophores has been prepared from a benzoselenadiazole core so that their spectroscopic, electrochemical, spectro-electrochemical, and spectro-electrofluorescence properties could be examined. The emission wavelengths and fluorescence quantum yields (Φfl ) of the N,N-dimethyl fluorophores were contingent on the solvent polarity and they ranged from 615 to 850 nm in aprotic solvents. The positive solvatochromism and the quenched Φfl in polar solvents were consistent with an intramolecular charge-transfer state (ICT). Meanwhile, a locally excited state (LE) was assigned in nonpolar solvents from the blue-shifted emission and high Φfl . The N,N-dimethylamine fluorophores examined could be both electrochemically oxidized and reduced, whereas the symmetric dinitro pull-pull derivative could be only reversibly reduced. Courtesy of their electrochemical reversibility, the fluorophores could reversibly change color from yellow to blue with an applied potential in addition to switching off their emission. The absorption of the electrochemically generated intermediates of the N,N-dimethyl derivatives spanned 500 nm over the visible and the NIR regions. The colors could be switched for upwards of two hours with applied potential, illustrating their potential use as electroactive materials in electrochromic devices.

Excitation-Wavelength-Dependent Light-Induced Electron Transfer and Twisted Intramolecular Charge Transfer in N,N-Bis(4'-tert-butylbiphenyl-4-yl)aniline Functionalized Borondipyrromethenes.

A series of bis(4'-tert-butylbiphenyl-4-yl)aniline (BBA) functionalized borondipyrromethene (BODIPY) dyads, Dyads 1-3, containing the BBA group tethered to BODIPY moiety either directly or through a phenyl or alkynyl phenyl spacers are synthesized, and the light-mediated charge transfer within the chromophores has been systematically investigated. The crystal structure of Dyad-1 showed a tilt of 44.2° between the BODIPY and BBA molecular planes and intermolecular C-H···π interactions with these moieties. Cyclic voltammetric and computational studies showed that the BBA moiety can act as the electron donor (D) and BODIPY as the electron acceptor (A) and the optical absorption studies revealed that an increase in the conjugation of the linker from Dyad-1 to Dyad-2 resulted in bathochromic shifts. Steady-state fluorescence studies involving photoexcitation of the BBA moiety at 326 nm resulted in the decrease in fluorescence intensity of the BBA, indicating the possibility of sequential occurrence of faster photoinduced energy transfer (PEnT) followed by the photoinduced electron transfer (PET) or solely PET within the dyads, and the driving forces of the charge separation were calculated to be exothermic in all of the employed solvents. Parallel time-resolved fluorescence experiments involving the excitation of BBA moiety also supported the occurrence of charge separation in these dyads. Interestingly, excitation of the BODIPY moiety of Dyad-1 and Dyad-2 at 490 nm in solvents of increasing polarity leads to a red-shifted BODIPY emission with weakened intensity. This spectral behavior indicated the occurrence of emission from the locally excited (LE) state in nonpolar solvents, whereas formation of an LE state followed by the rotation of the chromophores at the D-A bond leads to a low energy twisted intramolecular charge transfer state (TICT), resulting in a charge-separated state BBA+•-BODIPY-• in polar solvents. Furthermore, the hydrophobicity studies involving the solutions of dyads in admixtures of polar tetrahydrofuran (THF) and nonpolar hexanes revealed that when the fraction of hexanes in these mixtures is increased, the emission of BODIPY moiety was observed to be blue-shifted and exhibited enhanced intensity supporting the occurrence of TICT in these dyads.

Twisted Intramolecular Charge Transfer State of a "Push-Pull" Emitter.

The excited state Raman spectra of 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) in the locally-excited (LE) and the intramolecular charge transfer (ICT) states have been separately measured by time-resolved stimulated Raman spectroscopy. In a polar dimethylsulfoxide solution, the ultrafast ICT of DCM with a time constant of 1.0 ps was observed in addition to the vibrational relaxation in the ICT state of 4–7 ps. On the other hand, the energy of the ICT state of DCM becomes higher than that of the LE state in a less polar chloroform solution, where the initially-photoexcited ICT state with the LE state shows the ultrafast internal conversion to the LE state with a time constant of 300 fs. The excited-state Raman spectra of the LE and ICT state of DCM showed several major vibrational modes of DCM in the LE and ICT conformer states coexisting in the excited state. Comparing to the time-dependent density functional theory simulations and the experimental results of similar push-pull type molecules, a twisted geometry of the dimethylamino group is suggested for the structure of DCM in the S1/ICT state.

Characterization of Locally Excited and Charge-Transfer States of the Anticancer Drug Lapatinib by Ultrafast Spectroscopy and Computational Studies.

Lapatinib (LAP) is an anticancer drug, which is metabolized to the N- and O-dealkylated products (N-LAP and O-LAP, respectively). In view of the photosensitizing potential of related drugs, a complete experimental and theoretical study has been performed on LAP, N-LAP and O-LAP, both in solution and upon complexation with human serum albumin (HSA). In organic solvents, coplanar locally excited (LE) emissive states are generated; they rapidly evolve towards twisted intramolecular charge-transfer (ICT) states. By contrast, within HSA only LE states are detected. Accordingly, femtosecond transient absorption reveals a very fast switching (ca. 2 ps) from LE (λmax =550 nm) to ICT states (λmax =480 nm) in solution, whereas within HSA the LE species become stabilized and live much longer (up to the ns scale). Interestingly, molecular dynamics simulation studies confirm that the coplanar orientation is preferred for LAP (or to a lesser extent N-LAP) within HSA, explaining the experimental results.

Multiscale Conformational Sampling Reveals Excited-State Locality in DNA Self-Repair Mechanism.

Ultraviolet (UV) irradiation is known to be responsible for DNA damage. However, experimental studies in DNA oligonucleotides have shown that UV light can also induce sequence-specific self-repair. Following charge transfer from a guanine adenine sequence adjacent to a cyclobutane pyrimidine dimer (CPD), the covalent bond between the two thymines could be cleaved, recovering the intact base sequence. Mechanistic details promoting the self-repair remained unclear, however. In our theoretical study, we investigated whether optical excitation could directly lead to a charge-transfer state, thereby initiating the repair, or whether the initial excited state remains localized on a single nucleobase. We performed conformational sampling of 200 geometries of the damaged DNA double strand solvated in water and used a hybrid quantum and molecular mechanics approach to compute excited states at the complete active space perturbation level of theory. Analysis of the conformational data set clearly revealed that the excited-state properties are uniformly distributed across the fluctuations of the nucleotide in its natural environment. From the electronic wavefunction, we learned that the electronic transitions remained predominantly local on either adenine or guanine, and no direct charge transfer occurred in the experimentally accessed energy range. The investigated base sequence is not only specific to the CPD repair mechanism but ubiquitously occurs in nucleic acids. Our results therefore give a very general insight into the charge locality of UV-excited DNA, a property that is regarded to have determining relevance in the structural consequences following absorption of UV photons.

Enhanced photophysical properties of silica-methylene blue@amorphous carbon as fluorochromes with modulated Raman-active vibration modes

As a kind of near-infrared dye, methylene blue (MB) molecule is proneness to form excimers in solid powder or solution at high concentration, which results in weak photoluminescence intensity during clinic diagnostic and cell labeling applications. Aiming to improve the quantum yield of MB, silica-MB particles are herein prepared and treated hydrothermally (HT) to synthesize silica-MB-HT and silica-MB@amorphous carbon with glucose as carbon source (silica-MB@AmC). Significantly, hydrothermal treatment causes destruction of Si-O-Si network, modulation of aggregation state, variations of energy gap and release behavior of MB. In comparison with that of silica-MB, emission intensities of both silica-MB-HT and silica-MB@AmC increase greatly and their photoluminescence peaks are blue-shifted, which are closely related with Raman-active vibration modes of MB in a locally-excited state. Enhanced emissions of both silica-MB-HT120 and silica-MB@AmC120 are observed with increased intensities of Raman peaks assigned to in-plane bending vibrations of CH bond during skeletal deformation of CC in ring. In contrast, the in-plane bending vibration for CH bonds at end group of MB is considered as a way of radiation decay, and strong vibration of this Raman-active mode causes weak emission intensities of silica-MB-HT180 and silica-MB@AmC160. The stabilities of photophysical properties are compared after 72 h of incubation in buffered mediums or 5 h of illumination under ultraviolet light. The photoluminescence intensity of silica-MB@AmC120 fluorophore increase remarkably after illumination, meeting the requirements of reduced photobleaching, minized solvatochromic shift and increased fluorescent efficiency during versatile applications.