Energy Gap ΔEST Research Articles

Type-I Photodynamic Therapy Induced by Pt-Coordination of Type-II Photosensitizers into Supramolecular Complexes.

Platinum supramolecular complexes based on photosensitizers have garnered great interest in photodynamic therapy (PDT) due to Pt (II) centers as chemotherapeutic agents to eliminate tumor cells completely, which greatly improve the antitumor efficacy of PDT. However, in comparison to precursor photosensitizer ligand, the formed platinum supramolecular complexes typically exhibit inferior outcomes in terms of reactive oxygen species (ROS) generation. How to boost ROS generation in the formed platinum supramolecular complexes for enhanced PDT is an enticing yet highly challenging task. Here we report a Pt-coordination-based dimeric photosensitizer complex (Cz-BTZ-Py)2Pt(OTf)2. It is found that comparing with photosensitizer ligand Cz-BTZ-Py, the formed supramolecular complex exhibit redshifts of absorption wavelength as well as enhanced ROS generation efficiency. Moreover, type-I ROS generation (O2⋅-) is produced in the formed platinum supramolecular complexes mainly due to a reduced energy gap ΔEST resulting from exciton coupling between two photosensitizer ligands. And type-I ROS (O2⋅-) generation significantly amplifies the photodynamic therapy (PDT) outcomes. In vitro evaluation shows excellent photochemotherapy performance of (Cz-BTZ-Py)2Pt(OTf)2 nanoparticles. We anticipate this work would provide a novel approach to design type-I photosensitizers for efficient PDT.

Borylation and rearrangement reactions of azasilaanthracenes to afford B,N-doped nanographenes.

An air-stable B3,N3-containing dibenzobisanthene (8) was prepared in 29% yield by heating a 1,3,5-tri(azasilaanthryl)benzene (5) with BBr3 (180 °C). Under these conditions, the reaction does not stop after threefold SiMe2/BBr exchange but proceeds further via two rearrangement and two intramolecular C-H borylation steps. Some mechanistic details were unveiled by using smaller model systems and applying lower reaction temperatures. According to X-ray crystallography, compound 8 has a helically distorted scaffold. Due to its multiple resonance structure, it shows a narrow-band blue-green emission (λem = 493 nm; ΦPL = 84%; FWHM = 0.20 eV; THF); samples measured in PMMA gave prompt and delayed fluorescence lifetimes of 10.7 ns and 136 μs, respectively. The optical properties of 8 and of structurally related species were also investigated by quantum-chemical means: most of these compounds exhibit a small energy gap ΔEST between the lowest excited singlet (S1) and triplet (T1) states and a non-negligible spin-orbit coupling (SOC) between S1 and T1/T2, demonstrating their potential as thermally activated delayed fluorescence (TADF) emitters.

Theoretical study on the photophysical properties of thiophene-fused-type BODIPY series molecules in fluorescence imaging and photodynamic therapy.

As a class of photosensitizers (PSs) with dual functions of photodynamic therapy (PDT) and fluorescence imaging, the relationship between the structure and dual-function of thiophene-fused-type BODIPY dyes has not been studied in depth before. We found that the thiophene-fused-type BODIPY triplet photosensitizer is produced according to the energy level matching rule and the introduction of the thiophene ring significantly reduces the energy gap ΔEST between singlet and triplet states, as revealed by our investigation of the excited state structures and energies of thieno-fused BODIPY dyes. At the same time, a tiny ΔEST also results in a greatly enhanced intersystem crossing (ISC) rate, kISC. The kISC value of MeO-BODIPY, having the highest singlet oxygen quantum yield (ΦΔ), is the largest. Substitution with a strong electron donor N,N-dimethylaminophenyl (DMA) leads to the vertical configuration in the T1 state. The small ΔE (0.0029 eV) between the HOMO and HOMO-1 triggers the photo induced electron transfer (PET) of inhibiting ISC and fluorescence. When thieno-fused BODIPYs react with pyrrole, the increase of π-conjugation and smaller ΔEHOMO-LUMO explain the redshift in emission wavelength of thieno-pyrrole-fused BODIPY. The more planar configuration of the S1 state and the stronger oscillator intensity reflect a higher fluorescence quantum yield (ΦF). The extension of π-conjugation can cause molecules to transition to higher-level singlet excited states (Sn states, n ≥ 1) after absorbing energy and reduce the energy level of the excited state, resulting in multiple channels and favoring 1O2 production for thieno-pyrrole-fused BODIPYs with electron-withdrawing groups at the para-position of the phenyl groups. Due to ΔES0-T1 < 0.980 eV, the substitution of electron-donating groups cannot produce 1O2. In this work, we have revealed the mechanism of ISC and the fluorescence emission process in the thiophene-fused-type BODIPY dye, which has provided a theoretical foundation and guidance for the future design of BODIPY-based heavy-atom-free PSs for molecular applications in PDT.

Blatter Diradicals with a Spin Coupler at the N(1) Position.

Reactions of a benzo[e][1,2,4]triazine with dilithiobenzenes lead to di-Blatter diradicals connected at the N(1) positions via a spin coupling unit, 1,4-phenylene or 1,3-phenylene. Electrochemical analysis in MeCN revealed four one-electron redox processes separated by 0.1-0.3 V in both diradicals. Variable temperature EPR measurements in polystyrene (PS) solid solutions gave the singlet-triplet energy gaps ΔES-T =2 J of -3.02(11) and -0.16(1) kcal mol-1 for 1,4-phenylene and 1,3-phenylene derivatives, respectively. The latter negative value was attributed to conformational properties of the diradical in the PS solid solution. Results suggest a simple and efficient access to a family of stable Blatter diradicals with a controllable S-T gap through a judicious choice of the arylene coupling unit. DFT calculations indicate that the triplet state is stabilized by (het)arylenes with low LUMO.

Aggregation-Enhanced Thermally Activated Delayed Fluorescence Efficiency for Two-Coordinate Carbene-Metal-Amide Complexes: A QM/MM Study.

The two-coordinate carbene-metal-amide complexes have attracted a great deal of attention due to their remarkable thermally activated delayed fluorescence (TADF) properties, giving them promise in organic light-emitting diode application. To reveal the inherent mechanism, we take CAAC-Cu(I)-Cz and CAAC-Au(I)-Cz as examples to investigate the photophysical properties in solution and solid phases by combining quantum mechanics/molecular mechanics approaches for the electronic structure and the thermal vibration correlation function formalism for the excited-state decay rates. We found that both intersystem crossing (ISC) and its reverse (rISC) are enhanced by 2-4 orders of magnitude upon aggregation, leading to highly efficient TADF, because (i) the metal proportion in the frontier molecular orbitals increases, leading to an enhanced spin-orbit coupling strength between S1 and T1, and (ii) the reaction barriers for ISC and rISC are much lower in solution than in aggregate phases through a decrease in energy gap ΔEST and an increase in the relative reorganization energy through bending the angle ∠C2-Cu-N1 for T1. We propose a pump-probe time-resolved infrared spectroscopy study to verify the mechanism. These findings can clarify the ongoing dispute over the understanding of the high TADF quantum efficiency for two-coordinate metal complexes.

Theoretical insights on type I/II photoreactions of potential Zn(II) polypyridyl photosensitizers for two-photon photodynamic therapy

Large two-photon absorption cross-sections are vital to photosensitizers (PSs) in TP-PDT, which can be used to develop in-depth treatment for diseased cells and minimize the harm to surrounding cells. Here, we conduct a study about photophysical properties of one Ru(II) polypyridyl complex and two designed Zn(II) polypyridyl complexes by means of DFT and TD-DFT methods. The main results are as follows: firstly, the two-photon absorption spectrum of two designed complexes Zn-OMe and Zn-OCOOCH3 are all within the phototherapeutic window (550-900nm). Secondly, large SOC values and small energy gaps ΔES-T of these complexes guarantee the efficiency of ISC process. Thirdly, their T1 energy is greater than that required for generating 1O2 (0.98eV) via Type II photoreaction. In addition, the calculated results of vertical electron affinities (VEA) and vertical ionization potentials (VIP) show that these complexes are able to form superoxide ions O2(-) via Type I photoreaction. Specifically, both of two designed Zn-centric complexes have larger TPA cross-sections than that of Ru-centric complex. In a word, we are pleased to report two potential photosensitizers with excellent performance and reasonable price for Type I/II photoreactions. We expect our study will offer some theoretical guidance and help in TP-PDT.

The photophysical properties of three [M(phen)2dppz]2+ (M=Ru and Zn) derivatives for two-photon photodynamic therapy: Insights from theoretical investigations

Density Function Theory (DFT) and Time-Dependent Density Functional Theory (TD-DFT) are used to access the photophysical properties of [Ru(phen)2dppz]2+ and two designed [Zn(phen)2dppz]2+ derivatives for two-photon photodynamic therapy. The geometrical structures, electronic properties, one-photon and two photon absorption properties, spin-orbit matrix elements and energy gaps ΔES-T are calculated. The results demonstrate that the absorption wavelengths of these complexes are within the so-called phototherapeutic windows and have large two-photon absorption cross-sections to deeply permeate cancer tissue, especially for the [Zn(phen)2dppz]2+ derivatives with large π-conjugate surface. Furthermore, T1 energy of these three complexes is all higher than that required of generating singlet oxygen (0.98 eV). More importantly, the SOC constants and energy gap ΔES-T of these complexes indicating the ISC process can be effectively completed. Therefore, these three complexes all can be used as potential candidates in TP-PDT. In particular, the [Zn(phen)2dppz]2+ derivatives with large π-conjugate surface is proposed due to its low costing. Our study may help to rationalize the available experimental data and establish some guidelines for designing novel potential photosensitizers in two-photon photodynamic therapy.

Understanding the nature of quinoidal and zwitterionic states in carbazole-based diradicals.

We report two carbazole-based diradicals, out of which the m-isomer shows a large diradical character y0 (0.89) and a small singlet-triplet energy gap ΔES-T (-0.98 kcal mol-1), whereas the p-isomer exhibits smaller y0 (0.79) but a much larger ΔES-T (-6.16 kcal mol-1). DFT calculations reveal that this tendency is also suitable for nitrogen and carbon-centered diradicals.

Diradical Character Enhancement by Spacing: N-Heterocyclic Carbene Analogues of Müller's Hydrocarbon.

Two-fold C-C cross-coupling of N-heterocyclic carbenes [NHCs; SIPr=C(NArCH2 )2 , 1; IPr=C(NArCH)2 , 2; Me-IPr=C(NArCMe)2 , 3; Ar=2,6-iPr2 C6 H3 ] with 4,4''-diiodo-p-terphenyl under Ni catalysis furnished [(SIPr)(C6 H4 )3 (SIPr)](I)2 (4), [(IPr)(C6 H4 )3 (IPr)](I)2 (5), and [(Me-IPr)(C6 H4 )3 (Me-IPr)](I)2 (6). Two-electron reduction of 4-6 with KC8 readily afforded NHC analogues of Müller's hydrocarbon (MH), [(SIPr)(C6 H4 )3 (SIPr)] (7), [(IPr)(C6 H4 )3 (IPr)] (8), and [(Me-IPr)(C6 H4 )3 (Me-IPr)] (9), respectively, as highly colored crystalline solids. Quantum chemical calculations suggested that the singlet ground state for 7-9 possesses a vertical singlet-triplet energy gap ΔES-T of -7.24 to -7.60 kcal mol-1 , which is significantly lower compared to that of the NHC analogues of Thiele's (TH) and Chichibabin's (CH) (18-38 kcal mol-1 ) hydrocarbons. Importantly, the calculated diradical character (y) of 7-9 (y≈0.6) is considerably higher compared to that of the related TH and CH (y=0.1-0.4), suggesting the open-shell singlet character of 7-9.

N-Heterocyclic Carbene Analogues of Thiele and Chichibabin Hydrocarbons.

Stable N-heterocyclic carbene analogues of Thiele and Chichibabin hydrocarbons, [(IPr)(C6 H4 )(IPr)] and [(IPr)(C6 H4 )2 (IPr)] (4 and 5, respectively; IPr=C{N(2,6-iPr2 C6 H3 )}2 CHCH), are reported. In a nickel-catalyzed double carbenylation of 1,4-Br2 C6 H4 and 4,4'-Br2 (C6 H4 )2 with IPr (1), [(IPr)(C6 H4 )(IPr)](Br)2 (2) and [(IPr)(C6 H4 )2 (IPr)](Br)2 (3) were generated, which respectively afforded 4 and 5 as crystalline solids upon reduction with KC8 . Experimental and computational studies support the semiquinoidal nature of 5 with a small singlet-triplet energy gap ΔES-T of 10.7 kcal mol-1 , whereas 4 features more quinoidal character with a rather large ΔES-T of 25.6 kcal mol-1 . In view of the low ΔES-T , 4 and 5 may be described as biradicaloids. Moreover, 5 has considerable (41 %) diradical character.

Controlled Excited-State Dynamics and Enhanced Fluorescence Property of Tetrasulfone[9]helicene by a Simple Synthetic Process

Tetrasulfone[9]helicene (PTSH) was newly synthesized to improve and evaluate its fluorescence and excited-state dynamics through a single-step oxidation reaction of tetrathia[9]helicene (PTTH). In electrochemical measurements, the reduction potential of PTSH was shifted in a positive direction by approximately 1.0 V when compared to that of PTTH because of its electron-accepting sulfone units. The results of the electrochemical measurements agree with the energy levels calculated by density functional theory (DFT) methods and steady-state spectroscopic measurements. Furthermore, a significant enhancement of the absolute fluorescence quantum yield (ΦFL) was achieved. The absolute fluorescence quantum yield of PTSH attained 0.27, which is approximately 10 times larger than that of PTTH (ΦFL = 0.03). Such an enhancement of ΦFL can be successfully explained by the corresponding kinetic comparison. The reason is mainly the increased energy gap ΔEST between the lowest singlet (S1) and triplet (T1) excited state...

Carbenes and nitrenes. An overview

A brief survey of the structure, methods of generation and reactivity of the singlet and triplet carbenes and nitrenes is given. The DFT and CCSD(T) calculations of the singlet–triplet energy gap ΔEST for a large number of differently substituted carbenes and nitrenes are performed and the dependence of ΔEST on the substituent(s) at the sextet center is discussed.

Computational design of low singlet–triplet gap all-organic molecules for OLED application

It was recently reported that external quantum efficiency in organic LEDs can be substantially enhanced when triplet excitons are harvested through upconversion by E-type delayed fluorescence in materials with small singlet–triplet energy gap ΔEST, based on donor–acceptor (DA) chromophores. Furthermore, organic solar cells (OSCs) might profit from such materials in order to reduce recombination losses. However, targeted design rules for such materials are missing up to now. In this paper, we follow a facile (TD-)DFT-based computational design concept by engineering the fragment frontier orbitals in DA systems. The calculations show that optimized systems with very small ΔEST in the range of kT can be achieved by balancing the energetic offset between fragment MOs as well as through the nature of the DA connector. Application in OLED will additionally require small non-radiative rates, which recommends large bandgap materials. Utilization in polymeric DA systems with small ΔEST in OSCs requires the full exploration of the chain length dependence of the respective oligomers.

Diboration of the E═E Double Bond by [2]Metallocenophanes (E = N, P, As, Sb, and Bi): A Theoretical Study

Potential energy surfaces for cycloaddition reactions of group 15 Ph−E═E−Ph molecules to [2]boraferrocenophane have been studied using density functional theory (B3LYP/LANL2DZ). Five group 15 Ph−E═E−Ph species, where E = N, P, As, Sb, and Bi, have been chosen as model reactants in this work. Our findings reveal that the group 15 Ph−E═E−Ph species displays increasing distortions from trans-bent to trans geometry due to the accumulation of nonbonding electron density at the group 15 pnictogen elements. Additionally, our present theoretical work predicts that the relative chemical reactivity of group 15 Ph−E═E−Ph molecules decreases in the order N > P > As > Sb > Bi. That is, the lighter the atomic weight of group 15 elements E involved in the double bond of a Ph−E═E−Ph molecule, the easier its hybridization in the central pnictogen elements E, the larger the ∠EEC bond angle in the group 15 Ph−E═E−Ph molecule, the stronger its E═E π bond strength, and the smaller the singlet−triplet energy gap ΔEst of a Ph−E...