Intersystem Conversion Research Articles

Irradiated riboflavin over nonradiated one: Potent antimigratory, antiproliferative and cytotoxic effects on glioblastoma cells.

Riboflavin is a water-soluble yellowish vitamin and is controversial regarding its effect on tumour cells. Riboflavin is a powerful photosensitizer that upon exposure to radiation, undergoes an intersystem conversion with molecular oxygen, leading to the production of ROS. In the current study, we sought to ascertain the impact of irradiated riboflavin on C6 glioblastoma cells regarding proliferation, cell death, oxidative stress and migration. First, we compared the proliferative behaviour of cells following nonradiated and radiated riboflavin. Next, we performed apoptotic assays including Annexin V and caspase 3, 7 and 9 assays. Then we checked on oxidative stress and status by flow cytometry and ELISA kits. Finally, we examined inflammatory change and levels of MMP2 and SIRT1 proteins. We caught a clear antiproliferative and cytotoxic effect of irradiated riboflavin compared to nonradiated one. Therefore, we proceeded with our experiments using radiated riboflavin. In all apoptotic assays, we observed a dose-dependent increase. Additionally, the levels of oxidants were found to increase, while antioxidant levels decreased following riboflavin treatment. In the inflammation analysis, we observed elevated levels of both pro-inflammatory and anti-inflammatory cytokines. Additionally, after treatment, we observed reduced levels of MMP2 and SIRT. In conclusion, radiated riboflavin clearly demonstrates superior antiproliferative and apoptotic effects on C6 cells at lower doses compared to nonradiated riboflavin.

Tuning the Photophysical Properties of Ru(II) Photosensitizers for PDT by Protonation and Metallation: A DFT Study.

Due to their activity, photosensitizers with the Ru(II)-polypyridyl complex structure represent an intriguing class of photodynamic therapy agents used to treat neoplasms. However, their solubility is poor, intensifying experimental research into improving this property. One recently proposed solution is to attach a polyamine macrocycle ring. In this paper, the density functional theory (DFT) and time-dependent DFT (TD-DFT) studies on such derivative were performed to assess the impact of the protonation-capable macrocycle and its ability to chelate transition state metals, as illustrated by the Cu(II) ion, on the expected photophysical activity. These properties were determined by examining ultraviolet-visible (UV-vis) spectra, intersystem conversion, and type I and II photoreactions of all species possibly present in a tumor cell. For comparison purposes the structure devoid of the macrocycle was also examined. The results show that the subsequent protonation of amine groups improves the reactivity, with [H2L]4+/[H3L]5+ being borderline, whereas complexation appears to weaken the desired photoactivity.

Realizing External Quantum Efficiency over 25% with Low Efficiency Roll-Off in Polymer-Based Light-Emitting Diodes Synergistically Utilizing Intramolecular Sensitization and Bipolar Thermally Activated Delayed Fluorescence Monomer

Realizing External Quantum Efficiency over 25% with Low Efficiency Roll-Off in Polymer-Based Light-Emitting Diodes Synergistically Utilizing Intramolecular Sensitization and Bipolar Thermally Activated Delayed Fluorescence Monomer

High Steric‐Hindrance Windmill‐Type Molecules for Efficient Ultraviolet to Pure‐Blue Organic Light‐Emitting Diodes via Hybridized Local and Charge‐Transfer Excited‐State

AbstractHybridized local and charge‐transfer (HLCT) excited‐state compounds that enable full exciton utilization through a reverse intersystem conversion (RISC) from a high‐lying triplet to a singlet state have attracted attention. Developing high‐performance ultraviolet (UV) and blue organic light‐emitting diodes (OLEDs) is challenging due to difficulties acquiring HLCT molecules with a large energy bandgap and a high photoluminescence. Herein, a new strategy for excellent‐performance UV to blue emitters based on high steric‐hindrance windmill‐type structure is proposed. These emitters exhibit good thermal, morphological, and electrochemical stabilities, as well as HLCT excited‐state characteristics. Results suggest that OLED using CTPPI efficiently emits UV light (396 nm, CIEx,y = 0.16, 0.04) with a maximum external quantum efficiency (EQE) of 7.9% and currently ranks third in UV OLEDs. The lights of these devices are well modulated from UV/deep‐blue to pure‐blue with EQEs greater than 5% by regulating the locally excited (LE) and charge‐transfer (CT) components. Experimental and theoretical investigations indicate that harvesting triplet excitons afford high electroluminescence efficiencies via HLCT excited states in the devices. This study provides an efficient strategy to achieve high‐performance UV and blue OLEDs and offers great flexibility for material design.

Spectral-kinetic properties of transient absorption of hybrid silver–copper phthalocyanine nanostructures

In this paper, we studied the dynamics of transient absorption spectra of a hybrid nanostructure (Ag–CuPc)6 Ag based on island silver films and copper phthalocyanine thin films, as well as individual structural units of this system, by femtosecond transient absorption spectroscopy. It is found that the effects observed for a hybrid nanostructure reflect the existing mutual influence of the plasmonic and organic subsystems on the spectral-kinetic characteristics of each other. The characteristic time of the main component of induced optical density kinetics at a wavelength λ = 525 nm (τ ~ 15 ps) that is caused by singlet-triplet relaxation of the excited electronic states of the organic subsystem of the hybrid structure is noticeably shorter than the relaxation time (τ ~ 50 ps) at the same wavelength for a pure copper phthalocyanine film of 40 nm thickness, just the same as the total thickness of the organic subsystem in the hybrid structure. It is assumed that the presence of silver nanoparticles in the hybrid structure (Ag–CuPc)6 Ag, affects the probability of intersystem conversion in the organic subsystem, accelerating the transition of CuPc molecules to a long-lived triplet state due to a strong local field near the surface of the plasmon particle. The triplet-triplet absorption spectrum of copper phthalocyanine in the spectral range 470–750 nm was obtained.

Biomedical Platform Development of a Chlorophyll-Based Extract for Topic Photodynamic Therapy: Mechanical and Spectroscopic Properties.

Photodynamic therapy (PDT) is a therapeutic modality that has shown effectiveness in the inactivation of cancer cell lines and microorganisms. Treatment consists of activating the photosensitizer (PS) upon light irradiation of adequate wavelength. After reaching the excited state, the PS can handle the intersystem conversion through energy transfer to the molecular oxygen, generating reactive oxygen species. This especially applies to singlet oxygen (1O2), which is responsible for the selective destruction of the sick tissue. Photosensitizing compounds (chlorophylls and derivatives) existing in the spinach extract have applicability for PDT. This study aimed to develop and characterize the thermoresponsive bioadhesive system composed of Pluronic F127 20.0%- and Carbopol 934P 0.2% (w/w) (FC)-containing chlorophyll-based extract 0.5% (w/w) (FC-Chl). Mechanical and rheological properties, in vitro release, sol-gel transition temperature, and ex vivo permeability of the spinach extract PS components (through pig ear skin) were investigated. Furthermore, photodynamic activity of the system was accessed through uric acid and time-solved measurements. The sol-gel transition temperature obtained for the FC-Chl system was 28.8 ± 0.3 °C. Rheological and texture properties of the platform were suitable for use as a dermatological system, exhibiting easy application and good characteristics of retention in the place of administration. In vitro release studies showed the presence of two distinct mechanisms that reasonably obey the zero-order and first-order kinetics models. PS components presented skin permeability and reached a permeation depth of 830 μm (between the epidermis and dermis). The photodynamic evaluation of the FC-Chl system was effective in the degradation of uric acid. The quantum yield (ΦΔ1O2) and life time (τ1O2) of singlet oxygen showed similar values for the spinach extract and the isolated chlorophyll a species in ethanol. These results allowed for the classification of the FC-Chl platform as potentially useful for the delivery of the chlorophyll-based extract in the topic PDT, suggesting that it is worthy for in vivo evaluation.

Model and calculation of rate constants of nonradiative T-S intersystem conversion: Test by example of naphthalene and its chlorine derivatives

For in-plane spin states (s = z, y), the rate constants Kdgs of the nonradiative energy degradation T1 ⇝ S0 of the lowest triplet T1 state of naphthalene (NPH) and its dichloro-substituted derivatives at positions 1,4- and 2,3- of the molecule (1,4-NPH and 2,3-NPH) are calculated. A simple model is proposed for calculations that is based on the nonadiabatic approximation and uses all the out-of-plane vibrational modes of the molecule as promoting vibrational modes. As a result of calculations, the dependences of the rate constants Kdg and Kdgs on the positions of chlorine atoms in the molecule are obtained, which are consistent with the known data of magnetooptical measurements. The inversion of the ratio Kdgz: Kdgy in the 1,4-NPH and 2,3-NPH molecules is established.

A Quantum Chemical Study of Bis-(iminoquinonephenolate) Zn(II) Complexes

A computational DFT B3LYP*/6-311++G(d,p) study performed on bis-(iminoquinonephenolate) Zn(II) complex [Zn(II)(C(12)H(8)NO(2))(2)] has revealed a previously unexplored mechanism for valence tautomerism inherent in transition metal complexes with redox active (noninnocent) ligands. The occurrence of energy-close isomeric forms of the complex and their low energy barrier interconversion is caused not by the intramolecular electron transfer (IET) between the metal and ligand frontier orbitals, but the intersystem conversion within a redox active ligand without involvement of a metal center. This mechanism gives a new insight into the origin of the previously experimentally studied isomeric forms of bis-(iminoquinonephenolate) Zn(II) complexes that must be assigned to [Zn(II)((1)L(-1))(2)] (8) and [Zn(II)((1)L(-1))((3)L(-1))] (9) structures. The spin-forbidden transition between the two forms of the complex proceeds via a minimal energy crossing point (MECP) corresponding to the energy barrier of 8.9 kcal mol(-1) for the 9 --> 8 transformation in the gas phase.

DFT Studies on Reactions of Transition Metal Complexes with O2

DFT calculations have been carried out to study the mechanisms for reactions of O2 with a series of metal complexes, including d6 CpRuL2, d6 ML5, and d8 ML4 complexes. The calculation results indicate that the reaction is initiated by an end-on coordination of O2 to the metal center, which gives an (η1-O2)[M] intermediate. The uncoordinated oxygen atom of the η1-O2 ligand then approaches the metal center to give a new η1-O2 intermediate in which the η1-O2 ligand is oriented approximately the same as the one defined in the product. An intersystem conversion from the triplet to singlet energy surface (MECP) then occurs to enable the metal peroxide product to be formed.

Reaction of Ta(NMe2)5 with O2: Formation of Aminoxy and Unusual (Aminomethyl)amide Oxo Complexes and Theoretical Studies of the Mechanistic Pathways

Reaction of d0 Ta(NMe2)5 (1) with O2 yields two aminoxy complexes (Me2N)(n)Ta(eta2-ONMe2)(5-n) (n = 4, 2; 3, 3) as well as (Me2N)4Ta2[eta2-N(Me)CH2NMe2]2(mu-O)2 (4) and (Me2N)6Ta3[eta2-N(Me)CH2NMe2]2(eta2-ONMe2)(mu-O)3 (5) containing novel chelating (aminomethyl)amide-N(Me)CH2NMe2 ligands. The crystal structures of 2-5 have been determined by X-ray crystallography. (Me2N)4Ta(eta2-ONMe2) (2) converts to (Me2N)3Ta(eta2-ONMe2)2 (3) in its reaction with O2. In addition, the reaction of Ta(NMe2)5 with 3 gives 2 only at elevated temperatures. Density functional theory (DFT) calculations have been used to investigate the mechanistic pathways in the reactions of Ta(NMe2)5 (1) with triplet O2. Monomeric reaction pathways in the formation of 2-5 are proposed. A key step is the oxygen insertion into a Ta-N bond in 1 through an intersystem conversion from triplet to singlet energy surface to give an active peroxide complex (Me2N)4Ta(eta2-O-O-NMe2) (A4). The DFT studies indicate that the peroxide ligand plays an important role, including oxidizing an amide to an imine ligand through the abstraction of a hydride. Insertion of Me-N=CH2 into a Ta-amide bond yields the unusual -N(Me)CH2NMe2 ligands.

The Influence of Polymer Matrix Parameters on Intersystem Crossing in Dopant Molecules of Aromatic Amines

ABSTRACT The luminescence spectra of carbazole and phenyl-substituted amines in toluene solution and wide set of polymer matrix were studied at 5 and 295 K, as well as their parameters were obtained. The intersystem crossing was shown to be important process influencing on the energy relaxation of excited singlet state. Its efficiency depends not only on the chemical nature of substituents and spatial conformation of molecule, but also is conditioned by interaction with ambient medium. Therefore, the rate of intersystem conversion increases upon influence of external heavy atom, and also when the polymer matrix is changed or pressure up to 3 kbar is applied.

Theory of photon statistics in single-molecule Förster resonance energy transfer

We present the theory for the distribution of the number of donor and acceptor photons detected in a time bin and the corresponding energy-transfer efficiency distribution obtained from single-molecule Forster resonance energy-transfer measurements. Photon counts from both immobilized and freely diffusing molecules are considered. Our starting point is the joint distribution for the donor and acceptor photons for a system described by an arbitrary kinetic scheme. This is simplified by exploiting the time scale separation between fast fluorescent transitions and slow processes which include conformational dynamics, intersystem conversion to a dark state, and translational diffusion in and out of the laser spot. The fast fluorescent transitions result in a Poisson distribution of the number of photons which is then averaged over slow fluctuations of the local transfer efficiency and the total number of photons. The contribution of various processes to the distribution and the variance of the energy-transfer efficiency are analyzed.

Dependence of fluorescent ability on efficiency of intersystem conversion in psoralens

Results of theoretical research for seven psoralen molecules: psoralen, 5-methoxypsoralen, 8-methoxypsoralen, angelicin, pseudoisopsoralen, isopseudopsoralen, and allopsoralen, are presented. Schemes of photophysical processes in psoralens based on quantum chemical calculation by the INDO method are analyzed. Effects of isomerism and methoxysubstitution on the energy level diagram are demonstrated. The rate constants of intersystem and internal conversions are calculated. We obtained rather high intersystem conversion constant that influences on the fluorescent yield of psoralens.

Single molecule waiting time distribution functions in quantum processes

The statistics of single molecule blinking events often reveal underlying quantum mechanisms. The golden rule rate expression for quantum transitions is shown to be the inverse of the mean waiting time. The distribution function for the waiting time is related to the density of states such that simple power-law distribution functions can be predicted based on the functional form for the density of states. Explicit formulas are derived for waiting time distribution functions in three kinetic processes: Quantum tunneling, intersystem conversion, and nonstationary electron transfer.

SPECTRAL-LUMINESCENT CHARACTERISTICS AND PHOTOPROCESSES IN A PSORALEN SENSITIZER AND ITS ISOMERS AND METHOXYSUBSTITUTES

The quantum chemistry method of intermediate neglect of differential overlap (INDO) with spectroscopic parameterization is used to study the spectroscopic-luminescent and physical characteristics of psoralen and its isomers and methoxysubstitutes. Efficient intersystem conversion, which causes significant population of triplets, is observed for the examined compounds. Effects of isomerism and methoxysubstitution on the energy level diagram, photoelectron spectrum, and dipole moments are demonstrated. The efficiency of psoralen sensitizers is examined.

Intersystem Conversion between Singlet and Triplet Exciton States in ZnP2

Detailed excitation spectra for singlet and triplet luminescence in ZnP 2 are observed at 2 K by using a Ti:sapphire laser. When a singlet exciton is excited, conversion to a triplet exciton occurs over a wide excitation energy range. The spin memory of the exciton is easily lost under the band-to-band excitation condition. When the triplet exciton is excited, the excitation spectrum for the triplet luminescence shows a clear hydrogen-like series up to n =9, which reflects the absorption spectrum of a spin triplet exciton series with s - d doublet structure for n ≥3. The excitation spectrum for the singlet luminescence under the excitation of the triplet exciton is observed for the first time as a new exciton series appearing only for n ≥3. The new exciton series results from the contribution of the d -envelope function of the exciton, and the s -envelope function alone does not give rise to the singlet luminescence.

Exciton relaxation in β-ZnP 2 — Intersystem conversion between singlet and triplet states

Detailed excitation spectra for singlet and triplet luminescence bands in ZnP 2 are observed by using a Ti:sapphire laser. When the triplet exciton is excited, the excitation spectrum for the triplet luminescence shows clear hydrogenic series up to n=9, which reflects the absorption spectrum of the spin triplet exciton. The excitation spectrum for the singlet luminescence under the excitation of the triplet exciton is observed for the first time as a new exciton series. Magnetic field effects of the excitation spectra under the triplet exciton excitation are also shown.

Subpicosecond excitation of strongly coupled porphyrin–phthalocyanine mixed dimers

Electrostatic heterodimers are formed in the liquid phase by pairing a zinc porphyrin with either a copper or an aluminium phthalocyanine bearing oppositely charged substitutents. The ground-state absorption spectra of such heterodimers are drastically changed with respect to the corresponding monomers, indicating the existence of a strong interaction between the two chromophores. Irrespective of the nature of the metal ions and of the peripheral substituents, the heterodimers present very similar ground-state spectra.The photophysical properties of the two heterodimers are investigated with the use of femto- and nano-second absorption spectroscopies. Excitation of the zinc porphyrin–aluminium phthalocyanine heterodimer at 565 or 620 nm is followed by a very efficient electron transfer from the porphyrin to the phthalocyanine moiety while a very efficient intersystem conversion takes place in the excited zinc porphyrin–copper phthalocyanine, leading to the final ‘triplet’ excimer. The difference in behaviour is analysed and explained in terms of the thermodynamics of the transfer process and of the peculiar properties of the paramagnetic copper phthalocyanine.

HTFFR kinetic studies of the fate of excited BaO formed in the Ba/N2O chemiluminescent reaction

Rate coefficients for quenching kQ and intersystem conversion kT of excited BaO formed in the reaction Ba+N2O(1)→BaO+N2 have been measured for several collision partners. Methods are developed and applied here for obtaining quenching rate coefficients in chemiluminescent reactions involving a reservoir state. Experiments were performed between 450 and 1000 K at 1–120 Torr in a modified high-temperature fast-flow reactor (HTFFR). The rate coefficients (or their upper limits) are, in ml molecule−1 s−1 units: kArQ (600–1000 K) ?3×10−13; kHeQ (460 K) <3×10−13; kN2Q (600 K) = (4.8±2.0) ×10−12; kN2OQ (600 K) = (4.2±1.0) ×10−10; 2.5×10−10?kO2Q (600 K) ?7.0×10−10; kArT (600 K) ?1.5×10−11. Incidental data at ?1000 K indicate that k1?5×10−11, and that the Ba/O2 reaction has about the same rate coefficient. In addition to BaO emission, strong Ba atomic emission arising from energy transfer processes was observed for [Ba]≳1×1013 ml−1 at T?1000 K. Measurements of the spectral distributions of the Ba/N2O chemiluminescence as a function of [Ar], [He], and [N2] yield relations among vibrational relaxation rate coefficients: kv, kT, and kQ for specific BaO(A 1Σ) vibrational levels. The set of reactions currently in use to describe the Ba/N2O chemiluminescence is shown to be inadequate to account for the observed changes in BaO(A 1Σ−X 1Σ) spectral distribution with pressure.

Delayed Ionization of Excited Phenanthrene and Anthracene in Solution

The delayed ionization of excited states of phenanthrene and anthracene was observed in tetrahydrofuran solution. The measured photocurrent rose, attained a maximum, and decayed in time very long compared to flash duration. No currents were observable in hexane solution and current response was quenched on admission of O2 to the tetrahydrofuran solutions. The observed time dependence, dependence on flash intensity, and wavelength dependence are consistent with an interpretation based on 3B1 excitation via intersystem conversion from excited 1A1 and the subsequent interaction of two 3B1 molecules to yield current carriers.