Internal Heavy Atom Effect Research Articles

Halogen Impact on the Supramolecular Organization of Chiral Phthalimide EmittersDisplaying Room Temperature Phosphorescence.

Room temperature phosphorescence from organic materials has attracted an increasing attention in the recent years due to their potential application in various advancing technologies, notably in bioimaging and displays. In this context, heavy atoms such as halogen ones revealed useful tools to enhance the spin-orbit coupling (SOC) of molecular organic phosphors. However, the effect of halogen at the supramolecular level remains less understood, especially in the field of molecular crystals where additional factors can impact the phosphorescence emission. Here, we investigate external effect of halogens on the phosphorescence of chiral phthalimides molecular crystals. The results show that changing the nature of the halogen atom onto the phthalimide core leads to an evolution of the photophysical properties of the materials which does not necessarily follow the classical trend imposed by the expected internal heavy atom effect. Beyond this aspect, we showed that the halogen atom has a profound impact on the packing between the chromophores at the supramolecular level which is of paramount importance towards the optical properties (PLQY and lifetimes) of the different phosphors examined.

Harnessing Heavy-Atom Effects in Multiple Resonance Thermally Activated Delayed Fluorescence (MR-TADF) Sensitizers: Unlocking High-Performance Visible-to-Ultraviolet (Vis-to-UV) Triplet Fusion Upconversion.

Ultraviolet (UV) light plays a crucial role in various applications, but currently, the efficiency of generating artificial UV light is low. The visible-to-ultraviolet (Vis-to-UV) system based on the triplet-triplet annihilation upconversion (TTA-UC) mechanism can be a viable solution. Metal-free multiple resonance thermally activated delayed fluorescence (MR-TADF) materials are ideal photosensitizers (PSs) apart from the drawback of high photoluminescence quantum yields (PLQYs). Herein, we systematically investigated the impact of the heavy-atom effect (HAE) on the MR-TADF sensitizers. BNCzBr was then synthesized by incorporating a bromine atom into the skeleton of the precursor BNCz. Impressively, the internal HAE (iHAE) leads to a significantly decreased PLQY and a remarkably increased intersystem crossing quantum yield (ΦISC). Consequently, a higher upconversion quantum efficiency of 12.5% was realized. While the external HAE (eHAE) harms the UC performance. This work guides the further development of MR-TADF sensitizers for high-performance Vis-to-UV TTA-UC systems.

Internal Heavy-Atom Effect on Visible-Light-Induced Cyclization Reaction in Diarylethene-Perylenebisimide Dyads.

All-visible-light switchable diarylethene-perylenebisimide (DAE-PBI) dyads having bromine heavy atoms in the molecule were designed and synthesized. Very recently, we found a unique visible-light-induced cyclization reaction in a DAE-PBI dyad. The dyad exhibited reversible cyclization and cycloreversion reactions upon alternate irradiation with green (500-550 nm) and red (>600 nm) light. From the experimental results, it was suggested that the triplet state of DAE unit was generated via multiplicity conversion based on intramolecular energy transfer from the singlet excited state of PBI unit and that the cyclization reaction of DAE unit proceeded from the triplet state. In addition, it was revealed that the reactivity remarkably increased in a solvent containing heavy atoms such as carbon tetrachloride and iodoethane (i.e., external heavy-atom effect). Based on such results, in this study, we attempted to design and synthesize novel DAE-PBI dyads introducing bromine heavy atoms at different positions in the molecule. The synthesized dyads exhibited higher quantum yields of photocyclization reaction under visible-light irradiation even in a heavy-atom-free solvent compared to the previous dyad having no heavy atoms. The magnitude of enhancement well correlated to the contribution ratio of atomic orbital of bromine to the molecular orbital in LUMOs. These results indicated that the internal heavy atom effectively contributed to the visible-light-induced cyclization reaction in DAE-PBI dyads. Such an internal heavy-atom effect will pave the way for new molecular design to develop all-visible-light-activatable molecular switches.

The Internal Heavy Atom Effect on the Deactivation of Triplet States of Porphyrins in the Presence of Molecular Oxygen

The Internal Heavy Atom Effect on the Deactivation of Triplet States of Porphyrins in the Presence of Molecular Oxygen

Photodegradation of Flutamide and Halogen Derivatives of Nitrobenzotrifluoride: The NO Release Channel.

The photodegradation of the nonsteroidal antiandrogen drug flutamide has been long linked to the photoisomerization involving the nitro group. In this work, the dynamics of NO photoelimination upon photolysis at 266 nm of flutamide, nitrobenzotrifluoride, and its halogen derivatives were investigated. Similar to nitrobenzene and its derivatives, a bimodal translational energy distribution was observed for the NO photofragment indicating the presence of two distinct elimination channels resulting in slow and fast components. The trends in the slow/fast branching ratio show that halogen substitution at the para position increases the triplet state yield due to the internal heavy-atom effect leading to enhancement of the fast component. Furthermore, the topology of the triplet state potential energy surface showed that the minimum energy path favors the oxaziridine ring-type intermediate over the NO2 roaming mechanism in all five molecules investigated. The steric interaction between the NO2 group and the CF3 group, which are placed in the ortho position, lowers the barrier for the formation of the oxaziridine transition state compared to that of nitrobenzene.

Влияние центрального иона металла на люминесцентные и фотофизические параметры фталоцианинов

Based on the data on the spectral and photophysical parameters of the metal complexes of tetraazaporphyrins and phthalocyanines, the effect of the central metal ion on the manifestation of spin-orbit interaction is considered. For complexes of phthalocyanines with Mg(II), Zn(II), Pd(II), and Pt(II) ions, the absorption spectra, luminescence spectra, and luminescence excitation spectra were measured, and the lifetimes and quantum yields of luminescence were determined. It was shown that metal phthalocyanines with Mg(II) and Zn(II) ions, as well as Mg-tetaraazaporphyrin, exhibit intense fluorescence with high quantum yields. For single molecules of Mg-tetaraazaporphyrin in a polymer film at T=6 K, the fluorescence excitation spectra consisting both exclusively of zero-phonon lines and containing phonon wings were measured. A wide distribution of the measured zero-phonon line widths was observed. It is demonstrated, that the introduction of Pd(II) and Pt(II) ions into the center of phthalocyanine macrocycles leads to a decrease in fluorescence and the appearance of intense phosphorescence even at room temperature. Based on measurements of quantum yields of photosensitized generation of singlet molecular oxygen, the quantum yield of interconversion into triplet states for the compounds under study was defined. Deactivation of the lower triplet state of phthalocyanines with Pd(II) and Pt(II) ions indicates the influence of the internal heavy atom effect on the rate of triplet state deactivation at room temperature and at 77 K.

Tuning Deazaflavins Towards Highly Potent Reducing Photocatalysts Guided by Mechanistic Understanding - Enhancement of the Key Step by the Internal Heavy Atom Effect.

Deazaflavins are well suited for reductive chemistry acting via a consecutive photo‐induced electron transfer, in which their triplet state and semiquinone – the latter is formed from the former after electron transfer from a sacrificial electron donor – are key intermediates. Guided by mechanistic investigations aiming to increase intersystem crossing by the internal heavy atom effect and optimising the concentration conditions to avoid unproductive excited singlet reactions, we synthesised 5‐aryldeazaflavins with Br or Cl substituents on different structural positions via a three‐component reaction. Bromination of the deazaisoalloxazine core leads to almost 100 % triplet yield but causes photo‐instability and enhances unproductive side reactions. Bromine on the 5‐phenyl group in ortho position does not affect the photostability, increases the triplet yield, and allows its efficient usage in the photocatalytic dehalogenation of bromo‐ and chloroarenes with electron‐donating methoxy and alkyl groups even under aerobic conditions. Reductive powers comparable to lithium are achieved.

Корреляция между эффективностью генерации синглетного кислорода и параметрами люминесценции фотосенсибилизаторов

The quantum yield of interconversion to triplet states for free bases of 2,3,7,8,12,13,17,18-octaethylporphyrin and 5,10,15,20-tetraphenylporphyrin, as well as their metal complexes with Mg(II), Zn(II), Pd(II), and Pt(II) ions have been determined on the based on the data about efficiency of singlet oxygen generation. The quantum yields of singlet oxygen generation were measured for these compounds in several solvents. The maximum values of quantum yields of singlet oxygen generation are corresponding to compounds with heavy Pd(II) and Pt(II) ions, that is due to the high impact of the internal heavy atom on the spin-orbital coupling. High quantum yields of singlet oxygen generation for free bases of porphyrins indicate a significant role of spin-orbital coupling in the case without an internal heavy atom effect for these compounds. The effect of the internal heavy atom on the rates of deactivation of the lower triplet states for a porphyrins with Pd(II) and Pt(II) ions were detected at ambient and liquid nitrogen temperatures .

Understanding the internal heavy-atom effect on thermally activated delayed fluorescence: application of Arrhenius and Marcus theories for spin–orbit coupling analysis

The mechanism of heavy-atom effect on organic blue TADF emitters is investigated using a combined Arrhenius and Marcus theories approach. Basic molecular design principles of such hybrid organic TADF materials are formulated.

Enhancement of Reverse Intersystem Crossing in Charge‐Transfer Molecule through Internal Heavy Atom Effect (Adv. Funct. Mater. 50/2021)

Internal Heavy Atom Effect In article number 2104646, Jihoon Lee, Min Chul Suh, Seunghyup Yoo, and co-workers show how incorporating the internal heavy atom (IHA) effect can impact the reverse intersystem crossing (RISC) and device performance. The spectroscopic and theoretical results indicate that a fast RISC may not be the sole factor important for reducing efficiency roll-off and that the spin-flip cycles considering both T1 → S1 and S1 → T1 should be carefully taken into account to derive a complete picture of the IHA effect on efficiency roll-off behavior.

Enhancement of Reverse Intersystem Crossing in Charge‐Transfer Molecule through Internal Heavy Atom Effect

AbstractThermally activated delayed fluorescence (TADF) is beneficial for improving the efficiency of organic light‐emitting diodes (OLEDs) by providing pathways to convert non‐emissive triplet excitons into singlet excitons. To ensure TADF is efficient, it is critical to enhance the reverse intersystem crossing (RISC) rate. To this end, most approaches propose thus far have focused on reducing the energy difference between S1 and T1 states. The present study explores how incorporating the internal heavy atom (IHA) effect can impact the RISC and device performance. By introducing a series of halogen atoms to charge‐transfer molecules, TADF molecules exhibiting RISC over 7 × 107 s−1 are realized. These molecules are then applied to OLEDs, and the effect of incorporating these moieties is investigated. The results show that efficiency roll‐off is still significant even with RISC‐enhanced TADF emitters. Spectroscopic and theoretical results indicate that a fast RISC may not be the sole factor important for reducing efficiency roll‐off and that the spin‐flip cycles considering both T1→S1 and S1→T1 should be carefully taken into account to derive a complete picture of the IHA effect on efficiency roll‐off behavior.

Heteroatom Role in the Formation of Spectral-Luminescent Properties of 21-Thia- and 21,23-Dithia-5,10,15,20-Tetraphenylporphyrin in Solutions

The spectral and luminescent properties of 21-thia- and 21,23-dithia-5,10,15,20-tetraphenylporphyrin were studied in solutions at 293 K. The origin of the spectral shifts of the absorption bands upon heterosubstitution was discussed based on the Gouterman four-orbital model. Fluorescence quenching of the heteroporphyrins was shown to be due to an internal heavy-atom effect of the thiophene-ring heteroatom.

Synthesis and photophysical properties of monomeric and dimeric halogenated aza-BODIPYs

In this study, the effects of incorporation of halogen atoms at different positions and dimerization on the photophysical properties of the aza-BODIPYs were investigated. Aza-BODIPY derivative carrying chlorine atoms on the para-position of the distal phenyl groups (4) and its core-brominated analogue (5) were prepared to establish the external and internal heavy atom effects. Dimerization of aza-BODIPY 4 through oxidative homocoupling with FeCl3 yielded the target homonuclear aza-BODIPY dimer (6). All of the aza-BODIPY derivatives prepared in this study were fully characterized by using UV–Vis, Fluorescence, NMR (1H, 13C, 11B and 19F), Mass and X-Ray spectroscopy techniques. The photophysical properties were fully characterized and compared to those of the tetra-phenyl aza-BODIPY derivative (A). Singlet oxygen generation experiments were also performed. The aza-BODIPY derivative possessing bromine atoms at the central core displayed the utmost efficiency for singlet oxygen generation.

Synthesis and spectroscopic properties of axial phenoxide and para amino phenoxide incorporated indium (III) porphyrins

Tetrakis-4-methoxyphenyl indium(III) porphyrins containing phenolic and para amino phenolic moieties in the apical sites have been successfully prepared and characterized spectroscopically. The phases and structure evolutions of prepared porphyrins were deduced from powder X-ray diffraction (PXRD). The axial coordination of phenolic moieties induces slight shift of the 1H NMR signals of indium porphyrin macrocycles. The measurements of UV–Visible absorptions of porphyrins in CH2Cl2 and CH3COOC2H5 reflects that axially ligated metal chelates at neutral pH display hyperchromicity of absorption bands compared to unligated metal porphyrin. The most striking feature was the structural stability of indium porphyrins under basic (pH = 10) as well as acidic media (pH = 4). On the other hand, the metal free ligand 5, 10, 15, 20-Tetrakis-(4-methoxyphenyl) porphyrin, H2TMP (I) undergoes J type aggregation in acidic media (pH = 4). The robustness of indium porphyrins especially in acidic media are in strong contrary to other sitting atop (SAT) or out of plane (OOP) complexes which are usually demetallized under acidic pH. All the prepared macro rings exhibit negative solvatochromism reflected by the blue shift of absorption bands in CH3COOC2H5 (more polar) than in CH2Cl2 (less polar). The photoluminescence emissions in indium(III) species are significantly quenched relative to metal free ligand (I) due to internal heavy atom effect and distortion of ligand plane. The quenching of fluorescence bands are more pronounced in axially ligated metal porphyrins. The lesser excited state lifetimes of indium(III) porphyrins relative to free base analog (I) may be ascribed to spin orbit coupling induced by indium(III) ion in the respective metal chelates.

Influence of the heavy-atom effect on singlet fission: a study of platinum-bridged pentacene dimers.

The process of singlet fission (SF) produces two triplet excited states (T1 + T1) from one singlet excited exciton (S1) and a molecule in its ground state (S0). It, thus, possesses the potential to boost the solar cell efficiency above the thermodynamic Shockley-Queisser limit of 33%. A key intermediate in the SF mechanism is the singlet correlated triplet pair state 1(T1T1). This state is of great relevance, as its formation is spin-allowed and, therefore, very fast and efficient. Three fundamentally different pathways to formation of 1(T1T1) have been documented so far. The factors that influence which mechanism is associated with which chromophore, however, remain largely unknown. In order to harvest both triplet excitons independently, a decorrelation of the correlated triplet pair state to two individual triplets is required. This second step of the SF process implies a change in the total spin quantum number. In the case of a dimer, this is usually only possible if the coupling between the two pentacenes is sufficiently weak. In this study, we present two platinum-bridged pentacene dimers in which the pentacenes are coupled strongly, so that spin-decorrelation yielding (T1 + T1) was initially expected to be outcompeted by triplet-triplet annihilation (TTA) to the ground state. Both platinum-bridged pentacene dimers undergo quantitative formation of the (T1T1) state on a picosecond timescale that is unaffected by the internal heavy-atom effect of the platinum. Instead of TTA of (T1T1) to the ground state, the internal heavy-atom effect allows for 1(T1T1)-3(T1T1) and 1(T1T1)-5(T1T1) mixing and, thus, triggers subsequent TTA to the (T1S0) state and minor formation of (T1 + T1). A combination of transient absorption and transient IR spectroscopy is applied to investigate the mechanism of the (T1T1) formation in both dimers. Using a combination of experiment and quantum chemical calculations, we are able to observe a transition from the CT-mediated to the direct SF mechanism and identify relevant factors that influence the mechanism that dominates SF in pentacene. Moreover, a combination of time-resolved optical and electron paramagnetic resonance spectroscopic data allows us to develop a kinetic model that describes the effect of enhanced spin-orbit couplings on the correlated triplet pair state.

Experimental and theoretical analysis of a rare nitrato bridged 3d-4f complex containing LaZn2 core synthesized from a Zn(II) metalloligand

A trinulcear Zn2La Schiff base complex was synthesized using slow-solvent evaporation technique from a Zn(II) mononuclear metalloligand by 2:1 addition with La(NO3)3 salt. Single crystal XRD analysis revealed a rare nitrato bridged trinuclear entity which is seldom seen in these class of ligand systems. Qualitative and quantitative analysis of intermolecular interactions/short contacts were done using Hirshfeld surface and 2D finger print analysis. The thermally stable, blue luminescent compound exhibits internal heavy atom effect thereby quenching the emission intensity of the ligand. DFT calculations were performed on the compound to analyze frontier orbitals and also ESP plots were used to monitor nucleophilic/electrophilic regions on the compound and its implications on hydrogen bonding. A comparison of the bond orders and atomic charges on the trinuclear compound and the Zn(II) metalloligand precursor was performed to substantiate the formation of the trinuclear product through ligand exchange.

(Invited) Excited State of Y-Nitride Clusterfullerene: Luminescence and EPR Spectroscopy Study

Empty fullerenes are known to have very fast intersystem crossing with near 100% yield of the T1 state, which results in very low quantum yield of fluorescence and relatively short fluorescence lifetimes (less than 1 ns). In endohedral metallofullerenes, the intersystem crossing is even faster due to the internal heavy-atom effect, and hence fluorescence can hardly be expected. However, Y3N@C80 is known to exhibit reasonably high yield of fluorescence (1%) and unusually long lifetimes of 800 ns.1, 2 Such unusual photophysical parameters indicate that simple fluorescence mechanism cannot explain experimental facts. In this contribution we describe steady-state and lifetime luminescence as well as light-induced electron paramagnetic resonance studies of Y3N@C80 at different temperatures aimed at clarification of photophysical properties of Y3N@C80. 1. Bharadwaj, L.; Novotny, L., Plasmon-Enhanced Photoemission from a Single Y3N@C80 Fullerene. J. Phys. Chem. C 2010, 114, 7444-7447. 2. Toth, K.; Molloy, J. K.; Matta, M.; Heinrich, B.; Guillon, D.; Bergamini, G.; Zerbetto, F.; Donnio, B.; Ceroni, P.; Felder-Flesch, D., A Strongly Emitting Liquid-Crystalline Derivative of Y3N@C80: Bright and Long-Lived Near-IR Luminescence from a Charge Transfer State. Angew. Chem.-Int. Edit. Engl. 2013, 52, 12303-12307.

Halogen-induced internal heavy-atom effect shortening the emissive lifetime and improving the fluorescence efficiency of thermally activated delayed fluorescence emitters

A halogenating electron acceptor is demonstrated as a feasible strategy for shortening emissive lifetimes while improving the fluorescence efficiency of TADF emitters.

The internal heavy-atom effect on 3-phenylselanyl and 3-phenyltellanyl BODIPY derivatives studied by transient absorption spectroscopy.

Three monosubstituted 3-phenylselanyl and 3-phenyltellanyl BODIPY derivatives were synthesized and their spectroscopic properties were characterized and compared to those of iodine and chlorine-atoms containing analogues as well as an unsubstituted BODIPY derivative. The fluorescence quantum yields were found to decrease, whereas the intersystem crossing quantum yields (ΦISC), determined by transient spectroscopy, increased in the order of the H → Cl → Se/I → Te substitution. The maximum ΦISC, found for the 3-phenyltellanyl derivative, was 59%. The results are interpreted in terms of the internal heavy-atom effect of the substituents.

Internal heavy atom effects in phenothiazinium dyes: enhancement of intersystem crossing via vibronic spin-orbit coupling.

The effect of substituting the intra-cyclic sulphur of thionine by oxygen (oxonine) and selenium (selenine) on the intersystem crossing (ISC) efficiency has been studied using high level quantum mechanical methods. The ISC rate constants are considerably increased when going from O towards Se while the fluorescence rate constants remain unchanged. For the three dyes, all accessible ISC channels are driven by vibronic spin-orbit coupling (SOC) between ππ* states. The interplay between the ground and low-lying excited states has been investigated in order to determine the dominant relaxation pathways. In oxonine the relaxation to the ground state after photoexcitation in water proceeds essentially via fluorescence from the S1(πHπL*) bright state (kF = 2.10 × 10(8) s(-1)), in agreement with the high experimental fluorescence quantum yield. In aqueous solution of thionine, the ISC rate constant (kISC ∼ 1 × 10(9) s(-1)) is one order of magnitude higher than fluorescence (kF = 1.66 × 10(8) s(-1)) which is consistent with its high triplet quantum yield observed in water (ϕT = 0.53). Due to a stronger vibronic SOC in selenine, the ISC rate is very high (kISC ∼ 10(10) s(-1)) and much faster than fluorescence (kF = 1.59 × 10(8) s(-1)). This suggests selenine-based dyes as very efficient triplet photosensitizers.