Nonradiative Deactivation Research Articles

A theoretical study on vibronic spectra and photo conversation process of protonated naphthalenes

The equilibrium structures and vibrational frequencies of the ground state and several singlet low-lying excited states of alpha- and beta-protonated naphthalenes (α- and β-HN+) have been studied by time-dependent density-functional theory (TD-DFT). Within the Franck-Condon approximation, vibronic absorption spectra of α-HN+ and β-HN+, together with the vibronic emission spectrum of α-HN+, have been calculated. The obtained good agreement between the theoretical and experimental spectra enables to correctly assign vibronic features in both absorption and emission spectra. Moreover, the non-radiative deactivation pathway from the low-lying excite states to the ground state in α-HN+ and β-HN+, as well as the photo-induce proton transfer pathway, are investigated at the CASPT2/CASSCF/6-31G* level. Our study is helpful for understanding the photochemical behavior of these important polycyclic aromatic hydrocarbon molecules.

Two Macrocycles in One Shot: Synthesis, Spectroscopy, Photophysics, and Tautomerism of 23-Oxahemiporphycene and 21-Oxacorrole-5-carbaldehyde.

The synthesis of 23-oxahemiporphycene, the first monooxa analogue of hemiporphycene, a structural isomer of porphyrin, is reported. Its generation under McMurry reaction conditions is surprisingly accompanied by the appearance of a formyl derivative of oxacorrole, 21-oxacorrole-5-carbaldehyde. A mechanism for the formation of the latter is proposed, relying on pinacol rearrangement of titanium pinacolate. The structures of the most stable tautomeric forms are established for both compounds based on IR and NMR spectra combined with DFT calculations. Spectral and photophysical characteristics are compared with those of structurally similar macrocycles. Replacement of one nitrogen by oxygen in hemiporphycene has only a minor impact. In contrast, for corrole it leads to the enhancement of stability and to strongly reduced rates of nonradiative deactivation of the lowest excited singlet state. This is explained by the planarity of oxacorroles, achieved by removing one of the inner hydrogen atoms from the inner cavity. Unusual crystal packing is observed for the protonated form of 23-oxahemiporphycene, which exhibits a π-π stacked columnar alignment of positively charged macrocycle units.

Gas-Phase Photoluminescence and Photodissociation of Silver-Capped Hexagold Clusters.

We report on the radiative and nonradiative deactivation pathways of selected charge states of the stoichiometric hexagold phosphine-stabilized ionic clusters, [(C)(AuDppy)6Ag2·(BF4) x](4- x)+ with x = 2 and 3 (Dppy = diphenylphosphino-2-pyridine), combining gas-phase photoluminescence and photodissociation with quantum chemical computations. These clusters possess an identical isostructural core made of a hyper-coordinated carbon at their center octahedrally surrounded by six gold ions, and two silver ions at their apexes. Their luminescence and fragmentation behavior upon photoexcitation was investigated under mass and charge control in an ion trap. The experimental and computational results shed light on the electronic states involved in the optical transitions as well as on their core, ligand, or charge transfer character. Gas-phase results are discussed in relation with condensed phase measurements, as well as previous observations in solution and on metal-organic frameworks. The monocationic species ( x = 3) is found to be less stable than the dicationic one ( x = 2). In the luminescence spectrum of the monocationic species, a shoulder at short wavelength can be observed and is assigned to fragment emission. This fragment formation appears to be favored for the monocation by the existence of a low lying singlet state energetically overlapping with the triplet state manifold, which is populated quickly after photoexcitation.

Tetra-Au(I) Complexes Bearing a Pyrene Tetraalkynyl Connector Behave as Fluorescence Torches

A pyrene tetraalkynyl ligand has been used for the preparation of three different tetraalkynyl Au(I) complexes. Two of these complexes display fluorescent emission in CH2Cl2 solution, with quantum yields exceeding 90%. Although the emission is mainly due to ligand-centered excited states, the presence of the metal center is key to reaching such excellent quantum yield values, providing an extra rigidity to the system and therefore, minimizing the nonradiative deactivation pathways. To the best of our knowledge, these quantum yields lie among the highest reported for metal-based luminophores in solution, a quality that makes them resemble molecular torches. Preliminary studies on healthy cheek cells show that one of the complexes is efficiently and rapidly taken up into the cell.

Strategies for the design of bright upconversion nanoparticles for bioanalytical applications

In recent years upconversion nanoparticles (UCNPs) received great attention because of their outstanding optical properties. Especially in bioanalytical applications this class of materials can overcome limitations of common probes like high background fluorescence or blinking. Nevertheless, the requirements for UCNPs to be applicable in biological samples, e.g. small size, water-dispersibility, excitation at low power density are in contradiction with the demand of high brightness. Therefore, a lot of attention is payed to the enhancement of the upconversion luminescence. This review discuss the recent trends and strategies to boost the brightness of UCNPs, classified in three main directions: a) improving the efficiency of energy absorption by the sensitizer via coupling to plasmonic or photonic structures or via attachment of ligands for light harvesting; b) minimizing non-radiative deactivation by variations in the architecture of UCNPs; and c) changing the excitation wavelength to get bright particles at low excitation power density for applications in aqueous systems. These strategies are critically reviewed including current limitations as well as future perspectives for the design of efficient UCNPs especially for sensing application in biological samples or cells.

Computational Studies of Aromatic and Photophysical Properties of Expanded Porphyrins.

Magnetically induced current densities and ring-current pathways have been calculated at density functional theory (DFT) and second-order Møller-Plesset perturbation theory (MP2) levels of theory for a set of expanded porphyrins consisting of five or six pyrrolic rings. The studied molecules are sapphyrin, cyclo[6]pyrrole, rubyrin, orangarin, rosarin, and amethyrin. Different functionals have been employed to assess the functional dependence of the ring-current strength susceptibility. Vertical singlet and triplet excitation energies have been calculated at the second-order approximate coupled cluster (CC2), expanded multiconfigurational quasi-degenerate perturbation theory (XMC-DPT2), and time-dependent density functional theory levels. The lowest electronic transition of the antiaromatic molecules was found to be pure magnetic transitions providing an explanation for the large paratropic contribution to the total current density. Rate constants for different nonradiative deactivation channels of the lowest excited states have been calculated yielding lifetimes and quantum yields of the lowest excited singlet and triplet states. The calculations show that the spin-orbit interaction between the lowest singlet ( S0) and triplet ( T1) states of the antiaromatic molecules is strong, whereas for the aromatic molecule the spin-orbit coupling vanishes. The experimentally detected fluorescence from S2 to S0 of amethyrin has been explained. The study shows that there are correlations between the aromatic character and optical properties of the investigated expanded porphyrins.

Non-radiative deactivation of cytosine derivatives at elevated temperature

ABSTRACTIn this work, we simulate the non-radiative deactivation process of three cytosine derivatives with known S0/S1 conical intersections (cytosine, 5-fluorocytosine and 5-methylcytosine). We use quantum chemistry methods to compute the potential energy profile of each derivatives and estimate the energy barrier height between the minimum of the S1 state and the conical intersection. Although the topology of the potential surface seems to play a role in the deactivation process, we show that the magnitude of the barrier is too high to explain the picosecond timescale reported for this reaction. Instead, rates in agreement with experiments are predicted only when incorporating dynamical factors via ab-initio molecular dynamics and a generalised master equation approach. In particular, we find that the energy fluctuations experienced by the system after photoexcitation are key to realistically model the relaxation dynamics. In gas phase, the cytosine derivatives remain vibrationally ‘hot’ for long after the excitation, raising the effective temperature of the system. We argue that it is this elevated temperature that allows for the crossing of the energy barrier. Further, we show that the reaction kinetics are not actually dominated by the conical intersection as it is enough for the system to find an avoided crossing.

Solvent dependence of the emission intensities in photoluminescent mononuclear europium(III) complexes with tetradentate Schiff base ligands

Mononuclear tetradentate Schiff base–europium(III) complexes with different counter cations, X[Eu(3,5-Clsalen)2] {X = (C2H5)3NH+, (C2H5)4N+, K+; H2(3,5-Clsalen): N,N′-bis-3,5-dichlorosalicylideneethanediamine}, were prepared and photoluminescence properties of the europium(III) complexes in organic solvents (CH3CN, DMSO, DMF, (CH3)2CO, CH3OH, N-methylformamide (NMF), CH2Cl2) were investigated. All the emission spectra of the complexes displayed similar emission spectral patterns based on the f–f transitions by excitation with 365 nm in the solvents. In contrast, the emission intensities of the complexes varied greatly in the different solvents. All the complexes X[Eu(3,5-Clsalen)2] {X = (C2H5)3NH+, (C2H5)4N+, K+} displayed strong emission intensities in polar aprotic solvents (ϕ = 0.24–0.42 in CH3CN, DMF, DMSO, and (CH3)2CO), and weak emission intensities in polar protic solvents (ϕ = 0.052–0.10 in CH3OH and NMF) at 298 K. The emission intensities of (C2H5)3NH[Eu(3,5-Clsalen)2] and (C2H5)4N[Eu(3,5-Clsalen)2] in CH2Cl2 at 298 K are significantly different (ϕ = 0.016 for (C2H5)3NH[Eu(3,5-Clsalen)2] and ϕ = 0.19 for (C2H5)4N[Eu(3,5-Clsalen)2]). Mononuclear yttrium(III) and gadolinium(III) complexes X[Y(3,5-Clsalen)2] and X[Gd(3,5-Clsalen)2] {X = (C2H5)3NH+, (C2H5)4N+} were prepared. X-ray crystal structure analysis of (C2H5)3NH[Gd(3,5-Clsalen)2] was performed. Measurements of the 1H and 13C NMR spectra of the yttrium(III) complexes were performed to elucidate the structure of the complexes in the solvents. Measurement of the luminescence spectrum of the gadolinium(III) complex in ethanol/methanol (1:1) glass at 77 K was performed to estimate the energy level of the triplet state. The solvent effect on the emission intensities of the complexes is explained by the vibrational nonradiative deactivation caused by the OH, NH, CH vibrations of the solvent molecules, or alkylammonium cations, around the mononuclear europium(III) complex anion.

Efficient persistent room temperature phosphorescence achieved through Zn 2+ doped sodium carboxymethyl cellulose composites

Abstract Efficient room temperature phosphorescence (RTP) is rarely observed from rare metal or halogen free polymer materials. However, we have newly observed that Zn2+ doped sodium carboxymethyl cellulose emitting long-lasting phosphorescence at room temperature. It is ascribed to restricted molecular motions (RMM) from strong ionic band and abundant hydrogen bond in crystalline states. Such intermolecular and intramolecular interactions greatly rigidify the molecular conformation and drastically decrease the nonradiative deactivation channels of the triplet excitons, thus phosphorescent emission is greatly boosted at room temperature. The photoluminescence quantum yield and phosphorescence life time of Zn2+ doped carboxymethyl cellulose composites are up to 16.5% and 281 ms, respectively.

Chirality and spatially pre-organized multi-porphyrinoids

We report herein that chiral and enantiopure compounds such nucleosides and peptides can pre-organize multi-porphyrinic systems and influence their properties. The first example given concerns star-shaped mutli-porphyrins with chiral and enantiopure nucleosidic linkers. If the configuration is indeed a star-shaped nanomolecule, it appears that the induced conformation is nothing as expected. The four peripheral Zn(II) porphyrins collapse over the free-base central one, inducing totally different photo-physical properties. Despite a minor expected light energy harvesting behavior, the principal capability of this system is to quench the collected light energy and convert it from radiative to non-radiative de-activation. The second example concerns polypeptides with pendant porphyrins. The peptidic backbone confers to the systems, after a certain degree of oligomerization, a 3[Formula: see text] right handed helical conformation which induces cavities within the multi-porphyrinc architecture, ready to welcome guests and render, for example, the complexation of C[Formula: see text] much easier. We thus have constructed novel organic photovoltaic systems using supramolecular complexes of porphyrin–peptide oligomers with fullerene clusters. The composite cluster OTE/SnO[Formula: see text] electrode prepared with (P(ZnP)[Formula: see text] C[Formula: see text], exhibits an impressive incident photon-to-photocurrent efficiency (IPCE) with values reaching as high as 56%. The power conversion efficiency of the (P(H[Formula: see text]P)[Formula: see text] C[Formula: see text] modified electrode reaches 1.6%, which is 40 times higher than the value (0.043%) of the porphyrin monomer (P(H[Formula: see text]P)[Formula: see text] [Formula: see text] C[Formula: see text] modified electrode. Thus, the organization approach between porphyrins and fullerenes with polypeptide structures is promising, and may make it possible to further improve the light energy conversion properties by using a larger number of porphyrins in a polypeptide unit.

Solvent-dependent investigation of carbazole benzonitrile derivatives: does the LE3−CT1 energy gap facilitate thermally activated delayed fluorescence?

The photophysical properties of six types of carbazole benzonitrile (CzBN) derivatives are investigated in different solvents to examine the thermally activated delayed fluorescence (TADF) activation via reducing the energy gap between the singlet charge-transfer and triplet locally excited states, ΔE ST(LE) . Relative to the ΔE ST(LE) values for the CzBN derivatives in the low polarity solvent toluene ( ϵ ∼ 2 ), a reduction of ΔE ST(LE) for the CzBN derivatives in the polar solvent acetonitrile ( ϵ ∼ 37 ) was confirmed while maintaining fairly constant Δ E ST values. Notably, TADF activation was observed in acetonitrile for some CzBN derivatives that are TADF inactive in toluene. A numerical analysis of various rate constants revealed the cause of TADF activation as an increase in the reverse intersystem crossing rate and a suppression of the non-radiative decay rate of the triplet states. The positive effect of ΔE ST(LE) was limited, however, as an excessive decrease in ΔE ST(LE) facilitates the nonradiative deactivation of the triplet states, leading to a loss of the TADF efficiency. This paper shows that ΔE ST(LE) provides a measure of TADF activation and that appropriate regulation of ΔE ST(LE) is required to achieve high TADF efficiency.

Design of Near-Infrared Luminescent Lanthanide Complexes Sensitive to Environmental Stimulus through Rationally Tuning the Secondary Coordination Sphere.

The design of near-infrared (NIR) emissive lanthanide (Ln) complexes sensitive to external stimulus is fundamentally important for the practical application of Ln materials. Because NIR emission from Ln is extremely sensitive to X-H (X = C, N and O) bond vibration, we herein report to harness the secondary coordination sphere to design NIR luminescent lanthanide sensors. Toward this goal, we designed and synthesized two isomeric [(η5-C5H5)Co{(D3CO)2P = O}3]-Yb(III)-7,8,12,13,17,18-hexafluoro-5,10,15,20-tetrakis(pentafluorophenyl)porpholactol NIR emitters, Yb-up and Yb-down, based on the stereoisomerism of porphyrin peripheral β-hydroxyl group. Yb-up, in which β-OH is at the same side of Yb(III) center, can form an intramolecular hydrogen bond with the axial Kläui ligand, whereas Yb-down cannot because its β-OH is opposite to Yb(III) center. X-ray crystal structures and photophysical studies suggested that the intramolecular hydrogen bond plays important roles on the NIR luminescence of ytterbium(III), which shortens the distance between β-OH and Yb(III) and facilitates the nonradiative deactivation of Ln excited state. Importantly, Yb-up/down were demonstrated to be highly sensitive toward temperature and viscosity. The PMMA polymer using Yb-up as the dopant NIR emitter showed thermosensitivity up to 6.0% °C-1 in the wide temperature range of 77-400 K, higher than that of Yb-down (3.8% °C-1). These complexes were also explored as the first NIR viscosity sensor, revealing their potential applications as optical sensors without visible light interference. This work demonstrates the importance of secondary coordination sphere on designing NIR Ln luminescent functional materials.

Enhanced phosphorescence properties of a Pt-porphyrin derivative fixed on the surface of nano-porous glass

The room-temperature phosphorescence chromophore, Pt(II) coproporphyrin I (PtCP), was fixed on the surface of a 3D-network of nanoscale pores of porous glass through ion-exchange reaction. The absorption and phosphorescence spectra indicated that PtCP can be loaded while maintaining monomeric dispersion at concentrations well beyond solubility limits of PtCP in solution. The phosphorescence quantum yield of PtCP fixed on the surface was also found to have double the enhancement of solution. The extended lifetime of phosphorescence of PtCP bonded on the surface compared to that in solution clearly indicated that suppression of nonradiative deactivation plays a key role in high quantum yield and long triplet lifetime. This hybridization with nano-porous glass provides opportunities for various potential applications.

Solvation and hydrogen bonding aided efficient non-radiative deactivation of polar excited state of 5-aminoquinoline.

The fluorescence of 5-aminoquinoline (5AQ) is quenched strongly in alcoholic solvents, in stark contrast to the enhancement in fluorescence observed earlier for its positional isomer, 3-aminoquinoline (3AQ). This phenomenon has been explained by the involvement of a highly dipolar excited state, which is manifested in the solvatochromism of 5AQ. A marked wavelength dependence of fluorescence decays of 5AQ in alcoholic solvents is ascribed to the ultrafast solvation of the highly dipolar excited state in these solvents. The resultant stabilization of this state leads to a decrease in the gap between its energy and lower lying triplet as well as ground singlet states, resulting in an efficient non-radiative relaxation and consequently, fluorescence quenching. Solvation dynamics reported for 5AQ is somewhat slower than earlier reports with coumarin probes, due to the involvement of solute-solvent hydrogen bonds, especially in the excited state of the solute. At lower temperatures, solvation is slowed down. An extreme case of this phenomenon is the absence of solvent relaxation at liquid nitrogen temperature. The fluorescence lifetime increases from tens of picoseconds at room temperature to tens of nanoseconds at cryogenic temperature, lending credence to the proposed model.

Theoretical Study on the Pathway of Nonradiative Deactivation in Indigo and Its Isomers

Theoretical Study on the Pathway of Nonradiative Deactivation in Indigo and Its Isomers

Comparative photophysical investigation of doubly-emissive photochromic-fluorescent diarylethenes.

Diarylethene molecules showing photochromism and fluorescence properties in both open and closed forms, associated with two different emission colors, are very promising for applications involving ratiometric emissive photoswitches. We report here a complete study on the competition between the multiple photophysical processes involved in the excited states for two sulfone derivatives of benzothiophene-based diarylethene molecules, only differing by the substituent groups on their reactive carbon (methyl for DAE-Me and ethyl for DAE-Et). Steady-state and time-resolved spectroscopy, combined with DFT and TD-DFT calculations, allow a complete determination of the kinetic constants leading to fluorescence and photoreaction pathways in different solvents, and enlighten the specific role of the substituent group in the photophysical properties due to a shielding effect against the solvation environment. The predominant role of the non-radiative deactivation processes in such a family of molecules is shown, and a tentative excited state mechanistic scheme is proposed based on femtosecond transient absorption experiments performed on the closed forms.

Ultralong-lived room temperature triplet excitons: molecular persistent room temperature phosphorescence and nonlinear optical characteristics with continuous irradiation

Heavy atom-free molecular materials with ultralong-lived room-temperature triplet excitons display nonlinear absorption using ordinary light and persistent room-temperature emission characteristics.

Confinement of Long‐Lived Triplet Excitons in Organic Semiconducting Host–Guest Systems

AbstractLong‐lived triplet excitons on organic molecules easily deactivate at room temperature because of the presence of thermally activated nonradiative pathways. This study demonstrates long‐lived phosphorescence at room temperature resulting from suppression of the nonradiative deactivation of triplet excitons in conventional organic semiconducting host–guest systems. The nonradiative deactivation pathway strongly depends on the triplet energy gap between the guest emitting molecules and the host matrices. The triplet energy gap required to confine the long‐lived triplet excitons (≈0.5 eV) is much larger than that of conventional host–guest systems for phosphorescent emitters. By effectively confining the triplet excitons, this study demonstrates long‐lived room‐temperature phosphorescence under optical and electrical excitation.

Fluorescent Phase-Pure Zero-Dimensional Perovskite-Related Cs4PbBr6 Microdisks: Synthesis and Single-Particle Imaging Study.

Quantum-confined perovskites are a new class of promising materials in optoelectronic applications. In this context, a zero-dimensional perovskite-related substance, Cs4PbBr6, having high exciton binding energy can be an important candidate, but its photoluminescence (PL) is a topic of recent debate. Herein, we report an ambient condition controlled synthesis of Cs4PbBr6 microdisks of different shapes and dimensions which exhibit fairly strong green PL (quantum yield up to 38%, band gap ∼2.43 eV) in the solid state. Using confocal fluorescence microscopy imaging of the single particles, we show that the fluorescence of Cs4PbBr6 microdisks is inherent to these particles. Fluorescence intensity and lifetime imaging of the microdisks reveals significant spatial heterogeneity with a bright central area and somewhat dimmer edges. This intensity and lifetime distribution is attributed to enhanced trap-mediated nonradiative deactivation at the edges compared to the central region of the microdisks. Our results, which unambiguously establish the PL of these Cs4PbBr6 and suggest its possible origin, brighten the potential of these materials in photon-emitting applications.

7-Dialkylaminocoumarin Oximates: Small Molecule Fluorescent "Turn-On" Chemosensors for Low-Level Water Content in Aprotic Organic Solvents.

The water sensing properties of two efficient two-component fluorescent “turn-on” chemo-sensors based on the 7-dialkylaminocoumarin oxime acid-base equilibrium were investigated. Interestingly, although simple frontier orbital analysis predicts an intramolecular photoinduced electron transfer quenching pathway in conjugated oximates, TD-DFT (Time-dependent density functional theory) quantum chemical calculations support non-radiative dark S1 excited state deactivation as a fluorescence quenching mechanism. Due to the acid-base sensing mechanism and sensitive “turn-on” fluorescent response, both studied coumarin aldoxime chemosensors exhibit rapid response to low-level water content in polar aprotic solvents, with detection limits comparable to chemodosimeters or chemosensors based on interpolymer π-stacking aggregation.