New Ru(II) chromophores with extended excited-state lifetimes

We describe the synthesis, electrochemical, and photophysical properties of two new luminescent Ru(II) diimine complexes covalently attached to one and three 4-piperidinyl-1,8-naphthalimide (PNI) chromophores, [Ru(bpy)(2)(PNI-phen)](PF(6))(2) and [Ru(PNI-phen)(3)](PF(6))(2), respectively. These compounds represent a new class of visible light-harvesting Ru(II) chromophores that exhibit greatly enhanced room-temperature metal-to-ligand charge transfer (MLCT) emission lifetimes as a result of intervening intraligand triplet states ((3)IL) present on the pendant naphthalimide chromophore(s). In both Ru(II) complexes, the intense singlet fluorescence of the pendant PNI chromophore(s) is nearly quantitatively quenched and was found to sensitize the MLCT-based photoluminescence. Excitation into either the (1)IL or (1)MLCT absorption bands results in the formation of both (3)MLCT and (3)IL excited states, conveniently monitored by transient absorption and fluorescence spectroscopy. The relative energy ordering of these triplet states was determined using time-resolved emission spectra at 77 K in an EtOH/MeOH glass where dual emission from both Ru(II) complexes was observed. Here, the shorter-lived higher energy emission has a spectral profile consistent with that typically observed from (3)MLCT excited states, whereas the millisecond lifetime lower energy band was attributed to (3)IL phosphorescence of the PNI chromophore. At room temperature the data are consistent with an excited-state equilibrium between the higher energy (3)MLCT states and the lower energy (3)PNI states. Both complexes display MLCT-based emission with room-temperature lifetimes that range from 16 to 115 micros depending upon solvent and the number of PNI chromophores present. At 77 K it is apparent that the two triplet states are no longer in thermal equilibrium and independently decay to the ground state.     

Triplet Excited States and Singlet Oxygen Production by Analogs of Red Wine Pyranoanthocyanins

During the maturation of red wines, the anthocyanins of grapes are transformed into pyranoanthocyanins, which possess a pyranoflavylium cation as their basic chromophore. Photophysical properties of the singlet and triplet excited states of a series of synthetic pyranoflavylium cations were determined at room temperature in acetonitrile solution acidified with 0.10 mol dm^?3 trifluoroacetic acid (TFA, to inhibit competitive excited state proton transfer) and at 77 K in a rigid TFA‐acidified isopropanol glass. In solution, the triplet states of these pyranoflavylium cations are efficiently quenched by molecular oxygen, resulting in sensitized formation of singlet oxygen, as confirmed by direct detection of the triplet‐state decay by laser flash photolysis and of singlet oxygen monomol emission in the near infrared. The strong visible light absorption, the relatively small singlet‐triplet energy differences, the excited state redox potentials and the reasonably long lifetimes of pyranoflavylium triplet states in the absence of molecular oxygen suggest that they might be useful as triplet sensitizers and/or as cationic redox initiators in polar aprotic solvents like acetonitrile.     

Electronic spectra and luminescence properties of 1,2-benzotetraphene in crystalline matrices at 77 K

Quasilinear absorption and luminescence spectra of 1,2-benzotetraphene were obtained in polycrystalline matrices at 77 K. Tne energies of successive excited singlet states as well as the energy of the lowest excited triplet state were found experimentally and compared with those calculated by the PPP CI method. The fluorescence lifetime and quantum yield were determined experimentally. Moreover, the radiationless transition probabilities, lifetime of triplet state and phosphorescence quantum yield were estimated employing the Siebrand-Williams model. The results obtained suggest that radiationless ISC processes are the main deactivation channel of the S₁ and T₁ states. The vibrational analysis of quasilinear absorption and luminescence spectra was performed and fundamental frequencies of ground and first excited singlet states were determined.     

ACTIVATED OXYGEN: SINGLET MOLECULAR OXYGEN AND SUPEROXIDE ANION

Abstract— Elusive processes associated with molecular oxygen in chemical and biological systems are interpreted in terms of two activated oxygen species, singlet molecular oxygen (¹Σ⁺_g/¹Δ_g) and superoxide anion (X²π_g). The generation and deactivation of singlet oxygen by interaction with organic triplet states are discussed within a comprehensive theoretical framework. Experimental results indicate the anomalous molecular oxygen enhanced luminescence from organic chromophores in polymer matrices results from the deactivation of singlet (¹Δ_g) oxygen by energy transfer to electronically excited states of the chromophore, and three types of oxygen enhanced luminescence have been identified in these systems. Properties of the superoxide anion relevant to its solution chemistry are briefly discussed. Electron transfer theory is used to theoretically examine the generation of singlet oxygen in disproportionation reactions of the superoxide anion, predicting that, depending on the number of water molecules present, the disproportionation reaction is a proficient source of singlet oxygen. A competing quenching process imposes a limit to the steady state concentration of singlet oxygen in most chemical systems. Available experimental results on the quenching of singlet oxygen by superoxide anion are in good agreement with theoretical results obtained via application of electron transfer theory.     

DELAYED EMISSION OF CHLOROPHYLL a AGGREGATES AND RHODAMINE 6G EMBEDDED IN POLYMER MATRIX*

Delayed luminescence (in the microsecond time range) of the chlorophyll (Chl) a“dry” form as well as hydrated dimers located in a polyvinylalcohol film was measured from room temperature down to 8 K. In the same matrix the delayed luminescence of rhodamine 6G (Rhod) was investigated. The delayed emission both of Chl a and Rhod is probably due to the formation and delayed recombination of a radical pair. It seems that this process occurs without participation of triplet states, as it does not reflect their well-known sensitivity to oxygen. The temperature dependence of the delayed luminescence of vanous Chl forms is different. In the region around 678 nm (dry monomer) delayed luminescence needs a thermal activation energy of about 0.03 eV, whereas at 740 nm (wet aggregates) delayed luminescence intensity increases linearly with decreasing temperature. Its assignment as a-type delayed luminescence from the low-lying triplet state can consistently be excluded from both the weak temperature dependence of the delayed fluorescence and its large intensity as compared to the prompt fluorescence. Delayed luminescence of Rhod is almost independent of temperature between 8 K and 300 K. The dependence of delayed luminescence intensity on exciting light intensity is linear at lower intensities and tends to saturation at higher. Therefore the delayed luminescence is not related to exciton annihilation. Positions and intensities of the Chl delayed luminescence bands show that it is not phosphorescence (β-type delayed luminescence). The aggregation of both Chl and Rhod molecules strongly influences delayed luminescence since it differs in several properties if excited in the monomer or in the aggregate absorption range. Every aggregational form of dye emits its characteristic delayed luminescence band.     

ROOM TEMPERATURE TIME-RESOLVED IN μs RANGE DELAYED LUMINESCENCE OF CHLOROPHYLL a AND CHLOROPHYLL-LUTEIN MIXTURE SOLUTIONS

Using time-resolved in μS range luminescence spectroscopy, we observed at 20°C the emission of chlorophyll a, pheophytin a and chlorophyll a-lutein mixture solutions. This delayed emission exhibits several maxima in the650–750 nm region. The positions and kinetics of decay of delayed emission bands depend on chlorophyll concentration, and vary as a result of pheophytinization and addition of lutein. Our results can be explained by supposition that upon excitation, charge transfer species are formed in various pigment complexes. The back electron transfer reactions yield chlorophyll excited singlet states contributing to observed delayed emission. Delay in emission seems to be due also to the trapping of excitation on the triplet states of various forms of pigment and its detrapping with the participation of thermal energy followed by energy transfer to the forms of pigment characterized by different decay times.     

Electron vs energy transfer in arrays featuring two Bodipy chromophores axially bound to a Sn(IV) porphyrin via a phenolate or benzoate bridge

In this report we describe the synthesis of multichromophore arrays consisting of two Bodipy units axially bound to a Sn(IV) porphyrin center either via a phenolate (3) or via a carboxylate (6) functionality. Absorption spectra and electrochemical studies show that the Bodipy and porphyrin chromophores interact weakly in the ground state. However, steady-state emission and excitation spectra at room temperature reveal that fluorescence from both the Bodipy and the porphyrin of 3 are strongly quenched suggesting that, in the excited state, energy and/or electron transfer might occur. Indeed, as transient absorption experiments show, selective excitation of Bodipy in 3 results in a rapid decay (τ ≈ 2 ps) of the Bodipy-based singlet excited state and a concomitant rise of a charge-separated state evolving from the porphyrin-based singlet excited state. In contrast, room-temperature emission studies on 6 show strong quenching of the Bodipy-based fluorescence leading to sensitized emission from the porphyrin moiety due to a transduction of the singlet excited state energy from Bodipy to the porphyrin. Emission experiments at 77 K in frozen toluene reveal that the room-temperature electron transfer pathway observed in 3 is suppressed. Instead, Bodipy excitation in 3 and 6 results in population of the first singlet excited state of the porphyrin chromophore. Subsequently, intersystem crossing leads to the porphyrin-based triplet excited state.     

Triplet photophysics of gold(III) porphyrins

Gold porphyrins are often used as electron-accepting chromophores in donor-acceptor complexes for the study of photoinduced electron transfer, and they can also be involved in triplet-triplet energy-transfer interactions with other chromophores. Since the lowest excited singlet state is very short-lived (240 fs), the triplet state is usually the starting point for the transfer reactions, and it is therefore crucial to understand its photophysics. The triplet state of various gold porphyrins has been reported to have a lifetime of around 1.5 ns at room temperature and to have a biexponential decay both in emission and in transient absorption with decay times of around 10 and 100 micros at 80 K. In this paper, the triplet photophysics of two gold porphyrins (Au(III) 5,15-bis(3,5-di-tert-butylphenyl)-2,8,12,18-tetraethyl-3,7,13,17-tetramethylporphyrin and Au(III) 5,10,15,20-tetra(3,5-di-tert-butylphenyl)porphyrin) are studied by steady-state and time-resolved absorption and emission spectroscopy over a wide temperature range (4-300 K). The study reveals the existence of a dark state with an approximate lifetime of 50 ns, which was not previously observed. This state acts as an intermediate between the short-lived singlet and the triplet state manifold. In addition, we present DFT calculations, in which the core electrons of the central metal were replaced by a pseudopotential to account for the relativistic effects, which suggest that the lowest excited singlet state is an optically forbidden ligand-to-metal charge-transfer (LMCT) state. This LMCT state is an obvious candidate for the experimentally observed dark state, and it is shown to dictate the photophysical properties of gold porphyrins by acting as a gate for triplet state formation versus direct return to the ground state.     

Phosphorescence Energy Transfer: Ambient Afterglow Fluorescence from Water‐Processable and Purely Organic Dyes via Delayed Sensitization

Ambient afterglow luminescence from metal‐free organic chromophores would provide a promising alternative to the well‐explored inorganic phosphors. However, the realization of air‐stable and solution‐processable organic afterglow systems with long‐lived triplet or singlet states remains a formidable challenge. In the present study, a delayed sensitization of the singlet state of organic dyes via phosphorescence energy transfer from organic phosphors is proposed as an alternative strategy to realize “afterglow fluorescence”. This concept is demonstrated with a long‐lived phosphor as the energy donor and commercially available fluorescent dyes as the energy acceptor. Triplet‐to‐singlet Förster‐resonance energy‐transfer (TS‐FRET) between donor and acceptor chromophores, which are co‐organized in an amorphous polymer matrix, results in tuneable yellow and red afterglow from the fluorescent acceptors. Moreover, these afterglow fluorescent hybrids are highly solution‐processable and show excellent air‐stability with good quantum yields.     

Phosphorescence Energy Transfer: Ambient Afterglow Fluorescence from Water-Processable and Purely Organic Dyes via Delayed Sensitization

Ambient afterglow luminescence from metal-free organic chromophores would provide a promising alternative to the well-explored inorganic phosphors. However, the realization of air-stable and solution-processable organic afterglow systems with long-lived triplet or singlet states remains a formidable challenge. In the present study, a delayed sensitization of the singlet state of organic dyes via phosphorescence energy transfer from organic phosphors is proposed as an alternative strategy to realize “afterglow fluorescence”. This concept is demonstrated with a long-lived phosphor as the energy donor and commercially available fluorescent dyes as the energy acceptor. Triplet-to-singlet Förster-resonance energy-transfer (TS-FRET) between donor and acceptor chromophores, which are co-organized in an amorphous polymer matrix, results in tuneable yellow and red afterglow from the fluorescent acceptors. Moreover, these afterglow fluorescent hybrids are highly solution-processable and show excellent air-stability with good quantum yields.     

Density functional theory study of 1,2‐dioxetanone decomposition in condensed phase

The decomposition of 1,2‐dioxetanone into a CO₂ molecule and into an excited state formaldehyde molecule was studied in condensed phase, using a density functional theory approach. Singlet and triplet ground and excited states were all included in the calculations. The calculations revealed a novel mechanism for the chemiluminescence of this compound. The triplet excitation can be explained by two intersystem crossings (ISCs) with the ground state, while the singlet excitation can be accounted by an ISC with the triplet state. The experimentally verified small excitation yield can then be explained by the presence of an energy barrier present in the potential energy surface of the triplet excited state, which will govern both triplet and singlet excitation. It was also found that the triplet ground state interacts with both the triplet excited and singlet ground states. A MPWB1K/mPWKCIS approach provided results in agreement with the existent literature. © 2012 Wiley Periodicals, Inc.     

Direct observation of the fourth MLCT triplet state in ruthenium(II) tris(2,2'-bipyridine)

The luminescence properties of ruthenium(ii) tris(2,2'-bipyridine) have been recorded in butyronitrile solution and in a transparent KBr disk over a reasonable temperature range. In solution, spectral curve fitting routines indicate that emission arises solely from an ensemble of triplet states, each of which is of Metal-to-Ligand, Charge-Transfer (MLCT) character and of closely comparable energy. At ambient temperature, dual emission is observed for the KBr disk and interpreted in terms of luminescence from both the ensemble and the fourth MLCT triplet state that lies at slightly higher energy. Relative reorganisation energies, energies, Huang-Rhys factors and radiative rate constants have been calculated for the two emissive states. It is confirmed that the fourth MLCT triplet state possesses more singlet character than the ensemble.     

Influence of energy transfer on the photoionization of tryptophan and tyrosine in basic media

Abstract— The luminescence studies of the delayed isothermal and light stimulated recombinations of photoejected electrons with photoionized chromophores at 77°K show that in the peptide tryptophyltyrosine, where energy transfer occurs at the singlet level from tryptophan to tyrosine and at the triplet level from tyrosine to tryptophan, tryptophan photoionization is greatly enhanced. This increase in photoionization efficiency is attributed to triplet-triplet transfer.     

Boron photochemistry : XI. A study of the unusual luminescence and spectral properties of anilinodimesitylboranes

Anilinodimesitylboranes fluoresce showing the largest Stokes shifts yet reported, which correspond to energy losses of 35.3–61.8 kcal/mole. The dependence of wavelengths of fluorescence on the polarity of the solvent indicates that the first excited singlet state is highly dipolar in nature. A scheme involving an excited-state dipolar

species is used to explain the large loss of energy corresponding to the Stokes shifts for the anilinodimesitylboranes. Delayed emission found at 77 K corresponding to fluorescence is attributed to electron ejection—recombination-type luminescence. No emission was found at longer wavelengths corresponding to triplet emission. It is proposed that the photochemical rearrangements of anilinodimesitylboranes in the presence of iodine occur by interception of an excited singlet or excited charge-transfer state.     

Promising fast energy transfer system via an easy synthesis: Bodipy-porphyrin dyads connected via a cyanuric chloride bridge, their synthesis, and electrochemical and photophysical investigations

The boron dipyrrin (Bodipy) chromophore was combined with either a free-base or a Zn porphyrin moiety (H(2)P and ZnP respectively), via an easy synthesis involving a cyanuric chloride bridging unit, yielding dyads Bodipy-H(2)P (4) and Bodipy-ZnP (5). The photophysical properties of Bodipy-H(2)P (4) and Bodipy-ZnP (5) were investigated by UV-Vis absorption and emission spectroscopy, cyclic voltammetry, and femtosecond transient absorption spectroscopy. The comparison of the absorption spectra and cyclic voltammograms of dyads Bodipy-H(2)P (4) and Bodipy-ZnP (5) with those of their model compounds Bodipy, H(2)P, and ZnP shows that the spectroscopic and electrochemical properties of the constituent chromophores are essentially retained in the dyads indicating negligible interaction between them in the ground state. In addition, luminescence and transient absorption experiments show that excitation of the Bodipy unit in Bodipy-H(2)P (4) and Bodipy-ZnP (5) into its first singlet excited state results in rapid Bodipy to porphyrin energy transfer-k(4) = 2.9 × 10(10) s(-1) and k(5) = 2.2 × 10(10) s(-1) for Bodipy-H(2)P (4) and Bodipy-ZnP (5), respectively-generating the first porphyrin-based singlet excited state. The porphyrin-based singlet excited states give rise to fluorescence or undergo intersystem crossing to the corresponding triplet excited states. The title complexes could also be used as precursors for further substitution on the third chlorine atom on the cyanuric acid moiety.     

Excited state character of Cibalackrot-type compounds interpreted in terms of Hückel-aromaticity: a rationale for singlet fission chromophore design

The exact energies of the lowest singlet and triplet excited states in organic chromophores are crucial to their performance in optoelectronic devices. The possibility of utilizing singlet fission to enhance the performance of photovoltaic devices has resulted in a wide demand for tuneable, stable organic chromophores with wide S₁–T₁ energy gaps (>1 eV). Cibalackrot-type compounds were recently considered to have favorably positioned excited state energies for singlet fission, and they were found to have a degree of aromaticity in the lowest triplet excited state (T₁). This work reports on a revised and deepened theoretical analysis taking into account the excited state Hückel-aromatic (instead of Baird-aromatic) as well as diradical characters, with the aim to design new organic chromophores based on this scaffold in a rational way starting from qualitative theory. We demonstrate that the substituent strategy can effectively adjust the spin distribution on the chromophore and thereby manipulate the excited state energy levels. Additionally, the improved understanding of the aromatic characters enables us to demonstrate a feasible design strategy to vary the excited state energy levels by tuning the number and nature of Hückel-aromatic units in the excited state. Finally, our study elucidates the complications and pitfalls of the excited state aromaticity and antiaromaticity concepts, highlighting that quantitative results from quantum chemical calculations of various aromaticity indices must be linked with qualitative theoretical analysis of the character of the excited states.

Cibalackrot-type compounds are Hückel instead of Baird aromatic in their first triplet states (T₁). By choice of substituents and additional benzannelation we adjust the T₁ energies, providing a new strategy for singlet fission chromophore design.     

The gold porphyrin first excited singlet state

Gold porphyrins are often used as electron-accepting chromophores in artificial photosynthetic constructs. Because of the heavy atom effect, the gold porphyrin first-excited singlet state undergoes rapid intersystem crossing to form the triplet state. The lowest triplet state can undergo a reduction by electron donation from a nearby porphyrin or another moiety. In addition, it can be involved in triplet-triplet energy transfer interactions with other chromophores. In contrast, little has been known about the short-lived singlet excited state. In this work, ultrafast time-resolved absorption spectroscopy has been used to investigate the singlet excited state of Au(III) 5,15-bis(3,5-di-t-butylphenyl)-2,8,12,18,-tetraethyl-3,7,13,17-tetramethylporphyrin in ethanol solution. The excited singlet state is found to form with the laser pulse and decay with a time constant of 240 fs to give the triplet state. The triplet returns to the ground state with a life-time of 400 ps. The lifetime of the singlet state is comparable with the time constants for energy and photoinduced electron transfer in some model and natural photosynthetic systems. Thus, it is kinetically competent to take part in such processes in suitably designed supermolecular systems.     

Vibronic effects accelerate the intersystem crossing processes of the through-space charge transfer states in the triptycene bridged acridine–triazine donor–acceptor molecule TpAT-tFFO

Quantum chemical studies employing combined density functional and multireference configuration interaction methods suggest five excited electronic states to be involved in the prompt and delayed fluorescence emission of TpAT-tFFO. Three of them, a pair of singlet and triplet charge transfer (CT) states (S₁ and T₁) and a locally excited (LE) triplet state (T₃), can be associated with the (Me → N) conformer, the other two CT-type states (S₂ and T₂) form the lowest excited singlet and triplet states of the (Me → Ph) conformer. The two conformers, which differ in essence by the shearing angle of the face-to-face aligned donor and acceptor moieties, are easily interconverted in the electronic ground state whereas the reorganization energy is substantial in the excited singlet state, thus explaining the two experimentally observed time constants of prompt fluorescence emission. Forward and reverse intersystem crossing between the singlet and triplet CT states is mediated by vibronic spin–orbit interactions involving the LE T₃ state. Low-frequency vibrational modes altering the distance and alignment of the donor and acceptor π-systems tune the S₁ and T₃ states (likewise S₂ and T₃) into and out of resonance. The enhancement of intersystem crossing due to the interplay of vibronic and spin–orbit coupling is considered a general feature of organic through-space charge-transfer thermally activated delayed fluorescence emitters.

DFT/MRCI quantum chemical studies suggest five excited electronic states to be involved in the prompt and delayed fluorescence emission of TpAT-tFFO.     

Detection of the photochemical intermediate processes in phycobiliproteins

The delayed luminescence was applied to detect the intermediate processes of the excitedstate decay in the selectively excited phycobiliproteins. Phosphorescence spectra of the five types of phycobiliproteins, R-PE, CPC, APC, R-PC, PEC were reported in this article. The five phycobiliproteins showed different phosphorescence yields, the sequence of which was the same as that of the singlet oxygen yields. Based on the observation, it can be concluded that each of the chromophores possesses a characteristic phosphorescence emission. The delayed luminescence spectra of APC at different aggregation states (trimer, monomer and denatured APC) are researched. The lower aggregation APC showed less phosphorescence because of relative loose structures and less interaction with the surrounding proteins, while the denatured APC showed delayed fluorescence instead of phosphorescence because of triplet-triplet annihilation.     

Blue-light emission of Cu(I) complexes and singlet harvesting

Strongly luminescent neutral copper(I) complexes of the type Cu(pop)(NN), with pop = bis(2-(diphenylphosphanyl)phenyl)ether and NN = bis(pyrazol-1-yl)borohydrate (pz(2)BH(2)), tetrakis(pyrazol-1-yl)borate (pz(4)B), or bis(pyrazol-1-yl)-biphenyl-borate (pz(2)Bph(2)), are readily accessible in reactions of Cu(acetonitrile)(4)(+) with equimolar amounts of the pop and NN ligands at ambient temperature. All products were characterized by means of single crystal X-ray diffractometry. The compounds exhibit very strong blue/white luminescence with emission quantum yields of up to 90%. Investigations of spectroscopic properties and the emission decay behavior in the temperature range between 1.6 K and ambient temperature allow us to assign the emitting electronic states. Below 100 K, the emission decay times are in the order of many hundreds of microseconds. Therefore, it is concluded that the emission stems from the lowest triplet state. This state is assigned to a metal-to-ligand charge-transfer state (3MLCT) involving Cu-3dand pop-π* orbitals. With temperature increase, the emission decay time is drastically reduced, e.g. to 13 μs [corrected] (Cu(pop)-(pz(2)Bph(2))), at ambient temperature. At this temperature, the complexes exhibit high emission quantum yields, as neat material or doped into poly(methyl methacrylate) (PMMA). This behavior is assigned to an efficient thermal population of a singlet state (being classified as (1)MLCT), which lies only 800 to 1300 cm(-1) above the triplet state, depending on the individual complex. Thus, the resulting emission at ambient temperature largely represents a fluorescence. For applications in OLEDs and LEECs, for example, this type of thermally activated delayed fluorescence (TADF) creates a new mechanism that allows to harvest both singlet and triplet excitons (excitations) in the lowest singlet state. This effect of singlet harvesting leads to drastically higher radiative rates than obtainable for emissions from triplet states of Cu(I) complexes.