Triplet-state and singlet oxygen formation in fluorene-based alternating copolymers

Data are reported on the triplet states of a series of fluorene-based A-alt-B type alternating copolymers based on pulse radiolysis-energy transfer and flash photolysis experiments. From the pulse radiolysis experiments, spectra are given for eight copolymers involving phenylene, thiophene, benzothiadiazole, and oligothienylenevinylene groups. Quantum yields for triplet-state formation (PhiT) have been obtained by flash photolysis following laser excitation and in one case by photoacoustic calorimetry. In addition, yields of sensitized formation of singlet oxygen have been determined by time-resolved phosphorescence and are, in general, in excellent agreement with the PhiT values. In all cases, the presence of thiophene units is seen to increase intersystem-crossing quantum yields, probably because of the presence of the heavy sulfur atom. However, with the poly[2,7-(9,9-bis(2'-ethylhexyl)fluorene)-alt-1,4-phenylene] (PFP), thiophene S,S-dioxide (PFTSO2) and benzothiadiazole (F8BT) copolymers, low yields of triplet formation are observed. With three of the copolymers, the energies of the triplet states have been determined. With PFP, the triplet energy is virtually identical to that of poly[2,7-(9,9-bis(2'-ethylhexyl)fluorene)]. In contrast, with fluorene-thiophene copolymers PFaT and PF3T, the triplet energies are closer to those of thiophene oligomers, indicating that there is significant conjugation between fluorene and thiophene units but also that there is a more localized triplet state than with the homopolymers.     

Photophysical properties of closely-coupled, binuclear ruthenium(II) bis(2,2':6',2''-terpyridine) complexes

The photophysical properties of closely-coupled, binuclear complexes formed by connecting two ruthenium(II) bis(2,2':6',2'-terpyridine) complexes via an alkynylene group are compared to those of the parent complex. The dimers exhibit red-shifted emission maxima and prolonged triplet lifetimes in deoxygenated solution. Triplet quantum yields are much less than unity and the dimers generate singlet molecular oxygen with low quantum efficiency. Temperature dependence emission studies indicate coupling to higher-energy triplet states while cyclic voltammetry shows that the metal centres are only very weakly coupled but that extensive electron delocalization occurs upon one-electron reduction. The radiative rate constants derived for these dimers are relatively low, because the lowest-energy metal-to-ligand, charge-transfer states possess increased triplet character. In contrast, the rate constants for nonradiative decay of the lowest-energy triplet states are kept low by extended electron delocalization over the polytopic ligand. The poor triplet yields are a consequence of partitioning at the second triplet level.     

Oligothiophene catenanes and knots. Part II. Mono and dications. A theoretical study

Mono- and dications of catenanes and knots containing 16, 22, and 28 thiophene units have been studied at the BHandHLYP/3-21G* level of theory. The polaron localization and relaxation energies of monoionized molecules increase with dihedral angle between thiophene fragments, being higher for catenanes and knots compared to linear oligomers. Monoionization of catenanes results in the polaron localization at one macrocycle leaving another one intact. In all diionized oligomers, polaron pairs were found to be more stable than corresponding bipolarons. The energy difference between bipolaron and polaron pairs increases with the number of repeating units in oligomers for all studied molecular architectures. Singlet polaron pairs are more stable than triplet ones. The energy difference between triplet and singlet states does not exceed 7-8 kcal/mol and decreases with the number of thiophene units in oligomers. Two different singlet minima were found for diionized catenanes. In the first one (the most stable), each macrocycle loses one electron, and in the other one, the polaron pairs are located at one macrocycle, leaving another intact. The energy difference between two minima decreases with the number of repeating units in catenanes.     

Resolving electron injection from singlet fission-borne triplets into mesoporous transparent conducting oxides

Photoinduced electron transfer into mesoporous oxide substrates is well-known to occur efficiently for both singlet and triplet excited states in conventional metal-to-ligand charge transfer (MLCT) dyes. However, in all-organic dyes that have the potential for producing two triplet states from one absorbed photon, called singlet fission dyes, the dynamics of electron injection from singlet vs. triplet excited states has not been elucidated. Using applied bias transient absorption spectroscopy with an anthradithiophene-based chromophore (ADT-COOH) adsorbed to mesoporous indium tin oxide (nanoITO), we modulate the driving force and observe changes in electron injection dynamics. ADT-COOH is known to undergo fast triplet pair formation in solid-state films. We find that the electronic coupling at the interface is roughly one order of magnitude weaker for triplet vs. singlet electron injection, which is potentially related to the highly localized nature of triplets without significant charge-transfer character. Through the use of applied bias on nanoITO:ADT-COOH films, we map the electron injection rate constant dependence on driving force, finding negligible injection from triplets at zero bias due to competing recombination channels. However, at driving forces greater than −0.6 eV, electron injection from the triplet accelerates and clearly produces a trend with increased applied bias that matches predictions from Marcus theory with a metallic acceptor.

The rate of photoinduced electron transfer from triplet excited states after singlet fission in molecules adsorbed to mesoporous oxide substrates is shown through transient absorption studies to depend systematically on applied bias.     

Photophysics of Push–Pull Distyrylfurans,Thiophenes and Pyridines by Fast and Ultrafast Techniques

Time‐resolved transient absorption and fluorescence spectroscopy with nano‐ and femtosecond time resolution were used to investigate the deactivation pathways of the excited states of distyrylfuran, thiophene and pyridine derivatives in several organic solvents of different polarity in detail. The rate constant of the main decay processes (fluorescence, singlet–triplet intersystem crossing, isomerisation and internal conversion) are strongly affected by the nature [locally excited (LE) or charge transfer (CT)] and selective position of the lowest excited singlet states. In particular, the heteroaromatic central ring significantly enhances the intramolecular charge‐transfer process, which is operative even in a non‐polar solvent. Both the thiophene and pyridine moieties enhance the S₁→T₁ rate with respect to the furan one. This is due to the heavy‐atom effect (thiophene compounds) and to the ¹(π,π)*→³(n,π)* transition (pyridine compounds), which enhance the spin‐orbit coupling. Moreover, the solvent polarity also plays a significant role in the photophysical properties of these push–pull compounds: in fact, a particularly fast ¹LE*→¹CT* process was found for dimethylamino derivatives in the most polar solvents (time constant, τ≤400 fs), while it takes place in tens of picoseconds in non‐polar solvents. It was also shown that the CT character of the lowest excited singlet state decreased by replacing the dimethylamino side group with a methoxy one. The latter causes a decrease in the emissive decay and an enhancement of triplet‐state formation. The photoisomerisation mechanism (singlet/triplet) is also discussed.     

A Spin-orbit Coupling Study on the Quinoline-and Porphyrin-based Photosensitizers

The spin-orbit coupling(SOC) of four porphyrin- and quinoline-based compounds has been studied using Pauli-Breit SOC operator with one- and two-electron terms. The results revealed that the yield of singlet oxygen is affected by the spin-orbit coupling matrix element involving the emitting triplet and the perturbing singlet state. Investigated quinoline-based compounds have more high SOC values than those porphyrin-based compounds due to spin parallel electron pairs of oxygen. The open shell d8 of metal Pt can induce the stronger exchange interactions than the closed shell p6 of metal Mg, resulting in bigger SOC matrix element in quinoline-based Pt complex than in the quinoline-based Mg complex. Simultaneously, potential energy curves of the first excited sate and the first triplet sate have been calculated, which proves that all investigated complexes can induce singlet oxygen. These computational findings support quinolin-based compounds have high singlet oxygen yields and provide a rigorous basis for predicting the probability of singlet oxygen yields in plane-type molecules.     

分子形貌所指示的羟基卡宾及其衍生物的质子转移反应

分子形貌(Molecular face, MF)定义分子的内禀电子转折边界面, 同时在其上计算并描绘出前沿电子密度(MFED). MF不仅能显示分子的形状和大小, 还能够指示分子的化学反应性. 应用M06-2X/6-311++G(d,p)理论方法, 对单线态和三线态羟基卡宾分子及其衍生物的质子化反应进行研究并计算了这些反应的活化能. 结果表明, 吸电性和供电性较强的取代基,均使单线态反应活化能增大,只有吸电性较强的─CN才能使三线态反应活化能增大. 应用分子形貌理论研究了上述反应, 不仅展示出分子的形貌变化、 与反应位点的关联, 以及有关物理量的变化倾向, 而且还定量地显示出, 单线态羟基卡宾及其衍生物分子边界面上前沿电子密度极大值与极小值的差值, 与其质子转移反应的活化能之间存在线性相关.     

Triplet states of furan,thiophene, and pyrrole

Low-lying triplet electronic states have been detected in furan, thiophene, and pyrrole by the method of variable-angle, electron-impact spectroscopy. Singlet → triplet transitions occur with maximum intensity at 3.99 eV and 5.22 eV in furan, 3.75 eV and 4.62 eV in thiophene, and 4.21 eV in pyrrole. A weak transition at 5.22 eV in pyrrole is assigned as the lowest observed singlet → singlet excitation in that molecule.     

Intersystem crossing at singlet conical intersections

The energy of the lowest triplet state of organic molecules is intermediate between the ground state and the first excited singlet. At the S₁/S₀ conical intersection, the two singlet states are degenerate. It is shown that for some molecules (ethylene, benzene, toluene and pyrrole) the T₁ state is also degenerate with the two singlet states. Moreover, the spin orbit coupling matrix element at this structure is necessarily large, so that intersystem crossing can be quite efficient. If the lowest triplet state is repulsive (as in the studied molecules) it may significantly contribute to the dissociation yield under certain experimental conditions.     

Hole-induced quenching of triplet and singlet excitons in conjugated polymers

Quantitative information on the mechanisms and rates of hole (radical cation)-induced quenching of triplet and singlet excitons in the conjugated polymer poly[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylene vinylene] has been acquired by a new technique, fluorescence-voltage time-resolved single molecule spectroscopy (FV-TR-SMS). FV-TR-SMS measures the fluorescence intensity of a single conjugated polymer molecule that is embedded in a capacitor-like device while simultaneously modulating the bias on the device and the irradiation intensity. The results demonstrate that triplet excitons are efficiently quenched by holes in conjugated polymers for hole densities >10(16) charges/cm(3), while singlet excitons are quenched with a much lower efficiency. Detailed kinetic analysis shows that the greater efficiency for quenching of triplets by holes (compared to that for singlets) is due to a >10(6) times longer exciton lifetime for triplets. In fact, the results suggest that while singlet quenching is less efficient due to a much shorter singlet lifetime, the rate constant for the quenching of singlets by holes actually exceeds that for triplets by several orders of magnitude.     

Femtosecond stimulated Raman spectroscopy – guided library mining leads to efficient singlet fission in rubrene derivatives

Chromophores undergoing singlet fission are promising candidates for harnessing solar energy as they can generate a pair of charge carriers by the absorption of one photon. However, photovoltaic devices employing singlet fission are still lacking practical applications due to the limitations within the existing molecules undergoing singlet fission. Chemical modifications to acenes can lead to efficient singlet fission devices, but the influence of changes to molecular structure on the rate of singlet fission is challenging to model and predict. Using femtosecond stimulated Raman spectroscopy we have previously demonstrated that the triplet separation process during singlet fission in crystalline rubrene is associated with the loss of electron density from its tetracene core. Based on this knowledge, we mined a library of new rubrene derivatives with electron withdrawing substituents that prime the molecules for efficient singlet fission, without impacting their crystal packing. Our rationally chosen crystalline chromophores exhibit significantly improved singlet fission rates. This study demonstrates the utility and strength of a structurally sensitive spectroscopic technique in providing insights to spectroscopy-guided materials selection and design guidelines that go beyond energy arguments to design new singlet fission-capable chromophores.

In the race to find efficient singlet fission materials, picking a winner is not easy. Femtosecond stimulated Raman spectroscopy can help us choose the best candidates, as demonstrated here in choosing from a library of rubrene derivatives.     

Excitation energy shuttling in oligothiophene-diketopyrrolopyrrole-fullerene triads

The photophysical properties of a thiophene-diketopyrrolopyrrole oligomer linked to two fullerene units via alkyl linkers of different lengths have been investigated in solution. The molecules exhibit excitation energy shuttling between the singlet and triplet photoexcited states. Photoexcitation of the oligomer is followed by singlet energy transfer to the fullerene, intersystem crossing to the triplet state, and then triplet energy transfer back to the oligomer. Competing electron transfer reactions, followed by recombination to the triplet state, are energetically possible and cannot be ruled out but were not observed and seem to have a small contribution in solution.     

Ultrafast fluorescence detection in tris(2,2'-bipyridine)ruthenium(II) complex in solution: relaxation dynamics involving higher excited states

The excited-state dynamics of a transition metal complex, tris(2,2'-bipyridine)ruthenium(II), [Ru(bpy)(3)](2+), has been investigated using femtosecond fluorescence upconversion spectroscopy. The relaxation dynamics in these molecules is of great importance in understanding the various ultrafast processes related to interfacial electron transfer, especially in semiconductor nanoparticles. Despite several experimental and theoretical efforts, direct observation of a Franck-Condon singlet excited state in this molecule was missing. In this study, emission from the Franck-Condon excited singlet state of [Ru(bpy)(3)](2+) has been observed for the first time, and its lifetime has been estimated to be 40 +/- 15 fs. Biexponential decays with a fast rise component observed at longer wavelengths indicated the existence of more than one emitting state in the system. From a detailed data analysis, it has been proposed that, on excitation at 410 nm, crossover from higher excited (1)(MLCT) states to the vibrationally hot triplet manifold occurs with an intersystem crossing time constant of 40 +/- 15 fs. Mixing of the higher levels in the triplet state with the singlet state due to strong spin-orbit coupling is proposed. This enhances the radiative rate constant, k(r), of the vibrationally hot states within the triplet manifold, facilitating the upconversion of the emitted photons. The vibrationally excited triplet, which is emissive, undergoes vibrational cooling with a decay time in the range of 0.56-1.3 ps and relaxes to the long-lived triplet state. The results on the relaxation dynamics of the higher excited states in [Ru(bpy)(3)](2+) are valuable in explaining the role of nonequilibrated higher excited sensitizer states of transition metal complexes in the electron injection and other ultrafast processes.     

PHOTOELECTRIC EFFECTS FROM CHLOROPHYLL a IN BILAYER MEMBRANES

Abstract— The photophysical and photochemical properties of thiophene derivatives have been studied by fluorescence and by 353 nm laser flash spectroscopy. α-Terthienyl and its derivatives show a moderate fluorescence quantum yield (less than 0.1) in cyclohexane, ethanol, or TritonX–100 micelles. An additional thiophene ring increases this value to 0.2 in ethanol or micelles. The transient triplet state of the six thiophenes is characterized by strong absorptions (ε⋍ 50000 M ^-1 cm^-1) in the visible region. These triplet states are very long lived. They react with oxygen, producing singlet oxygen very efficiently because of their high quantum yield of triplet formation (0.1 to 0.3). They do not react with excellent hydrogen or electron donors such as indole, N-acetyl tryptophanamide or cysteine. The hydrophobic thiophenes investigated are, therefore, Type II photodynamic agents almost exclusively.     

A triplet mechanism for the formation of thymine-thymine (6-4) dimers in UV-irradiated DNA

The reaction pathway for the photochemical formation of thymine-thymine (6-4) dimers in DNA is explored using hybrid density functional theory techniques in gas and in bulk solvent. It is concluded that the photo-induced cycloaddition displays favorable energy barriers in the triplet excited state. The stepwise cycloaddition in the triplet excited state involves the initial formation of a diradical followed by ring closure via singlet-triplet interaction. The key geometric features and electron spin densities are also discussed. The difference in barriers of H3' transfer for the lowest-lying triplet and singlet states shows that the singlet oxetane intermediate could catch the second photon to accelerate the rate of proton transfer, leading to formation of the Dewar structure. The present results provide a rationale for the formation of thymine-thymine (6-4) dimers in the triplet excited states.     

Synthesis of thiophene/phenylene co‐oligomers. V. Functionalization at molecular terminals toward optoelectronic device applications

We report the synthesis of various thiophene/phenylene co‐oligomers with a total number of thiophene and benzene (phenylene) rings of 5 and 6 with various terminal groups. Those terminal groups have been chosen from among alkyl groups, methoxy groups, trifluoromethyl groups, and cyano groups. The molecular backbone of these compounds comprises phenyl‐ or biphenylyl‐capped thiophene (or oligothiophene) or an alternating co‐oligomer. The synthesis is based on either the Suzuki coupling reaction or the Negishi coupling reaction. These reaction schemes enabled us to obtain the target compounds in high quality. In particular, the latter coupling method turned out to produce the compounds at a high yield. The terminal groups are expected to produce various functionalities based upon their electron donating character (alkyl groups and methoxy groups) or electron withdrawing character (trifluoromethyl groups and cyano groups). Additionally some of these groups bring about enhanced solubility. This will lead to the production of a diversity of modified compounds of thiophene/phenylene co‐oligomers. To give an example that demonstrates usefulness of the target compounds, we present optoelectronic data that are associated with their device applications.     

Singlet fission in a hexacene dimer: energetics dictate dynamics

Singlet fission (SF) is an exciton multiplication process with the potential to raise the efficiency limit of single junction solar cells from 33% to up to 45%. Most chromophores generally undergo SF as solid-state crystals. However, when such molecules are covalently coupled, the dimers can be used as model systems to study fundamental photophysical dynamics where a singlet exciton splits into two triplet excitons within individual molecules. Here we report the synthesis and photophysical characterization of singlet fission of a hexacene dimer. Comparing the hexacene dimer to analogous tetracene and pentacene dimers reveals that excess exoergicity slows down singlet fission, similar to what is observed in molecular crystals. Conversely, the lower triplet energy of hexacene results in an increase in the rate of triplet pair recombination, following the energy gap law for radiationless transitions. These results point to design rules for singlet fission chromophores: the energy gap between singlet and triplet pair should be minimal, and the gap between triplet pair and ground state should be large.

We report the synthesis and photophysical characterization of highly exoergic singlet fission in a hexacene dimer revealing exciton dynamics that follow the energy gap law.     

Triplet acetylenes as synthetic equivalents of 1,2-bicarbenes: phantom n,pi state controls reactivity in triplet photocycloaddition

Diaryl acetylenes, in which one of the aryl groups is either a pyridine or a pyrazine, undergo efficient triplet state photocycloaddition to 1,4-cyclohexadiene with formation of 1,5-diaryl substituted tetracyclo[3.3.0.0(2,8).0(4,6)]octanes (homoquadricyclanes). In the case of pyrazinyl acetylenes, the primary homoquadricyclane products undergo a secondary photochemical rearangement leading to diaryl substituted tricyclo[3.2.1.0(4,6)]oct-2-enes. Mechanistic and photophysical studies suggest that photocycloaddition proceeds through an electrophilic triplet excited state whereas the subsequent rearrangement to the tricyclooctenes proceeds through a singlet excited state. Chemical and quantum yields for the cycloaddition, in general, correlate with the electron acceptor character of aryl substituents but are attenuated by photophysical factors, such as the competition between the conversion of acetylene singlet excited state into the reactive triplet excited states (intersystem crossing: ISC) and/or to the radical-anion (photoelectron transfer from the diene to the excited acetylene: PET). Dramatically enhanced ISC between pi-pi S(1) state and "phantom" n,pi triplet excited state is likely to be important in directing reactivity to the triplet pathway. The role of PET can be minimized by the judicious choice of reaction conditions (solvent, concentration, etc.). From a practical perspective, such reactions are interesting because "capping" of the triple bond with the polycyclic framework orients the terminal aryl (4-pyridyl, 4-tetrafluoropyridyl, phenyl, etc.) groups in an almost perfect 60 degrees angle and renders such molecules promising supramolecular building blocks, especially in the design of metal coordination polymers.     

Singlet exciton trapping and heterofission in tetracene doped anthracene crystals

In tetracene doped anthracene, the magnetic field modulation of prompt tetracene fluorescence following excitation into the anthracene singlet manifold has been measured as a function of the magnetic field orientation and optical excitation energy. The results show that this modulation with low energy excitation is caused by singlet heterofission into one anthracene triplet exciton and one tetracene triplet. With higher excitation energies this modulation is due to both the singlet heterofission and also singlet homofission into a pair of anthracene triplet excitons. Heterofission occurs mainly from anthracene molecules next to a tetracene and competes with the singlet trapping. From the singlet trapping rate and from the magnetic modulation of tetracene prompt fluorescence the heterofission rate is estimated as ≈10^?11s^?1.     

PHOTOCHEMISTRY OF THE BOTANICAL PHOTOTOXIN, α-TERTHIENYL AND SOME RELATED COMPOUNDS*

The photochemistry of α-terthienyl (αT) and its mono- and dodo derivatives has been examined using nanosecond laser Hash photolysis techniques. The triplet states of these intermediates have been characterized, and show strong triplet-triplet absorptions with maxima in the 450 to 490 nm region. The triplet lifetimes are normally reduced by efficient triplet-triplet annihilation and self-quenching both of which approach diffusion control. Triplet lifetimes in methanol obtained by extrapolation to zero laser dose and zero concentration are 30, 12.5 and 9.4 μs for αT and its mono- and dodo derivatives, respectively; the effect of iodo substitution on the lifetimes is attributed to heavy atom effects. The triplet states are efficiently quenched by oxygen and the electron acceptor methyl viologen, while amines tire very poor triplet quenchers. The iodo derivatives are photolabile. undergoing C-I bond cleavage from the singlet state, a process that was studied in benzene solvent, where the complex between iodine atoms and benzene can be readily characterized. Modification of αT by replacement of the central thiophene ring by an aromatic ring (i.e. DTB) causes drastic changes in the triplet and singlet state kinetic and spectroscopic characteristics.