Singlet Heterofission in Tetracene–Pentacene Thin-Film Blends

Heterofission is a photophysical process of fundamental and applied interest whereby an excited singlet state is converted into two triplets on chemically distinct chromophores. The potential of this process lies in the tuning of both the optical band gap and the splitting between singlet and triplet energies. Herein, we report the time-domain observation of heterofission in mixed thin films of the prototypical singlet fission chromophores pentacene and tetracene using excitation wavelengths above and below the tetracene band gap. We found a time constant of 26 ps for endothermic heterofission of a singlet exciton on pentacene in blends with low pentacene fractions, which was outcompeted by pentacene homofission for increasing pentacene concentrations. Direct excitation of tetracene lead to fast energy transfer to pentacene and subsequent singlet fission, which prevented homo- or heterofission of a singlet exciton on tetracene.     

Eosin-sensitized photoreduction of benzil with 1-benzyl-1,4-dihydronicotinamide

The mechanism of eosin-sensitized photoreduction of benzil with 1-benzyl-1,4-dihydronicotinamide — a model compound of NAD(P)H and the behavior of the excited states of eosin have been investigated. The effect of anthracene as a diffusion-controlled quencher of the photoreaction indicates that both excited triplet state and an unquenchable excited singlet state of eosin participated in the sensitized photoreaction. From the Stern-Volmer plot of quantum yield vs. anthracene concentration, the triplet reaction rate constant has been calculated to be 0.78 × 10⁸ L M^?1S^?1 while the singlet reaction rate constant determined from quenching of eosin fluorescence by benzil is equal to 7.2 × 10⁹ L M^?1S^?1. The singlet and triplet quantum yields are also determined to be 0.09 and 0.18 respectively. Since both the singlet and triplet energies of eosin are lower than that of benzil, energy transfer sensitization is not feasible. It is proposed that electron transfer from the excited eosin to benzil is responsible for the initiation.     

On singlet exciton fission in anthracene and tetracene at 77°K

Relative variations of prompt flourescence yield, on application of a magnetic field are studied at 77°K for anthracene and tetracene crystals excited by light with wavenumbers up to 50,000 cm^-1. The results obtained for anthacene, as compared to those previously reported at 300°K, indicate a very small temperature dependence. The variations observed for tetracene at 77°K are comparable in magnitude to those for anthracene. The singlet exciton fission process, responsible for the experimental observations, does not involve a thermally relaxed lowest bound or charge transfer singlet exciton. The possible role of upper excited vibronic states is discussed.     

Homogeneous and heterogeneous triplet — triplet annihilation in anthracene-doped phenanthrene crystals

Delayed and prompt fluorescence spectra and delayed fluorescence decay data for 10^?5?10^?6 M/M crystals of anthracene in phenanthrene are reported. It is shown that these data indicate that a heterogeneous (host-dopant) annihilation process takes place which produces the excited singlet state of the host molecule.     

Vibrational relaxation by reversible intersystem crossing

The time- and energy-resolved fluorescence spectra of CO obtained under selective excitation of the A ¹Π (υ′ = 1) level have been studied. The apparent vibrational relaxation in the singlet manifold is shown to be mainly due to the three-step process: the singlet → triplet crossing from υ′ = 1 level, vibrational relaxation in the triplet manifold and reverse crossing to the A ¹Π (υ′ = 0) level. The decay form may be fitted by assuming the relaxation rate constants in triplet (and singlet) manifolds of the order of 10⁵ s^?1 Torr^?1 i.e. smaller by one order of magnitude than previously proposed.     

Rate constants of singlet exciton fission in a tetracene crystal determined from the rydmr spectral linewidth

The lifetime τ of a correlated pair of triplet excitons has been determined from the linewidth of the RYDMR spectrum. The value τ = 7 × 10^?9 s obtained has been used to calculate the rate constants of the singlet exciton fission into a pair of triplet excitons in a tetracene single crystal. The fission was shown to go via three steps: the first one is contact pair formation, the second step is its transformation into a diffusive pair, and the third one is the dissociation of the diffusive pair into free triplet excitons. Both the values of the rate constants of every step and the entropy growth in the transition from contact pair to diffusive pair have been evaluated.     

Pressure dependence of the Davydov splitting in tetracene single crystals

The reflection spectrum of the OO component of the first singlet transition of tetracene single crystal has been measured as a function of pressure in the range 1 atm–6.5 kbar. A discontinuous change in the Davydov splitting occurs near 3 kbar confirming the existence of a pressure-induced first order phase transition discovered recently by measurements of the magnetic field anisotropy in the single crystal tetracene fluorescence. The pressure-induced spectral red-shifts are larger in the low-pressure (LP) phase than those in the high-pressure (HP) crystal structure. The Davydov splitting of the OO band however increases with increasing pressure at a larger rate of 57 cm^?1/kbar for the HP phase as compared with 46 cm^?1/kbar for the LP phase.     

Magnetic resonance of short-lived triplet exciton pairs detected by fluorescence modulation at room temperature

The possibility of resonant electromagnetic field modulation of the rate of reactions between paramagnetic particles in a condensed medium lasting for 10^?8–10^?10 sand shorter than the spin-lattice relaxation time has been demonstrated experimentally. The EPR spectra of triplet exciton pairs in tetracene and charge transfer crystals have been recorded at room temperature.     

Singlet exciton interactions in crystalline naphthalene

The decay of prompt fluorescence in crystalline naphthalene at 300 K, excited by a picosecond 266 nm pulse, has been studied as a function of excitation intensity. Experimental decay curves can be fitted only when the exponential distribution in depth of excitation and the radial (gaussian) intensity profile of the excitation are both taken into account. From an analysis of the decay at early time (?5 ns) a best fit value of the singlet—singlet annihilation rate constant is found γ_SS = (4 ± 1) × 10^?10 cm³ s^?1. If the reaction is diffusion-limited, this rate implies an average singlet diffusivity D_S = (2 ± 1) × 10^?4 cm² s^?1.     

New Anthracene Derivatives as Triplet Acceptors for Efficient Green‐to‐Blue Low‐Power Upconversion

Three new anthracene derivatives [2‐chloro‐9,10‐dip‐tolylanthracene (DTACl), 9,10‐dip‐tolylanthracene‐2‐carbonitrile (DTACN), and 9,10‐di(naphthalen‐1‐yl)anthracene‐2‐carbonitrile (DNACN)] were synthesized as triplet acceptors for low‐power upconversion. Their linear absorption, single‐photon‐excited fluorescence, and upconversion fluorescence properties were studied. The acceptors exhibit high fluorescence yields in DMF. Selective excitation of the sensitizer Pd^IIoctaethylporphyrin (PdOEP) in solution containing DTACl, DTACN, or DNA‐CN at 532 nm with an ultralow excitation power density of 0.5 W cm^?2 results in anti‐Stokes blue emission. The maximum upconversion quantum yield (Φ_UC=17.4 %) was obtained for the couple PdOEP/DTACl. In addition, the efficiency of the triplet–triplet energy transfer process was quantitatively studied by quenching experiments. Experimental results revealed that a highly effective acceptor for upconversion should combine high fluorescence quantum yields with efficient quenching of the sensitizer triplet.     

The role of the triplet state in the photosensitization of cationic polymerizations by anthracene

The photosensitization mechanism for cationic polymerizations initiated by diaryliodonium salts photosensitized by anthracene was investigated using fluorescence and phosphorescence spectroscopy. In situ photosensitizer fluorescence measurements confirmed that the photosensitization reaction proceeds by an electron transfer process. Transient phosphorescence studies demonstrated that electron transfer occurred from the triplet excited state of anthracene to the initiator, with an intrinsic kinetic rate constant of 2 × 10⁸ L/mol s. Further evidence for the role of the triplet state was provided by an observed seven-fold decrease in the polymerization rate upon addition of a triplet state quencher. Finally, numerical solution of the photophysical kinetic equations indicated that the triplet state concentration was approximately three orders of magnitude higher than that of the singlet state, and that 94-96% of the active cationic centers are produced by reaction of the initiator with the triplet state. These results indicate that the electron transfer occurs primarily from the triplet state of anthracene, with the singlet state providing only a minor contribution to the photosensitization reaction. © 1995 John Wiley & Sons, Inc.     

Fluorescence of the antharacenes following Tn → T1 excitation studied by a double excitation method

The fluorescence from the lowest excited singlet state following excitation of the lowest triplet state was observed for anthracene, 9-methylanthracene, and 9-phenylanthracene in ethanol by a newly devised double excitation method which is essentially the combination of flash and laser photolysis. The quantum yield of intersystem crossing from the excited triplet state, T_n(n ? 2), to the lowest excited singlet state was markedly increased by methyl- and phenyl-substitution at the meso-position.     

The electronic relaxation of biacetyl in the vapor phase

The emissions of biacetyl after pulsed dye-laser excitation were studied at pressures down to 0.05 mtorr. At all energies the time-resolved fluorescence was composed of a nanosecond and a microsecond component. At “zero” pressure the long lived phosphorescence was absent while the “hot” phosphorescence has the same time characteristics as the slow fluorescence. By increasing the pressure the slow fluorescence was quenched while the milisecond phosphorescence was induced. We determined the low-pressure emission characteristics and the pressure effects as a function of excitation energy.From our data we obtained the parameters describing the intermediate type singlet-triplet coupling, the radiative and non-radiative relaxation rates from the singlet and triplet levels and the cross sections for the slow fluorescence quenching, all as a function of energy. Strong evidence is obtained for the participation of rotational states in the intra-molecular relaxation. The important difference between the situation where the singlet levels are isolated (low energy) and where the singlet level widths overlap (at higher energies) is demonstrated. In the former situation very large fluorescence quenching cross sectios were found. It is further shown that for high energies at least two effective collisions are needed to obtain a thermalized triplet; the mean energy removed per effective collision is 2200 cm^?1.     

Intramolecular photoexcitation dynamics and magnetic field effects in an intermediate-case molecule

Molecular vibration and rotation play a significant role in the intramolecular photoexcitation dynamics of the so-called intermediate-case molecule, and the fluorescence intensity, decay and polarization of s-triazine vapor are shown to depend on the excited rovibronic level of the S₁ state. Fluorescence characteristics are interpreted by assuming three zero-order states: (1) a zero-order singlet state that carries the absorption intensity and emits fluorescence with sharp structure; (2) zero-order singlet states that do not carry the absorption intensity but emit broad fluorescence; and (3) zero-order triplet states. The interaction among these states depends not only on the vibrational level but also on the rotational level excited. It is suggested that the number of triplet states coupled to the singlet state increases with increasing excess vibrational energy. It is also suggested that K-scrambling occurs both in the triplet manifold following intersystem crossing (ISC) and in the singlet manifold following intramolecular vibrational energy redistribution (IVR). The fluorescence intensity and decay of s-triazine vapor are significantly influenced by a magnetic field, and the field effects are interpreted in terms of the spin decoupling in the triplet manifold following ISC; the role of external magnetic fields is to mix the spin sublevels of different rovibronic levels coupled to the excited singlet state. Magnetic depolarization of fluorescence also occurs because of the efficient interaction between the excited singlet state and the triplet state.     

Electron paramagnetic resonanace of the photoexcited triplet state of electron—donor—acceptor complex crystals: Naphthalene—tetrachlorophthalic anhydrides

The EPR spectra and kinetics of the photoexcited triplet state of naphthalene—tetrachlorophthalic anhydride (N—TCPA) complex crystals are reported. The phosphorescent state of N—TCPA is naphthalene-like with 10% charge-transfer character at 4.2 K. Detailed temperature and orientational studies show that donors and acceptors reorientate along the stack axis at very low temperatures (20 K and below) to gain a maximum overlapping (X-trap). The activation energy of the detrapping process is 60 ± 10 cm^?1. At higher temperatures (50 K and above), the state of the system is best described as an excitation jumping between a localized state and a thermally accessible higher delocalized state (exciton). The activation energy of the excitation jumping is 150 ± 10 cm^?1. The upper limit of the average time spent in the exciton state is 8 × 10^?10 s. The transient studies yield the triplet population and decay rate constants of N—TCPA complexes which differ from those of uncomplexed naphthalenes. The difference is attributed to the excitation to CT singlet state followed by intersystem crossing in different pathways.     

Stark effects on the low energy electronic states of p-benzoquinone single crystals

The Stark effect on the visible absorption spectra of p-benzoquinone single crystals has been measured using modulation techniques. The measurements of second order Stark effects confirm the presence of at least two close lying electronic nπ^* states in both the singlet and triplet systems. Both the origin of the first singlet-singlet and singlet—triplet transition appear as doublets separated by 3.5 and 18 cm^?1, respectively. For the triplet state the low energy component is Stark induced and has not been observed previously in absorption. The doublet components are mixed in the electric field showing that they are of different parity. They are tentatively assigned as inversion doublets of a symmetric double well potential produced by the vibronic interaction of two different electronic nπ^* states.     

Preparation and spectral characteristics of anthracene/tetracene mixed crystals

A series of tetracene-doped anthracene crystals with different doping concentrations (the highest molar ratio 100 1) are grown from solution. Crystal structures and optical characteristics of the above mixed crystals are investigated at room temperature. By changing the doping concentrations, the fluorescence can be adjusted from blue-green to green and even to yellow-green. The emission spectra of anthracene/tetracene (An/Te) mixed crystals reveal the sensitized fluorescence of tetracene and the partial quenching of anthracene emission. The data of transient photoluminescence (PL) decays illustrate that in An/Te mixed crystals, the decay of anthracene becomes faster, while the PL lifetime of tetracene is longer than that of the tetracene single crystals. All above experimental results suggest that there is excitation energy transfer from anthracene to tetracene in the mixed crystals. Supported by the National Natural Science Foundation of China (Grant No. 5057 3039) and the National Key Basic Research and Development Program of China (Grant No. 2006CB806200)     

Intramolecular Electronic Energy Transfer in a Supersonic Jet Expansion

Intramolecular electronic energy transfer (intra-EET) was investigated in an isolated bichromophoric naphthalene (N) and anthracene (A) 1:1 molecular cluster. Investigation of the spectroscopic properties of these chromophores, separately and loosely bound in a van der Waals complex, helps to understand the dependence of the EET rate on the initially excited vibronic level and on the cluster's interchromophoric orientation. Measurement of fluorescence excitation spectra of anthracene, at different anthracene pressures shows bands that can be assigned to dimers of anthracene. From measurement of the anthracene excitation spectrum at increasing naphthalene pressures one can identify other spectral features, characterized by different spectral shifts from excitations of the bare molecule. Some transitions are probably due to a 13.5 cm^?1 progression associated with an interchromophore cluster bond. Pressure dependence of fluorescence intensity gives evidence for 1:1 cluster composition, and for a slow intra-EET rate that is associated with an unfavoured orientation of the two chromophores in one of the two possible conformers of the A-N cluster, as supported by a calculation of the cluster geometry and by comparison with a recent study of intra-EET in the A-(CH₂)_n-N bichromophoric molecules.     

Dual fluorescence lifetimes of pyrimidine: Intermediate radiationless decay

We have observed a dual fluorescence decay from the lowest n → π* excited singlet state of pyrimidine. The vibronic states 0-0, 6a¹, 12¹, 6a¹12¹, 12² and 6a¹12² have two exponential decays with lifetimes ranging from 2.7-0.7 nsec and from 410-234 ns at 0.02 torr. The ratio of pre-exponentials is pressure independent but the long decay is very sensitive to collisions. The four lower energy states have effective impact diameters of 16 A and the highest energy state is quenched by gas kinetic collision diameters (≈ 5.5 Å). The dual fluorescence decay and collisional fluorescence quenching by rotational relaxation is consistent with the available models of singlet-triplet mixed state decay. Using these models we have computed the rates for singlet-triplet crossing, the number of coupled triplet levels, and the decay rates for internal conversion. The model used our measured fluorescence decay parameters and our estimate of a triplet loss rate. The estimated triplet loss varies from 0.2 to 2.0 × 10⁶ s^?1 and the singlet internal conversion rate varies from ≈ 0.4 to 56 × 10⁷ s^?1. The singlet-triplet radiationless rate suggests that 50–100 times more triplet levels are effective in the state mixing than can be expected from the triplet vibronic density. Such an enhanced coupling of ro-vibronic triplet levels is 5–10 times larger than previously observed for the dicarbonyls. The observation of reduced collisional quenching of higher energy vibronic levels is quantitatively interpreted by a different model than used previously for the dicarbonyls.     

Excited singlet yield in T-T annihilation, a comparative study of naphthalene and anthracene in solution

A method based on the study of both triplet absorption and delayed fluorescence (DF) decays after flash excitation has been developed to determine DF yields. Values of the excited singlet yield in the T-T annihilation process have been obtained for naphthalene and anthracene in solution at room temperature by application of this method. Triplet extinction coefficients and annihilation rate constants for these compounds were also determined. Excited singlet generation is the predominant annihilation channel for naphthalene whereas it is only of minor importance for anthracene. The difference in DF yields is ascribed to differences between these aromatics with respect to the efficiencies of the annihilation channels leading respectively to excited singlet and triplet states. The higher rate of excited singlet formation for naphthalene is qualitatively accounted for by considering the possibility of radiationless transitions from the bimolecular states formed in T-T interaction towards an upper excimer singlet state.