Akin Akdag

Singlet Exciton Fission for Solar Cell Applications: Energy Aspects of Interchromophore Coupling †

Singlet exciton fission, a process that converts one singlet exciton to a pair of triplet excitons, has the potential to enhance the efficiency of both bulk heterojunction and dye-sensitized solar cells and is understood in crystals but not well understood in molecules. Previous studies have identified promising building blocks for singlet fission in molecular systems, but little work has investigated how these individual chromophores should be combined to maximize triplet yield. We consider the effects of chemically connecting two chromophores to create a coupled chromophore pair and compute how various structural choices alter the thermodynamic and kinetic parameters likely to control singlet fission yield. We use density functional theory to compute the electron transfer matrix element and the thermodynamics of fission for several promising chromophore pairs and find a trade-off between the desire to maximize this element and the desire to keep the singlet fission process exoergic. We identify promising molecular systems for singlet fission and suggest future experiments.

Toward designed singlet fission: Solution photophysics of two indirectly coupled covalent dimers of 1,3-diphenylisobenzofuran

In order to identify optimal conditions for singlet fission, we are examining the photophysics of 1,3-diphenylisobenzofuran (1) dimers covalently coupled in various ways. In the two dimers studied presently, the coupling is weak. The subunits are linked via the para position of one of the phenyl substituents, in one case (2) through a CH2 linker and in the other (3) directly, but with methyl substituents in ortho positions forcing a nearly perpendicular twist between the two joint phenyl rings. The measurements are accompanied with density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations. Although in neat solid state, 1 undergoes singlet fission with a rate constant higher than 10(11) s(-1); in nonpolar solutions of 2 and 3, the triplet formation rate constant is less than 10(6) s(-1) and fluorescence is the only significant event following electronic excitation. In polar solvents, fluorescence is weaker because the initial excited singlet state S1 equilibrates by sub-nanosecond charge transfer with a nonemissive dipolar species in which a radical cation of 1 is attached to a radical anion of 1. Most of this charge transfer species decays to S0, and some is converted into triplet T1 with a rate constant near 10(8) s(-1). Experimental uncertainties prevent an accurate determination of the number of T1 excitations that result when a single S1 excitation changes into triplet excitation. It would be one if the charge-transfer species undergoes ordinary intersystem crossing and two if it undergoes the second step of two-step singlet fission. The triplet yield maximizes below room temperature to a value of roughly 9% for 3 and 4% for 2. Above ∼360 K, some of the S1 molecules of 3 are converted into an isomeric charge-transfer species with a shorter lifetime, possibly with a twisted intramolecular charge transfer (TICT) structure. This is not observed in 2.

Toward designed singlet fission: electronic states and photophysics of 1,3-diphenylisobenzofuran.

Single crystal molecular structure and solution photophysical properties are reported for 1,3-diphenylisobenzofuran (1), of interest as a model compound in studies of singlet fission. For the ground state of 1 and of its radical cation (1(+*)) and anion (1(-*)), we report the UV-visible absorption spectra, and for neutral 1, also the magnetic circular dichroism (MCD) and the decomposition of the absorption spectrum into purely polarized components, deduced from fluorescence polarization. These results were used to identify a series of singlet excited states. For the first excited singlet and triplet states of 1, the transient visible absorption spectra, S(1) --> S(x) and sensitized T(1) --> T(x), and single exponential lifetimes, tau(F) = approximately 5.3 ns and tau(T) = approximately 200 micros, are reported. The spectra and lifetimes of S(1) --> S(0) fluorescence and sensitized T(1) --> T(x) absorption of 1 were obtained in a series of solvents, as was the fluorescence quantum yield, Phi(F) = 0.95-0.99. No phosphorescence has been detected. The first triplet excitation energy of solid 1 (11,400 cm(-1)) was obtained by electron energy loss spectroscopy, in agreement with previously reported solution values. The fluorescence excitation spectrum suggests an onset of a nonradiative channel at approximately 37,000 cm(-1). Excitation energies and relative transition intensities are in agreement with those of ab initio (CC2) calculations after an empirical 3000 cm(-1) adjustment of the initial state energy to correct differentially for a better quality description of the initial relative to the terminal state of an absorption transition. The interpretation of the MCD spectrum used the semiempirical PPP method, whose results for the S(0) --> S(x) spectrum require no empirical adjustment and are otherwise nearly identical with the CC2 results in all respects including the detailed nature of the electronic excitation. The ground state geometry of 1 was also calculated by the MP2, B3LYP, and CAS methods. The calculations provided a prediction of changes of molecular geometry upon excitation or ionization and permitted an interpretation of the spectra in terms of molecular orbitals involved. Computations suggest that 1 can exist as two nearly isoenergetic conformers of C(2) or C(s) symmetry. Linear dichroism measurements in stretched polyethylene provide evidence for their existence and show that they orient to different degrees, permitting a separation of their spectra in the region of the purely polarized first absorption band. Their excitation energies are nearly identical, but the Franck-Condon envelopes of their first transition differ to a surprising degree.

Search for a small chromophore with efficient singlet fission: biradicaloid heterocycles.

Of the five small biradicaloid heterocycles whose S(1), S(2), T(1), and T(2) adiabatic excitation energies were examined by the CASPT2/ANO-L-VTZP method, two have been found to meet the state energy criterion for efficient singlet fission and are recommended to the attention of synthetic chemists and photophysicists.

The Stabilities of N−Cl Bonds in Biocidal Materials

N-halamine chemistry has been a research topic of considerable importance in these laboratories for over two decades. N-halamine compounds are useful in preparing biocidal materials. There are three N-Cl moieties available in cyclic N-halamine compounds:u2009 imide, amide, and amine. The stabilities toward the release of free halogen have been experimentally shown to decrease in the order amine > amide > imide. In this work, this generalization has been tested theoretically at the level of B3LYP/6-31+G(d) and using the conductor-like polarizable continuum aqueous solvation model with UAKS cavities. Excellent accord was observed between theory and experiment. It was also found that the imide and amide N-halamine stabilities on hydantoin rings could be reversed with substitution patterns at the 5 position.

The 16 CB11(CH3)n(CD3)(12-n)• radicals with 5-fold substitution symmetry: spin density distribution in CB11Me12(•).

The syntheses of all 16 CB(11)(CH(3))(n)(CD(3))(12-n)(•) radicals with 5-fold substitution symmetry are described. The variation in the width of their broad and featureless electron paramagnetic resonance signals as a function of the deuteriation pattern is used to deduce the relative values of the average hyperfine coupling constants a(H) of the hydrogen atoms in the ipso (1), ortho (2-6), meta (7-11), and para (12) methyl groups, a(H)(i):a(H)(o):a(H)(m):a(H)(p) = (0.18 ± 0.09):(0.71 ± 0.02):(1.00 ± 0.03):(0.52 ± 0.05), and these can be compared with ratios expected from a B3LYP/EPRII calculation, 0.04:0.55:1:0.51.

Toward singlet fission for excitonic solar cells

Sensitizer dyes capable of producing two triplet excited states from a singlet excited state produced by the absorption of a single photon would allow an increase of the efficiency of photovoltaic cells by up to a factor of 1.5, provided that each triplet injects an electron into a semiconductor such as TiO2. Although singlet fission in certain crystals and polymers was reported long ago, little is known about its efficiency in dyes suitable for use as sensitizers of photoinduced charge separation on semiconductors surfaces. Biradicaloids and large alternant hydrocarbons are desirable parent structures likely to meet the requirement E(T2), E(S1) > 2E(T1) for the excitation energies of the lowest excited singlet (S1) and the two triplet (T1, T2) states. We report results for 1,3-diphenylisobenzofuran, a model compound of the biradicaloid type. Its energy levels satisfy the desired relation, and in solution it shows no triplet formation by intersystem crossing. In the neat solid state, it forms triplets efficiently, and indirect evidence suggests that this is due to singlet fission. This appears to be the first compound displaying SF by design. When two such chromophores were combined into dimers, triplet formation yields of up to 9% were observed in polar solvents, possibly due to singlet fission, but possibly due to intersystem crossing. The triplet formation occurs in two steps, via an intermediate assigned as an intramolecular charge-transfer state and responsible for most of the observed excitation loss.

Excitation Localization/Delocalization Isomerism in a Strongly Coupled Covalent Dimer of 1,3-Diphenylisobenzofuran

Two isomers of both the lowest excited singlet (S1) and triplet (T1) states of the directly para, para-connected covalent dimer of the singlet-fission chromophore 1,3-diphenylisobenzofuran have been observed. In one isomer, excitation is delocalized over both halves of the dimer, and in the other, it is localized on one or the other half. For a covalent dimer in solution, such excitation isomerism is extremely rare. The vibrationally relaxed isomers do not interconvert, and their photophysical properties, including singlet fission, differ significantly.

Covalent Dimers of 1,3-Diphenylisobenzofuran for Singlet Fission: Synthesis and Electrochemistry

The synthesis of covalent dimers in which two 1,3-diphenylisobenzofuran units are connected through one phenyl substituent on each is reported. In three of the dimers, the subunits are linked directly, and in three others, they are linked via an alkane chain. A seventh new compound in which two 1,3-diphenylisobenzofuran units share a phenyl substituent is also described. These materials are needed for investigations of the singlet fission process, which promises to increase the efficiency of solar cells. The electrochemical oxidation and reduction of the monomer, two previously known dimers, and the seven new compounds have been examined, and reversible redox potentials have been compared with results obtained from density functional theory. Although the overall agreement is satisfactory, some discrepancies are noted and discussed.

Manganese(III) acetate oxidation of some ketones in the presence of 3-chloropropionic acid and theoretical investigation of the products

Abstract Manganese(III) acetate oxidation of some ketones in the presence of 3-chloropropionic acid was carried out. The products were characterized. The experimental results were compared with the theoretical ones which was based on AM1 (UHF) type semi-empirical calculations. The theoretical results disfavor any mechanism solely based on radical stability involving radicals originating from the reactants.