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Recent theories suggest that low frequency dynamic intramolecular and intermolecular motions in organic semiconductors (OSCs) are critical to determining the hole mobility. So far, however, it has not been possible to probe these motions directly experimentally and therefore no unequivocal and quantitative link exists between molecular-scale thermal disorder and macroscale hole mobility in OSCs. Here we use inelastic neutron scattering to probe thermal disorder directly by measuring the phonon spectrum in six different small molecule OSCs, which we accurately reproduce with first principles simulations. We use the simulated phonons to generate a set of electron-phonon coupling parameters. Using these parameters, the theoretical mobility is in excellent agreement with macroscopic measurements. Comparison of mobility between different materials reveals routes to improve mobility by engineering phonon and electron-phonon coupling.

The effect of the strong electron acceptor, 2,3,5,6-tetrafluoro-7,7,8,8- tetracyanoquinodimethane (F4-TCNQ), on poly(3-hexylthiophene) (P3HT) aggregates is studied. F4-TCNQ is commonly used as a dopant for P3HT, however, relatively little is currently known about its effect on polymer conformation and packing in the presence of fullerenes. Resonance Raman and optical spectra of pristine P3HT or blends with [6,6]-phenyl-C 61-butyric acid methyl ester (PCBM) doped with F4-TCNQ up to ∼10% w/w show a loss of pristine-type P3HT aggregates with increasing dopant concentration. Complexed P3HT chains possess greater backbone planarity due to hole injection which is corroborated from density functional theory (DFT) calculations of oligothiophene surrogates and F4-TCNQ. Morphologies of doped P3HT/PCBM systems are characterized using scanning transmission electron microscopy (STEM) with electron energy loss spectroscopy (EELS) detection and images reveal mixed clusters of P3HT/F4-TCNQ fibril-like domains that increase in size with dopant loading. The apparent preference of F4-TCNQ for P3HT aggregates is attributed to efficient charge separation stemming owing to the more polarizable nature of chains comprising the aggregate π-stack. © 2013 The Royal Society of Chemistry.

We investigate the ability of dynamic fluorescence probes to accurately track populations of multi-excitonic states in molecular dyads based on conjugated acenes capable of intramolecular singlet fission (iSF). Stochastic simulations of reported photophysical models from time-resolved spectroscopic studies of iSF dyads based on large acenes (e.g., tetracene and pentacene) are used to extrapolate population and fluorescence yield dynamics. The approach entails the use of repetitive rectangular-shaped excitation waveforms as a stimulus, with durations comparable to triplet lifetimes. We observe unique dynamics signatures that can be directly related to relaxation of multi-exciton states involved over the entire effective time of singlet fission in the presence and absence of an excitation light stimulus. In particular, time-dependent fluorescence yields display an abrupt decay followed by slower rise dynamics appearing as a prominent "dip" feature in responses. The initial fast decrease in the fluorescence yield arises from the formation of triplet pairs and separated triplets that do not produce emission resembling a complete ground state bleach effect. However, relaxation of one separated triplet allows the system to absorb, and in some cases, this increases the fluorescence yield, causing rise dynamics in the emissive response. Our approach also permits extrapolation of all multi-exciton state population dynamics up to steady state conditions in addition to the ability to explore consequences of alternative relaxation channels. The results demonstrate that it is possible to resolve unique signatures of singlet fission events from dynamic fluorescence studies, which can augment detection capabilities and extend sensitivity limits and accessible time scales. [ABSTRACT FROM AUTHOR]

Resolving the population dynamics of multiple triplet excitons on time scales comparable to their lifetimes is a key challenge for multiexciton harvesting strategies, such as singlet fission. We show that this information can be obtained from fluorescence quenching dynamics and stochastic kinetic modeling simulations of single nanoparticles comprising self-assembled aggregated chains of poly(3-hexylthiophene) (P3HT). These multichromophoric structures exhibit the elusive J-aggregate type excitonic coupling leading to delocalized intrachain excitons that undergo facile triplet formation mediated by interchain charge transfer states. We propose that P3HT J-aggregates can serve as a useful testbed for elucidating the presence of multiple triplets and understanding factors governing their interactions over a broad range of time scales. Stochastic kinetic modeling is then used to simulate discrete population dynamics and estimate higher order rate constants associated with triplet-triplet and singlet-triplet annihilation. Together with the quasi-CW nature of the experiment, the model reveals the expected amounts of triplets at equilibrium per molecule. Our approach is also amenable to a variety of other systems, e.g., singlet fission active molecular arrays, and can potentially inform design and optimization strategies to improve triplet harvesting yields. [ABSTRACT FROM AUTHOR]

Resolved vibronic structure in electronic spectra provides a detailed view into how molecular structure changes after absorption or emission of a photon. We report temperature- and pressure-dependent luminescence spectra of trans-[Re[O.sub.2][(pyridine).sub.4]I. Low-temperature spectra reveal long vibronic progressions in the totally symmetric O = Re = O (907 [cm.sup.-1] and Re-pyridine (211 [cm.sup.-1] stretching modes, indicating large structural displacements along these normal coordinates. The luminescence band maximum is at ca. 15 500 [cm.sup.-1]. Room-temperature spectra are somewhat less-resolved; however, intervals closely matching the O = Re = O frequency (~870 [cm.sup.-1] persist at higher temperatures. The variable pressure spectra exhibit distinct changes in the vibronic patterns, and luminescence energies decrease by 16 [+ or -] 2 [cm.sup.-1]/kbar (1 bar = 100 kPa). Low-temperature spectra are modeled using two-dimensional potential energy surfaces to represent the initial and final electronic states, from which the quantitative normal coordinate offsets can be determined. We then adapt this model to the room-temperature, pressure-dependent data where it is possible to determine how the offsets and other important spectroscopic parameters vary with the pressure-induced changes of the molecular structure. Key words: trans-[Re[O.sub.2] [(pyridine).sub.4] I, low-temperature luminescence spectroscopy, high-pressure luminescence spectroscopy, vibronic structure, emitting state distortions.

Luminescence spectra of trans-dioxo bis-(tetramethylethylenediamine)-rhenium(V) show pressure-dependent vibronic structure, frequency and the intensity distribution within the progression, involving the high-frequency Raman-active O=Re=O stretching mode. The influence of pressure on the shape and relative positions of the ground and emitting state potentials energy surfaces are calculated along the normal coordinate of the vibrational mode.

Optical emission spectra of poly[2-methoxy-5-[3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) in dilute solutions exhibit a vibronic progression interval (∼1225 cm-1) that does not correspond to any ground state vibrational mode frequency. This phenomenon is assigned as the missing mode effect (MIME) in which five key displaced polymer backbone vibrational modes in the range of 800-1600 cm-1 contribute to the MIME interval. Emission spectra are calculated by analytically solving the time-dependent Schrödinger equation using estimates of mode-specific vibrational displacements determined independently from preresonance Raman intensities. Emission spectra of MDMO-PPV thin films and nanoparticles are measured and lineshapes show an increase of the MIME frequency to ∼1340 cm-1 in addition to changes in vibronic intensity distributions and energies. Composite blend thin films consisting of MDMO-PPV and a fullerene derivative (1:1 w/w) exhibit a substantially larger MIME interval (∼1450 cm-1) that arises from an increase in polymer chain planarity. This structural change is most apparent from large decreases of the excited state displacement of an out-of-plane C[Single_Bond]H bending mode (961 cm-1) that becomes forbidden in the planar structure. [ABSTRACT FROM AUTHOR]

Fluorescence emission spectra were recorded for isolated polymer chains of the polyfluorene copolymer, F8BT [poly(9,9-di-n-octylfluorene-alt-benzothiadiazole)], at 298 and 20 K for two molecular weights (chain lengths). Condon analysis reveals a decrease in emitting state displacements between spectra measured at 298 and 20K, suggesting that temperature-induced structural displacements are responsible for the change in the bimodal emission.

Vibronic progressions in both metal-ligand and ligand-centered symmetric stretching modes are described. Low-temperature luminescence spectra are used to determine these progressions.

The luminescence properties of thiocyanate and selenocyanate platinum (II) and Palladium (II), that shows strong variations with temperature and pressure, are described. The temperature and pressure-dependant spectroscopic results show large variations from ambient conditions.