Vacancy control in acene blends links exothermic singlet fission to coherence.

The fission of singlet excitons into triplet pairs in organic materials holds great technological promise, but the rational application of this phenomenon is hampered by a lack of understanding of its complex photophysics. Here, we use the controlled introduction of vacancies by means of spacer molecules in tetracene and pentacene thin films as a tuning parameter complementing experimental observables to identify the operating principles of different singlet fission pathways. Time-resolved spectroscopic measurements in combination with microscopic modelling enables us to demonstrate distinct scenarios, resulting from different singlet-to-triplet pair energy alignments.

Pressure- and temperature-dependent inelastic neutron scattering study of the phase transition and phonon lattice dynamics in para-terphenyl.

Inelastic neutron scattering has been performed on para-terphenyl at temperatures from 10 to 200 K and under pressures from the ambient pressure to 1.51 kbar. The temperature dependence of phonons, especially low-frequency librational bands, indicates strong anharmonic phonon dynamics.

Using temperature dependent fluorescence to evaluate singlet fission pathways in tetracene single crystals.

The temperature-dependent fluorescence spectrum, decay rate, and spin quantum beats are examined in single tetracene crystals to gain insight into the mechanism of singlet fission. Over the temperature range of 250 K-500 K, the vibronic lineshape of the emission indicates that the singlet exciton becomes localized at 400 K. The fission process is insensitive to this localization and exhibits Arrhenius behavior with an activation energy of 550 ± 50 cm.

Using sulfur bridge oxidation to control electronic coupling and photochemistry in covalent anthracene dimers.

Covalently tethered bichromophores provide an ideal proving ground to develop strategies for controlling excited state behavior in chromophore assemblies. In this work, optical spectroscopy and electronic structure theory are combined to demonstrate that the oxidation state of a sulfur linker between anthracene chromophores gives control over not only the photophysics but also the photochemistry of the molecules. Altering the oxidation state of the sulfur linker does not change the geometry between chromophores, allowing electronic effects between chromophores to be isolated.

Sulfur-Bridged Terthiophene Dimers: How Sulfur Oxidation State Controls Interchromophore Electronic Coupling.

Symmetric dimers have the potential to optimize energy transfer and charge separation in optoelectronic devices. In this paper, a combination of optical spectroscopy (steady-state and time-resolved) and electronic structure theory is used to analyze the photophysics of sulfur-bridged terthiophene dimers. This class of dimers has the unique feature that the interchromophore (intradimer) electronic coupling can be modified by varying the oxidation state of the bridging sulfur from sulfide (S), to sulfoxide (SO), to sulfone (SO2).

A pressure sensor based on the orientational dependence of plasmonic properties of gold nanorods.

A novel pressure sensor has been developed by taking advantage of the orientational dependence of localized surface plasmon resonance of gold nanorods embedded in a polymer matrix. This stress-responsive material can be used to record the distribution and magnitude of pressure between two contacting surfaces by outputting optical response.

Pressure dependence of the forward and backward rates of 9-tert-butylanthracene Dewar isomerization.

9-tert-Butylanthracene undergoes a photochemical reaction to form its strained Dewar isomer, which thermally back-reacts to reform the original molecule. When 9-tert-butylanthracene is dissolved in a polymer host, we find that both the forward and reverse isomerization rates are pressure-dependent. The forward photoreaction rate, which reflects the sum of contributions from photoperoxidation and Dewar isomerization, decreases by a factor of 1000 at high pressure (1.

Pressure catalyzed bond dissociation in an anthracene cyclophane photodimer.

The anthracene cyclophane bis-anthracene (BA) can undergo a [4 + 4] photocycloaddition reaction that results in a photodimer with two cyclobutane rings. We find that the subsequent dissociation of the dimer, which involves the rupture of two carbon-carbon bonds, is strongly accelerated by the application of mild pressures. The reaction kinetics of the dimer dissociation in a Zeonex (polycycloolefin) polymer matrix were measured at various pressures and temperatures.

Vibrations of a chelated proton in a protonated tertiary diamine.

Vibrational spectra of the conjugate acid of Me(2)NCH(2)CH(2)CH(2)CH(2)NMe(2) (N,N,N',N'-tetramethylputrescine) have been examined in the gaseous and crystalline phases using Infrared Multiple Photon Dissociation (IRMPD) spectroscopy, Inelastic Neutron Scattering (INS), and high pressure Raman spectroscopy. A band observed near 530 cm(-1) is assigned to the asymmetric stretch of the bridging proton between the two nitrogens, based on deuterium substitution and pressure dependence. The NN distance measured by X-ray crystallography gives a good match to DFT calculations, and the experimental band position agrees with the value predicted from theory using a 2-dimensional potential energy surface.

Molecular simulation of the pressure-induced crystallographic phase transition of p-terphenyl.

The pressure- and temperature-induced polymorphic crystal phase transitions of p-terphenyl (PTP) have been modeled using a modified PCFF interaction force field. Modifications of the interaction potential were necessary to simultaneously model both the temperature-induced phase transition at ambient pressure and the pressure-induced phase transition at low temperature. Although the high-temperature and high-pressure phases are both characterized by flattening of the PTP molecule, the mechanisms of the temperature- and pressure-induced phase transitions are different.

High-pressure studies of optical dephasing in polymer glasses.

The effect of high pressure on the optical dephasing of chromophores in organic polymers at low temperature is evaluated within the stochastic sudden jump two-level-system (TLS) model. The approximations within the "standard" TLS model cannot account for the observed pressure dependence of the pure dephasing rate without ad hoc assumptions about changes in the TLS density of states. However, the photon echo model of Geva and Skinner for disordered systems can be used to model pressure-dependent optical dephasing results for a variety of doped polymer systems without assuming changes in the TLS density of states.

Characterization of tunneling systems in molecular versus polymer glasses by high-pressure photon echo spectroscopy.

Intrinsic differences between tunneling two-level systems (TLSs) in molecular versus polymeric glasses are revealed by studying the effect of compression on TLS dynamics. Photon echo studies under variable low-temperature (1.1-2.

Photoinduced kinetics of bacteriorhodopsin in a dried xerogel glass.

Time-resolved absorption measurements of the formation and decay kinetics of the M (deprotonated) photocycle intermediate of bR purple membranes entrapped within a dried xerogel glass have been investigated. The dramatic change observed for the M state decay time is in contrast to the relatively insensitive half life reported for the M intermediate of the D96N mutant entrapped within a dried sol-gel glass. The decay kinetics of the M intermediate was observed to slow by a factor of almost 100 when the solvent was removed from the wet-gel to form the dry xerogel glass.