Predicting aromatic exciplex fluorescence emission energies

PAH dimerization has been widely posited to play an important, even rate-determining role in soot nucleation, despite scanty experimental evidence of the existence of PAH dimers in flames. Laser-induced fluorescence (LIF) offers a promising in situ method of identifying PAH dimers, if dimer fluorescence can be distinguished from the fluorescence of the constituent monomers and other species present. Predicting transition energies for excited dimers (excimers) and excited complexes (exciplexes) represents a significant challenge for theory. Nonempirically tuned LC-BLYP functionals have been used to compute excited-state geometries and emission energies for a database of 81 inter- and intramolecular PAH excimers and exciplexes. Exciplex emission energies depend sensitively on the topology of the PAHs involved, but a linear relationship between the mean monomer bandgap and the computed exciplex emission means that dimer electronic properties can be predicted based on the properties of the constituent monomers. The range of fluorescence energies calculated for structures containing small to moderately-sized PAHs indicates that either noncovalent or aliphatically-linked complexes could generate the visible-range fluorescence energies observed in LIF experiments

Predicting the photoresponse of soot nuclei: Spectroscopic characteristics of aromatic aggregates containing five-membered rings

Establishing the mechanism for soot nucleation will require in situ experimental characterization of the identity and intermolecular interactions of the initial precursors, and electronic spectroscopy methods have the potential to do both. However, the optical response of polycyclic aromatic hydrocarbon (PAH) dimers and complexes differs significantly from that of the constituent monomers, and studies of soot precursor complexes have largely been limited to PAHs containing only six-membered aromatic rings. Hydrocarbons containing unsaturated five-membered rings are also present in high concentration in flames, and the photoresponse of complexes containing five-membered rings has not yet been examined. In this work, we elucidate the spectroscopic properties of small hydrocarbon complexes containing cyclopentadienyl groups or cyclopenta-fused groups, calculating ground- and excited-state binding energies, optimal excited-state geometries, repulsion energies, and fluorescence emission wavelengths. We show that excited-state distortion of cyclopentadienyl groups allows strong binding and low-energy fluorescence emission compared to similar-sized PAHs, and that a cyclopenta-fused group dramatically lowers the absorption and emission energies for acenaphthylene, dominating excited-state noncovalent interactions—findings that could shed light on the complex electronic properties of flames

Turning Performance in Squid and Cuttlefish: Unique Dual-Mode, Muscular Hydrostatic Systems

Although steady swimming has received considerable attention in prior studies, unsteady swimming movements represent a larger portion of many aquatic animals\u27 locomotive repertoire and have not been examined extensively. Squids and cuttlefishes are cephalopods with unique muscular hydrostat-driven, dual-mode propulsive systems involving paired fins and a pulsed jet. These animals exhibit a wide range of swimming behavior, but turning performance has not been examined quantitatively. Brief squid, Lolliguncula brevis, and dwarf cuttlefish, Sepia bandensis, were filmed during turns using high-speed cameras. Kinematic features were tracked, including the length-specific radius of the turn (R/L), a measure of maneuverability, and angular velocity (ω), a measure of agility. Both L. brevis and S. bandensis demonstrated high maneuverability, with (R/L)min values of 3.4x10(-3)+/- 5.9x10(-4) and 1.2x10(-3)+/- 4.7x10(-4) (mean +/- s.e.m.), respectively, which are the lowest measures of R/L reported for any aquatic taxa. Lolliguncula brevis exhibited higher agility than S. bandensis (ωa,max=725.8 versus 485.0 deg s-1), and both cephalopods have intermediate agility when compared with flexible-bodied and rigid-bodied nekton of similar size, reflecting their hybrid body architecture. In L. brevis, jet flows were the principal driver of angular velocity. Asymmetric fin motions played a reduced role, and arm wrapping increased turning performance to varying degrees depending on the species. This study indicates that coordination between the jet and fins is important for turning performance, with L. brevis achieving faster turns than S. bandensis and S. bandensis achieving tighter, more controlled turns than L. brevis

Turning Performance of Brief Squid Lolliguncula Brevis During Attacks on Shrimp and Fish

Although squid are generally considered to be effective predators, little is currently known of how squid maneuver and position themselves during prey strikes. In this study, high-speed video and kinematic analyses were used to study attacks by the brief squid Lolliguncula brevis on both shrimp and fish. Squid attack successwas high (\u3e80%) and three behavioral phases were identified: (1) approach, (2) strike and (3) recoil. Lolliguncula brevis demonstrated greater maneuverability (i.e. a smaller length-specific turning radius) and employed more body adjustments (i.e. mantle angle posturing) during approaches toward shrimp versus fish. Squid exhibited higher linear approach/strike velocities and accelerations with faster-swimming fish prey compared with slower shrimp prey. Agility (i.e. turning rate) during prey encounters was comparable to performance extremes observed during non-predatory turns, and did not differ according to prey type or distance. Despite having the ability to modulate tentacle extension velocity, squid instead increased their own swimming velocity rather than increasing tentacle velocity when targeting faster fish prey during the strike phase, but this was not the case for shrimp prey. Irrespective of prey type, L. brevis consistently positioned themselves above the prey target prior to the tentacle strike, possibly to facilitate a more advantageous downward projection of the tentacles. During the recoil, L. brevis demonstrated length-specific turning radii similar to those recorded during the approach despite vigorous escape attempts by some prey. Clearly, turning performance is integral to prey attacks in squid, with differences in attack strategy varying depending on the prey target

Exciplex Stabilization in Asymmetric Acene Dimers

Excimers play an important role in photochemical processes ranging from singlet fission to DNA damage, and the characteristic red-shift in fluorescence spectra associated with excimer formation can provide information about aggregate formation and the orientation of chromophores. When a mixture of chromophores is present, exciplex formation may lead to spectral characteristics distinct from those of either monomer or the corresponding excimers. To predict the effects of aggregation in a system containing a mixture of small acenes, binding energies and minimum-energy geometries have been calculated for three mixed S_1 exciplexes. Benchmark CASSCF/NEVPT2 multireference binding energies of 18.2, 27.7, and 49.3 kJ/mol are reported for the benzene-naphthalene, benzene-anthracene, and naphthalene-anthracene exciplexes, respectively. TDDFT calculations have been performed using a range of exchange-correlation functionals, showing that many functionals perform inconsistently, and the error in binding energy often depends on the character of the monomer excitation from which the exciplex state is derived. Moderate exciplex stabilization observed for the benzene-naphthalene and naphthalene-anthracene exciplexes results from a mixture of charge transfer and exciton delocalization

Predicting aromatic exciplex fluorescence emission energies

PAH dimerization has been widely posited to play an important, even rate-determining role in soot nucleation, despite scanty experimental evidence of the existence of PAH dimers in flames. Laser-induced fluorescence (LIF) offers a promising in situ method of identifying PAH dimers, if dimer fluorescence can be distinguished from the fluorescence of the constituent monomers and other species present. Predicting transition energies for excited dimers (excimers) and excited complexes (exciplexes) represents a significant challenge for theory. Nonempirically tuned LC-BLYP functionals have been used to compute excited-state geometries and emission energies for a database of 81 inter- and intramolecular PAH excimers and exciplexes. Exciplex emission energies depend sensitively on the topology of the PAHs involved, but a linear relationship between the mean monomer bandgap and the computed exciplex emission means that dimer electronic properties can be predicted based on the properties of the constituent monomers. The range of fluorescence energies calculated for structures containing small to moderately-sized PAHs indicates that either noncovalent or aliphatically-linked complexes could generate the visible-range fluorescence energies observed in LIF experiments

Exciplex Stabilization in Asymmetric Acene Dimers

Excimers play an important role in photochemical processes ranging from singlet fission to DNA damage, and the characteristic red-shift in fluorescence spectra associated with excimer formation can provide information about aggregate formation and the orientation of chromophores. When a mixture of chromophores is present, exciplex formation may lead to spectral characteristics distinct from those of either monomer or the corresponding excimers. To predict the effects of aggregation in a system containing a mixture of small acenes, binding energies and minimum-energy geometries have been calculated for three mixed S_1 exciplexes. Benchmark CASSCF/NEVPT2 multireference binding energies of 18.2, 27.7, and 49.3 kJ/mol are reported for the benzene-naphthalene, benzene-anthracene, and naphthalene-anthracene exciplexes, respectively. TDDFT calculations have been performed using a range of exchange-correlation functionals, showing that many functionals perform inconsistently, and the error in binding energy often depends on the character of the monomer excitation from which the exciplex state is derived. Moderate exciplex stabilization observed for the benzene-naphthalene and naphthalene-anthracene exciplexes results from a mixture of charge transfer and exciton delocalization

Multireference exciplex binding energies: Basis set convergence and error

In multichromophore systems, characterization of electronic structure requires characterization of exciplexes, electron‐hole pairs delocalized over multiple molecules. Computing exciplex binding energy requires an accurate description of both the noncovalent interactions between the chromophores and their excited electronic states. The critical role of basis set selection for accurate description of noncovalent interactions is well known, but for some of the most accurate excited‐state methods, basis set dependence is incompletely understood. In this work, the impact of basis set size and diffuseness on CASSCF/NEVPT2 binding energies is determined for three systems in their lowest singlet excited states: the benzene excimer, the cis‐butadiene‐benzene exciplex, and the benzene‐naphthalene exciplex. We demonstrate that excellent CBS binding energies may be obtained using the moderately‐sized jun‐cc‐pV(D + d)Z and jun‐cc‐pV(T + d)Z basis sets and a simple N^(−3) model. Repeating this procedure with the N = 3, 4 basis sets from the most diffuse basis set family applied to each system yields a binding energy of 56.6 ± 1.2 kJ/mol for the benzene excimer and binding energies of 11.1 ± 0.5 kJ/mol and 19.2 ± 1.7 kJ/mol for the cis‐butadiene‐benzene exciplex and the benzene‐naphthalene exciplex, respectively

Volumetric Flow Imaging Reveals the Importance of Vortex Ring Formation in Squid Swimming Tail-First and Arms-First

Squids use a pulsed jet and fin movements to swim both arms-first (forward) and tail-first (backward). Given the complexity of the squid multi-propulsor system, 3D velocimetry techniques are required for the comprehensive study of wake dynamics. Defocusing digital particle tracking velocimetry, a volumetric velocimetry technique, and high-speed videography were used to study arms-first and tail-first swimming of brief squid Lolliguncula brevis over a broad range of speeds [0-10 dorsal mantle lengths (DML) s-1) in a swim tunnel. Although there was considerable complexity in the wakes of these multi-propulsor swimmers, 3D vortex rings and their derivatives were prominent reoccurring features during both tail-first and arms-first swimming, with the greatest jet and fin flow complexity occurring at intermediate speeds (1.5-3.0 DML s-1). The jet generally produced the majority of thrust during rectilinear swimming, increasing in relative importance with speed, and the fins provided no thrust at speeds \u3e4.5 DML s-1. For both swimming orientations, the fins sometimes acted as stabilizers, producing negative thrust (drag), and consistently provided lift at low/intermediate speeds (\u3c2.0 DML s-1) to counteract negative buoyancy. Propulsive efficiency (η) increased with speed irrespective of swimming orientation, and eta for swimming sequences with clear isolated jet vortex rings was significantly greater (η=78.6 +/- 7.6%, mean +/- s.d.) than that for swimming sequences with clear elongated regions of concentrated jet vorticity (η=67.9 +/- 19.2%). This study reveals the complexity of 3D vortex wake flows produced by nekton with hydrodynamically distinct propulsors

Perceptions of the mental health impact of intimate partner violence and health service responses in Malawi

Background and objectives: This study explores the perceptions of a wide range of stakeholders in Malawi towards the mental health impact of intimate partner violence (IPV) and the capacity of health services for addressing these. Design: In-depth interviews (IDIs) and focus group discussions (FGDs) were conducted in three areas of Blantyre district, and in two additional districts. A total of 10 FGDs, 1 small group, and 14 IDIs with health care providers; 18 FGDs and 1 small group with male and female, urban and rural community members; 7 IDIs with female survivors; and 26 key informant interviews and 1 small group with government ministry staff, donors, gender-based violence service providers, religious institutions, and police were conducted. A thematic framework analysis method was applied to emerging themes. Results: The significant mental health impact of IPV was mentioned by all participants and formal care seeking was thought to be impeded by social pressures to resolve conflict, and fear of judgemental attitudes. Providers felt inadequately prepared to handle the psychosocial and mental health consequences of IPV; this was complicated by staff shortages, a lack of clarity on the mandate of the health sector, as well as confusion over the definition and need for ‘counselling’. Referral options to other sectors for mental health support were perceived as limited but the restructuring of the Ministry of Health to cover violence prevention, mental health, and alcohol and drug misuse under a single unit provides an opportunity. Conclusion: Despite widespread recognition of the burden of IPV-associated mental health problems in Malawi, there is limited capacity to support affected individuals at community or health sector level. Participants highlighted potential entry points to health services as well as local and national opportunities for interventions that are culturally appropriate and are built on local structures and resilience