We report on structure-dependent one- and two-photon induced photochemical dynamics of modified ortho-terphenyls (OTPs), model 6π electrocyclizing photoreactants that exhibit distinct branching between reactive and nonreactive nonradiative excited-state deactivation channels. Using both pump-probe transient absorption spectroscopy (TAS) and pump-repump-probe (PRP) TAS, we find an enhanced efficiency for cyclization over nonreactive deactivation with the addition of 4,4″ alkyl substitution on pendant rings. The nonreactive deactivation rate is largely unaffected by structural modifications, whereas the cyclization rate to form trans-4a,4b-dihydrotriphenylene (trans-DHT) decreases with increasing substituent size. Based on these observations, we conclude that the reactive and nonreactive deactivation channels involve distinctly different structural dynamics within low-lying electronic excited states to reach critical geometries (i.e., conical intersections). We show that the cis-DHT photoproduct yield obtained by resonant 1 + 1' excitation is sensitive to dynamics in the low-lying excited-states of OTP that are in turn sensitive to structural modifications. Within the sub-ps time regime, PRP and TAS signals evolve on significantly different timescales, highlighting that these methods have different contrasts for probing dynamics on low-lying potential energy surfaces.

We investigate the effects of incorporating the monomer 2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene (BTTT) into thin films of its corresponding polymer, poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT). We examine how this incorporation influences the film's morphology, charge storage capabilities, and dielectric properties. In tri-layer dielectric organic field-effect transistor devices with pentacene as the semiconductor layer, the addition of BTTT to the PBTTT-polystyrene dielectric layer results in increased drain currents and unique threshold voltage shift behaviors, indicating enhanced charge storage capabilities. The key step in this mechanism is that a constant portion of charge-stabilizing entities is generated continuously with the presence of applied voltage. Capacitance measurements show a peak in charge storage at low BTTT concentrations, followed by a decrease at higher concentrations. Notably, dielectric strength analysis using Weibull statistics indicates that films with 20% BTTT content exhibit higher voltage tolerance compared to pure PBTTT or polystyrene films. Spectroscopy and x-ray diffraction analysis reveal that BTTT addition compromises the original ordering of the PBTTT, with higher concentrations leading to more significant disruption, even though distinguishable BTTT domains are formed. We propose a mechanism where BTTT/PBTTT clusters form charge-stabilizing entities, leading to improved charge storage capability and dielectric strength. These findings provide insights into the distinct contributions of monomers in conjugated polymer films and their potential applications in organic electronic devices.

Artificial light harvesting, a process that involves converting sunlight into chemical potential energy, is considered to be a promising part of the overall solution to address urgent global energy challenges. Conjugated polyelectrolyte complexes (CPECs) are particularly attractive for this purpose due to their extended electronic states, tunable assembly thermodynamics, and sensitivity to their local environment. Importantly, ionically assembled complexes of conjugated polyelectrolytes can act as efficient donor-acceptor pairs for electronic energy transfer (EET). However, to be of use in material applications, we must understand how modifying the chemical structure of the CPE backbone alters the EET rate beyond spectral overlap considerations. In this report we investigate the dependence of the EET efficiency and rate on the electronic structure and excitonic wave function of the CPE backbone. To do so, we synthesized a series of alternating copolymers where the electronic states are systematically altered by introducing comonomers with electron withdrawing and electron-rich character while keeping the linear ionic charge density nearly fixed. We find evidence that the excitonic coupling may be significantly affected by the exciton delocalization radius, in accordance with analytical models based on the line-dipole approximation and quantum chemistry calculations. Our results imply that care should be taken when selecting CPE components for optimal CPEC EET. These results have implications for using CPECs as key components in water-based light-harvesting materials, either as standalone assemblies or as adsorbates on nanoparticles and thin films.

Diaryl thieno-[3,4-b]thiophenes (TT) are photoswitchable compounds that operate through reversible photoinduced cyclization/cycloreversion and have been designed specifically for integration within pi-conjugated polymers to switchably manipulate polymer electronic properties. Here we report on how cross conjugating the central TT moiety impacts photocyclization dynamics as interrogated using transient absorption spectroscopy (TAS) for a series of switches built with electron-rich substituents that have various electronic interaction strengths with the TT core. For cross-conjugated structures exhibiting a propensity to switch in steady-state photoconversion experiments, ultrafast TAS reveals signatures of rapid dynamics (occurring within <1-10 ps) similar to those observed for unsubstituted switches and that are consistent with photocyclization. In contrast, TAS reveals comparatively slower spectral dynamics (similar to 100 ps) that are not consistent with cyclization for switches that are cross-conjugated with substituents that have greater electronic interaction with the TT core and that exhibit no propensity to photoswitch in photoconversion experiments. Microsecond TAS confirms that photoinduced cyclization occurs for the former and that a metastable triplet state localized on the conjugated backbone is generated with the latter. We find that the balance of these two deactivation pathways is sensitive to the interaction strength of the conjugated substituents with the core, with select structures exhibiting signatures of both. These findings are consistent with prior work demonstrating that the LUMO character is delocalized over the switch backbone when there are strong interactions with cross-conjugating groups and reveal that the competition between deactivation pathways can be controlled structurally by weakening pi conjugation across the backbone.

Plasmonic nanoparticles are highly tunable light-harvesting materials with a wide array of applications in photonics and catalysis. More recently, there has been interest in using aerosolized plasmonic nanoparticles for cloud formation, airborne photocatalysts, and molecular sensors, all of which take advantage of the large scattering cross sections and the ability of these particles to support intense local field enhancement ("hot spots"). While extensive research has investigated properties of plasmonic particles in the solution phase, surfaces, and films, aerosolized plasmonics are relatively unexplored. Here, we demonstrate how the capping ligand, suspension solvent, and atomization conditions used for aerosol generation control the steady-state optical properties of aerosolized Silica@Au plasmonic nanoshells. Our experimental results, supported with spectral simulations, illustrate that ligand coverage and atomization conditions control the degree of solvent retention and thus the spectral characteristics and potential access to surfaces for catalysis in the aerosol phase, opening a new regime for tunable applications of plasmonic metamaterials.

Singlet fission, a process by which one singlet exciton is converted into two lower energy triplet excitons, is sensitive to the degree of electronic coupling within a molecular packing structure. Variations in molecular packing can be detrimental to triplet formation and triplet–triplet separation, ultimately affecting the harvesting of triplets for electricity in organic photovoltaic devices. Here, six phase‐pure molecular packing structures of 6,13‐bis(triisopropylsilylethynyl)pentacene (TIPS‐pentacene) with varying optoelectronic properties are isolated using 2D lead halide perovskites as tunable, crystalline surfaces for crystallization. Transient absorption spectroscopy reveals that while triplet formation is fast (<100 fs) regardless of template structure, the increased ordering in perovskite‐templated samples speeds up triplet–triplet separation and recombination, providing evidence that the benefits of phase‐purity offset minor variations in molecular packing. Molecular dynamics modeling of the interface reveals that perovskite‐templating allows for closer packing of TIPS‐pentacene molecules for all perovskite templates. With an extensive number of organic molecule‐perovskite pairings, this work provides a methodology to use ordered, periodic surfaces to elucidate structure–property relationships of small organic molecules in order to adjust structural or optoelectronic responses, such as molecular packing and singlet fission.

We demonstrate the use of sequence-dependent interactions between peptide-functionalized pi-conjugated pig-ments to tune exciton transport behavior in their supramolecular assemblies in aqueous solutions. Peptide sequences attached to a perylene diimide (PDI) core were selected based on their ability to foster a wide range of excitonic coupling strengths within organized assemblies, as reflected by experimentally measured steady-state absorption and emission spectra. Photoresponses of organized assemblies of weakly interacting PDIs closely resemble those observed for weakly assembled chromophores. In contrast, organized assemblies that support strong intermolecular coupling exhibit significant excitonic delocalization and transport, as observed through distinct transient signatures of fluence-dependent singlet-singlet annihilation and a short-lived biexcitonic state at high initial exciton densities. A one-dimensional (1D) diffusion model appropriately accounts for exciton-exciton encounters in assemblies, with effective exciton diffusion constants that scale with interchromophore coupling strength, ranging between 0 and 7 sites2/ps (0-1 nm2/ps or LD ranging from 0 to 40 nm). Variations in effective diffusion constant arise from peptide-tuned variations in interchromophore alignment and wavefunction overlap, with mixed Frenkel-CT exciton coupling identified as the key interaction that facilitates site-to-site exciton diffusion within PDI stacks. This work demonstrates the potential to use simple peptide sequence variation as a tool for rational engineering of exciton transport and other excited-state behaviors in supramolecular materials.

Molecular charge doping involves the formation of donor-acceptor charge-transfer complexes (CTCs) through integer or partial electron transfer; understanding how local chemical environment impacts complexation is important for controlling the properties of organic materials. We present steady-state and temperature-dependent spectroscopic investigations of the p-dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) complexed with the electron donor and hole transport material N,N'-diphenyl-N,N'-di-p-tolylbenzene-1,4-diamine (MPDA). Equilibrium formation constants (KCT) were determined for donor-acceptor pairs dissolved in a series of solvents covering a range of values of permittivity. A threshold for highly favorable complex formation was observed to occur at ϵ ∼ 8-9, with large (>104) and small (<103) values of KCT obtained in solvents of higher and lower permittivity, respectively, but with chloroform (ϵ = 4.81) exhibiting an anomalously high formation constant. Temperature-dependent formation constants were determined in order to evaluate the thermodynamics of complex formation. In 1,2-dichloroethane (ϵ = 10.36) and chlorobenzene (ϵ = 5.62), complex formation is both enthalpically and entropically favorable, with higher enthalpic and entropic stabilization in the solvent with higher permittivity. Complexation in chloroform is exothermic and entropically disfavored, indicating that specific, inner-shell solvent-solute interactions stabilize the charge-separated complex and result in a net increase in local solution structure. Our results provide insight into how modification to the chemical environment may be utilized to support stable integer charge transfer for molecular doping applications and requiring only modest changes in local permittivity.

Supramolecular complexes have great potential as light-harvestingmaterials, as intermolecular structural organization can be manipulatedthrough steric or electrostatic interactions to impact electroniccoupling between energy and charge donors and acceptors. Here, weexamine the relative rates and efficiencies of charge transfer inconjugated polymer electrolyte complexes (CPECs) based on a polythiopheneelectron donor (PTAK) and polyfluorene electron acceptor (PFPI). TheseCPECs are characterized by ordered polymer microstructures, as evidentfrom spectral signatures of strong excitonic coupling within the PTAKcomponent from steady-state UV-vis absorption spectra. We findthat PTAK polarons are generated within tens of picoseconds througha combination of prompt and delayed charge separation following directphotoexcitation or energy transfer from PFPI. Further, we find thatdecreasing the length of charged PTAK side chains or increasing excitationenergy increases the driving force for electron transfer to increasecharge separation rates and yields for polarons, with the greatestrelative yields observed at excitation energies that initiate PFPI-to-PTAKenergy transfer. Charge separation between components can be rationalizedfrom a canonical Marcus picture, whereby excess vibrational energyeffectively lowers the barrier for PTAK-to-PFPI charge separation.This contrasts with the recently reported ultrafast (<100 fs) chargeseparation in small-molecule/polythiophene electrolyte complexes thatis attributed to strong orbital mixing that gives rise to charge generationvia CT exciton states. These results provide insights into conditionsfor realizing charge separation in concert with energy transfer inCPECs as light-harvesting materials.

In this Viewpoint article, we describe an approach to enhance photosensitized homogeneous photocatalysis of the CO2 reduction reaction by leveraging ion-pairing interactions. Our goal was to link a cationic catalyst with an anionic photosensitizer to promote intermolecular electron transfer between the two, thereby enhancing catalytic turnover. Instead, we discovered that the cationic catalyst (complex 1) was photoreductively fragmented. Curiously, we still observed more active CO2 reduction in systems containing complex 1, when compared to control measurements. Herein describes the process of discovery and experiments, and the lessons acquired from the development of a system more prone to photoreductive degradation than catalytic turnover.

We synthesized polystyrene-block-polyanthrylaminomethylstyrene (PS-b-PAAS) via the RAFT reaction and side chain modification, where the PAAS block is 50%. By blending 2-aminoanthracene (2AA) with PS-b-PAAS, the coaggregation behavior of the PAAS block and 2AA within the thin film was observed by optical microscopy (OM) and X-ray diffraction. At 5 and 10 wt % 2AA within the thin film, OM results showed a clear morphological difference between the PS matrix and PS-b-PAAS matrix. Differences in the 2AA excited-state lifetimes in these matrices were also established by using transient absorption spectroscopy (TAS). Blends were used as gate dielectrics in the pentacene transistors. We compared characteristic voltage (V-c, an indicator of threshold voltage) before and after charging of PS-b-PAAS blended with unsubstituted PS and both polymers individually blended with 2AA. For blended PS-b-PAAS and PS, as the concentration of PS-b-PAAS increases, the decreased V-c especially with negative charging of the pentacene indicated a larger concentration of charge carriers induced by the dielectric film. The charging experiments also revealed that the unique structure formed by the 2AA/PS-b-PAAS blend significantly affected the charge storage capability of organic field-effect transistors (OFETs). This work demonstrates the novel coaggregation structure in the dielectrics and furthermore its impact on charge storage capability in pentacene OFETs.

Conjugated polyelectrolyte complexes (CPECs) are an artificial light-harvesting platform formed by pairing oppositely charged conjugated polyelectrolytes in solution. We demonstrate that selective pairing of poly[3-(potassium-4-alkanoate)thiophene-2,5-diyl] (PTAK) of various regioregularity and side-chain lengths with either methyl viologen or an electrolytic naphthalene diimide electron acceptor supports different PTAK microstructures based on specific donor-acceptor stacking relationships. Alteration in microstructure is signaled by distinct signatures of excitonic coupling in steady-state absorption spectra. More ordered PTAK microstructures are obtained in CPECs formed with regioregular PTAK and when the distance between charged groups on the acceptor and PTAK matches. Photoinduced dynamics in these CPECs are characterized by sub-100-fs PTAK-to-acceptor electron transfer with polaron-pair generation in PTAK quenched in higher-order complexes. Rates of subsequent multiphasic charge recombination on picosecond-to-nanosecond timescales are determined by structural characteristics associated with specific donor- acceptor pairings and acceptor-dependent driving force for recombination, with longer-lived charge pairs observed in highly ordered CPEC microstructures. CPECs also demonstrate structure-dependent sensitivity to excitation energy and intensity; excitation energies significantly exceeding PTAK band energy increase exciton delocalization, particularly in complexes with higher structural order. The structure dependence of charge-transfer behaviors in CPECs provides insights for inducing long-range charge separation in related materials for light-harvesting applications.

Conjugated polyelectrolyte complexes (CPECs) are promising aqueous-compatible materials for artificial light-harvesting applications that possess continuous one-dimensional pathways for exciton and charge delocalization and migration. We demonstrate how donor-acceptor composition in complexes of electrolytic polythiophene (PTAK) and naphthalene diimide (ENDI) impacts the microstructure of the polymer and donor-acceptor interaction, with consequences for photoinduced charge-pair formation and recombination. PTAK evolves from microstructures with H-like to random-coil to J-like excitonic coupling character with increasing ENDI/PTAK charge ratio, while ENDI exhibits weak J-like coupling at high acceptor densities that reflects ordering along PTAK strands. We observe sub-100-fs charge separation between PTAK and ENDI that implies close donor-acceptor orbital proximity and hot-exciton relaxation via charge-transfer exciton states. Multiphasic recombination is observed and reflects a distribution of charge-pair separation distances that is correlated with degree of exciton delocalization, which can be controlled with CPEC composition. Recombination timescales and the fraction of long-lived charge pairs increase with higher excitation energies, consistent with energy-dependent coupling between hot polymer and delocalized charge-transfer excitons. These results indicate that CPEC structure is characterized by an ordered donor-acceptor interface that could be used to induce long-range charge separation for the benefit of applications in artificial light harvesting and photosynthesis.

Multiphoton excitation promises opportunities for opening new photochemical reaction pathways and controlling photoproduct distributions. We demonstrate photonic control of the 6π photocyclization of ortho-terphenyl to make 4a,4b-dihydrotriphenylene (DHT). Using pump-repump-probe spectroscopy we show that 1 + 1' excitation to a high-lying reactant electronic state generates a metastable species characterized by a red absorption feature that accompanies a repump-induced depletion in the one-photon trans-dihydro product (trans-DHT); signatures of the new photoproduct are clearer for a structural analogue of the reactant that is sterically inhibited against one-photon cyclization. Quantum-chemical computations support assignment of this species to cis-DHT, which is accessible photochemically along a disrotatory coordinate from high-lying electronic states reached by 1 + 1' excitation. We use time-resolved spectroscopy to track photochemical dynamics producing cis-DHT. In total, we demonstrate that selective multiphoton excitation opens a new photoreaction channel in these photocyclizing reactants by taking advantage of state-dependent correlations between reactant and product electronic states.

We synthesized highly branched and electron-donating side chain subunits and attached them to polystyrene (PS) used as a dielectric layer in a pentacene field-effect transistor. The influence of these groups on dielectric function, charge retention, and threshold voltage shifts (ΔVth) depending on their positions in dielectric multilayers was determined. We compared the observations made on an N-perphenylated iminobisaniline side chain with those from the same side chains modified with ZnO nanoparticles and with an adduct formed from tetracyanoethylene (TCNE). We also synthesized an analogue in which six methoxy groups are present instead of two amine nitrogens. At 6 mol % side chain, hopping transport was sufficient to cause shorting of the gate, while at 2 mol %, charge trapping was observable as transistor threshold voltage shifts (ΔVth). We created three types of devices: with the substituted PS layer as single-layer dielectric, on top of a cross-linked PS layer but in contact with the pentacene (bilayers), and sandwiched between two PS layers in trilayers. Especially large bias stress effects and ΔVth, larger than those in the case of the hexamethoxy and previously studied dimethoxy analogues, were observed in the second case, and the effects increased with the increasing electron-donating properties of the modified side chains. The highest ΔVth was consistent with a majority of the side chains stabilizing the trapped charge. Trilayer devices showed decreased charge storage capability compared to previous work in which we used less donating side chains but in higher concentrations. The ZnO and TCNE modifications resulted in slightly more and less negative ΔVth, respectively, when the side chain polystyrene was not in contact with the pentacene and isolated from the gate electrode. The results indicate a likely maximum combination of molecular charge stabilizing activity and side chain concentration that still allows gate dielectric function.

We report on charge-transfer dynamics of newly designed acceptor-donor-acceptor organosilanes, with a specific focus on how donor-acceptor combination and local chemical environment can be used to control the lifetime for intramolecular charge-separation between silane electron donors and organic acceptors. In this work linear oligosilanes were capped with arene-vinyl end groups of variable electron-accepting strength: weak (diester vinyl), intermediate (ester,cyano vinyl), and strong (dicyanovinyl). Ultrafast transient absorption spectroscopy was used to characterize their structure-dependent charge-transfer and recombination behaviors. All structures exhibit similar photoinduced ultrafast spectral dynamics that we ascribe to relaxation of the nascent charge-separated excited state followed by a return to the ground state via charge recombination. We find that relaxation of the nascent "hot" charge-separated excited state scales with the strength of dipole-dipole interactions between solvent molecules and the polar arene-vinyl acceptor. Furthermore, electron-accepting strength governs whether electronic coupling dictates charge recombination rate: weak acceptors produce charge-separated states that exhibit relatively large electronic coupling for back-electron transfer (approaching the adiabatic limit) that result in fast recombination, whereas the strong and moderate-strength acceptors support more stable charge-separated states with weaker coupling and longer lifetimes. We find that recombination rates increase substantially for structures with weak and moderate-strength acceptors in cyclohexane (i.e., negligible solvent reorganization energy), which we attribute to an increased electronic coupling in a nonpolar solvent environment where charge pairs are weakly screened. In contrast, for structures with strong electron acceptors, the very low reorganization energy of cyclohexane places back-electron transfer even further into the Marcus inverted regime, with a resultant increase in charge-separation lifetime. Together these results provide critical insights on how to tune photoinduced charge-transfer behavior in organic-inorganic hybrids that have potential material applications in molecular electronics and optoelectronics.

Diarylperfluorocyclopentenes are a well-characterized class of molecular photoswitches that undergo reversible photocyclization. The efficiency of cycloreversion (<∼30%), in particular, is known to be limited by a competition with excited-state deactivation by internal conversion that is strongly impacted by the electron-withdrawing/donating character of pendant aryl groups. Here we present a first study to determine how varied structural motifs for the core bridge group impact excited-state dynamics that control cycloreversion quantum yields. Specifically, we compare photophysical behaviors of 3,3'-(perfluorocyclopent-1-ene-1,2-diyl)bis(2-methylbenzo[b]thiophene) with diarylethene derivatives possessing the same benzo[b]thiophene pendant group but with a rigid 1-methyl-1H-pyrrole-2,5-dione and a rigid/aromatic thieno[3,4-b]thiophene bridge (TT) core bridge group. We find that the flexible perfluorocyclopentene core undergoes cycloreversion 3-4× slower than the rigid core photoswitches (9 vs. 2-3 ps in acetonitrile, 25 vs. 5-6 ps in cyclohexane) despite comparable cycloreversion quantum yields. To distinguish effects induced by bridge vs. pendant groups, we also studied a series of photoswitches with the same thieno[3,4-b]thiophene bridging group, but with varied pendant groups including 2,5-dimethylthiophene and 2-(3,5-bis(trifluoromethyl)phenyl)-5-methylthiophene. Analysis of temperature-dependent excited-state lifetimes and cycloreversion quantum yields reveals that both the rates of nonreactive internal conversion and reactive cycloreversion increase with greater structural rigidity of the core. This difference is attributed to smaller energy barriers on the excited-state potential energy surface for both reactive and non-reactive deactivation from the 21A electronic state relative to the flexible perfluorocyclopentene switch, implying that a rigid core results in a net shallower excited-state potential energy surface.

F4TCNQ (2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane) is used widely as a hole-doping agent in photoresponsive organic semiconducting materials, yet relatively little is known about the pho...

Plasmonic aluminum nanoparticles have emerged as an exciting new materials platform due to the high natural abundance of aluminum, their ability to be synthesized in the solution phase, and the potential of these materials to be used for photocatalysis and sensing. However, the photothermal properties of solution-processed aluminum nanoparticles, in particular, how phonon energy transfer depends on the particle size and surface properties, are critical for practical applications and are currently unexplored. Here we use transient absorption spectroscopy, in combination with simulations of phonon and thermal energy dissipation, to investigate the photoresponses of aluminum nanoparticles of various diameters (54, 85, 121, and 144 nm) suspended in 2-propanol. Fast thermal-transfer rates to the solvent (170-280 ps) are observed for particles of all sizes and are facilitated by native oxide coverage, as verified by a two-interface thermal energy-transfer model. Size-dependent phonon "breathing"/vibrational modes are also observed as oscillations in the total cross-section. We find that both the oscillation frequency and the damping rate increase as the diameter of the particles decreases. On the basis of the results of finite element calculations, we attribute the damping strength and oscillation period observed to a combination of the noncrystalline nature of the native oxide shell and the presence of surface-bound ligands, both of which increase the vibrational mode damping rates relative to bare Al and Al particles with a bare crystalline oxide shell. These insights should guide future work on controlling energy transfer through the use of size and surface tuning in sustainable aluminum nanomaterial systems for applications in catalysis and sensing.

Bis(bithienyl)-1,2-dicyanoethene (4TCE) is a photoswitch that operates via reversible E/Z photoisomerization following absorption of visible light. cis-to-trans photoisomerization of 4TCE requires excitation below 470 nm, is relatively inefficient (quantum yield < 5%) and occurs via the lowest-lying triplet. We present excitation-wavelength dependent (565-420 nm) transient absorption (TA) studies to probe the photophysics of cis-to-trans isomerization to identify sources of switching inefficiency. TA data reveals contributions from more than one switch conformer and relaxation cascades between multiple states. Fast (∼4 ps) and slow (∼40 ps) components of spectral dynamics observed at low excitation energies (>470 nm) are readily attributed to deactivation of two conformers; this assignment is supported by computed thermal populations and absorption strengths of two molecular geometries (PA and PB) characterized by roughly parallel dipoles for the thiophenes on opposite sides of the ethene bond. Only the PB conformer is found to contribute to triplet population and the switching of cis-4TCE: high-energy excitation (<470 nm) of PB involves direct excitation to S2, relaxation from which prepares an ISC-active S1 geometry (ISC QY 0.4-0.67, kISC∼ 1.6-2.6 × 10-9 s-1) that is the gateway to triplet population and isomerization. We ascribe low cis-to-trans isomerization yield to excitation of the nonreactive PA conformer (75-85% loss) as well as loses along the PB S2→ S1→ T1 cascade (10-20% loss). In contrast, electrocyclization is inhibited by the electronic character of the excited states, as well as a non-existent thermal population of a reactive "antiparallel" ring conformation.