Core structure dependence of cycloreversion dynamics in diarylethene analogs.

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.

Energy- and conformer-dependent excited-state relaxation of an E/Z photoswitchable thienyl-ethene.

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.

BN Doping and the Photochemistry of Polyaromatic Hydrocarbons: Photocyclization of Hexaphenyl Benzene and Hexaphenyl Borazine.

Boron-nitrogen doping of polyaromatic hydrocarbon (PAH) materials can be used to tune their electronic properties while preserving the structural characteristics of pure hydrocarbons. Many multicycle PAHs can be synthesized photochemically; in contrast, very little is known about the photochemistry of their BN-doped counterparts. We present results of fs, ns, and µs time-resolved spectroscopic studies on the photoinduced dynamics of hexaphenyl benzene and hexaphenyl borazine in order to examine how BN doping alters photochemical C-C bond formation via 6π electrocyclization as well as the stability of resulting cyclized structures. Ultrafast measurements reveal photoinduced behaviors reflecting differences in excited-state decay pathways for the two molecules, with hexaphenyl borazine relaxing from its excited state with a rate that is 2 orders of magnitude faster than that of hexaphenyl benzene (3.0 vs 428 ps). Tetraphenyl dihydrotriphenylene generated from hexaphenyl benzene is observed to reopen with a ∼2 µs lifetime controlled by entropic stabilization of the cyclized structure; in contrast, photoinduced dynamics appear to be complete within 100 ps after excitation of hexaphenyl borazine. This significant difference in photochemical dynamics is reflected in the cyclodehydrogentation yields obtained for the two reactants (25 vs 0% for hexaphenyl benzene and borazine, respectively). Quantum-chemical computations predict that BN doping gives rise to energetic destabilization and increased singlet diradical character in cyclized structures. These findings indicate that the polarized BN bonds of the borazine core adversely impact photochemical bond formation relative to analogous hydrocarbons.

Excited-State Deactivation Pathways and the Photocyclization of BN-Doped Polyaromatics.

Boron-nitrogen doping of polyaromatic hydrocarbons (PAH), such as borazine-core hexabenzocoronene, presents possibilities for tuning the properties of organic electronics and nanographene materials while preserving structural characteristics of pure hydrocarbons. Previous photochemical studies have demonstrated extension of a borazine-core PAH network (1,2:3,4:5,6-tris(o,o'-biphenylylene)borazine, 1) by photoinduced cyclodehydrogenation. We present steady-state and femtosecond-to-microsecond resolved spectroscopic studies of the photophysics of 1 and a related borazine-core PAH in order to characterize competing excited-state relaxation pathways that determine the efficacy of bond formation by photocyclization. Transient spectra evolve on time scales consistent with S1 fluorescence lifetimes (1-3 ns) to features that persist onto microsecond time scales. Nanosecond-resolved oxygen-quenching measurements reveal that long-lived metastable states are triplets rather than cyclized products. Determination of fluorescence and triplet quantum yields reveal that photochemical bond formation is a minor channel in the relaxation of 1 (∼5% or less), whereas highly efficient fluorescence and intersystem crossing result in negligible photoinduced bond formation in more extended borazine-core networks. Results of computational investigations at the RICC2 level reveal sizable barriers to cyclization on the S1 potential energy surfaces consistent with quantum yields deduced from experiment. Together these barriers and competing photophysical pathways limit the efficiency of photochemical synthesis of BN-doped polyaromatics.

Photoinduced Electron Transfer within Supramolecular Donor-Acceptor Peptide Nanostructures under Aqueous Conditions.

We report the synthesis, self-assembly, and electron transfer capabilities of peptide-based electron donor-acceptor molecules and supramolecular nanostructures. These modified peptides contain π-conjugated oligothiophene electron donor cores that are peripherally substituted with naphthalene diimide electron acceptors installed via imidation of site-specific lysine residues. These molecules self-assemble into one-dimensional nanostructures in aqueous media, as shown through steady-state absorption, photoluminescence, and circular dichroism spectra, as well as transmission electron microscopy. Excitation of the oligothiophene donor moieties results in electron transfer to the acceptor units, ultimately creating polar, charge-separated states that persist for over a nanosecond as observed with transient absorption spectroscopy. This study demonstrates how transient electric fields can be engineered into aqueous nanomaterials of biomedical relevance through external, temporally controlled photonic inputs.

Aminofluorination of Cyclopropanes: A Multifold Approach through a Common, Catalytically Generated Intermediate.

We have discovered a highly regioselective aminofluorination of cyclopropanes. Remarkably, four unique sets of conditions-two photochemical, two purely chemical-generated the same aminofluorinated adducts in good to excellent yields. The multiple, diverse ways in which the reaction could be initiated provided valuable clues that led to the proposal of a "unifying" chain propagation mechanism beyond initiation, tied by a common intermediate. In all, the proposed mechanism herein is substantiated by product distribution studies, kinetic analyses, LFERs, Rehm-Weller estimations of ΔGET, competition experiments, KIEs, fluorescence data, and DFT calculations. From a more physical standpoint, transient-absorption experiments have allowed direct spectroscopic observation of radical ion intermediates (previously only postulated or probed indirectly in photochemical fluorination systems) and, consequently, have provided kinetic support for chain propagation. Lastly, calculations suggest that solvent may play an important role in the cyclopropane ring-opening step.

Photoinduced Charge Separation in Molecular Silicon.

Interest in molecular silicon semiconductors arises from the properties shared with bulk silicon like earth abundance and the unique architectures accessible from a structure distinctly different than rigid π-conjugated organic semiconductors. We report ultrafast spectroscopic evidence for direct, photoinduced charge separation in molecular silicon semiconductors that supports the viability of molecular silicon as donor materials in optoelectronic devices. The materials in this study are σ-π hybrids, in which electron-deficient aromatic acceptors flank a σ-conjugated silicon chain. Transient absorption and femtosecond-stimulated Raman spectroscopy (FSRS) techniques revealed signatures consistent with direct, optical charge transfer from the silane chain to the acceptor; these signatures were only observed by probing excited-state structure. Our findings suggest new opportunities for controlling charge separation in molecular electronics.

Quenching of pH-Responsive Luminescence of a Benzoindolizine Sensor by an Ultrafast Hydrogen Shift.

Fluorescent-sensor design requires consideration of how photochemical dynamics control properties of a sensing state. Transient absorption (TA) spectroscopy reveals an ultrafast net [1,3]-hydrogen shift following excitation of a protonated methoxy benzoindolizine (bzi) sensor in solution. These photochemical dynamics explain a quenched pH-responsive fluorescence shift and dramatically reduced fluorescence quantum yield relative to other (e. g. methyl) bzi compounds that do not tautomerize. Calculations predict the energetic and structural feasibility for rearrangement in protonated bzi compounds, such that interaction between the pi-network and strongly electron-donating methoxyl must lower the barrier for suprafacial H or H+ shift across an allylic moiety. As bzi compounds broadly exhibit pH-responsive emission shifts, chemical interactions that modulate this electronic interaction and suppress tautomerization could be used to facilitate binding- or surface-specific acid-responsive sensing.

Structural Control of Nonadiabatic Photochemical Bond Formation: Photocyclization in Structurally Modified ortho-Terphenyls.

Understanding how molecular structure impacts the shapes of potential energy surfaces and prospects for nonadiabatic photochemical dynamics is critical for predicting and controlling the chemistry of molecular excited states. Ultrafast transient absorption spectroscopy was used to interrogate photoinduced, nonadiabatic 6π cyclization of a collection of ortho-terphenyls (OTP) modified with alkyl substituents of different sizes and electron-donating/withdrawing character positioned on its central and pendant phenyl rings. OTP alkylated at the 4,4″ and 4',5' positions of the pendant and central rings, respectively, exhibiting biphasic excited-state relaxation; this is qualitatively similar to relaxation of OTP itself, including a fast decrease in excited-state absorption (τ1 = 1-4 ps) followed by formation of metastable cyclized photoproducts (τ2 = 3-47 ps) that share common characteristic spectroscopic features for all substitutions despite variations in chemical nature of the substituents. By contrast, anomalous excited-state dynamics are observed for 3',6'dimethyl-OTP, in which the methyl substituents crowd the pendant rings sterically; time-resolved spectral dynamics and low photochemical reactivity with iodine reveal that methylation proximal to the pendant rings impedes nonadiabatic cyclization. Results from transient measurements and quantum-chemical calculations are used to decipher the nature of excited state relaxation mechanisms in these systems and how they are perturbed by mechanical, electronic, and steric interactions induced by substituents.

Molecular Switching via Multiplicity-Exclusive E/Z Photoisomerization Pathways.

Mutual exclusivity in the nature of forward and reserve isomerization pathways holds promise for predictably controlling responses of photoswitchable materials according to molecular structure or external stimuli. Herein we have characterized the E/Z photoisomerization mechanisms of the visible-light-triggered switch 1,2-dithienyl-1,2-dicyanoethene (4TCE) in chlorobenzene with ultrafast transient absorption spectroscopy. We observe that switching mechanisms occur exclusively by relaxation through electronic manifolds of different spin multiplicity: trans-to-cis isomerization only occurs via electronic relaxation within the singlet manifold on a time scale of 40 ps; in contrast, cis-to-trans isomerization is not observed above 440 nm, but occurs via two rapid ISC processes into and out of the triplet manifold on time scales of ∼2 ps and 0.4 ns, respectively, when excited at higher energies (e.g., 420 nm). Observation of ultrafast ISC in cis-4TCE is consistent with photoinduced dynamics of related thiophene-based oligomers. Interpretation of the photophysical pathways underlying these isomerization reactions is supported by the observation that cis-to-trans isomerization occurs efficiently via triplet-sensitized energy transfer, whereas trans-to-cis isomerization does not. Quantum-chemical calculations reveal that the T1 potential energy surface is barrierless along the coordinate of the central ethylene dihedral angle (θ) from the cis Franck-Condon region (θ = 175°) to geometries that are within the region of the trans ground-state well; furthermore, the T1 and S1 surfaces cross with a substantial spin-orbital coupling. In total, we demonstrate that E/Z photoswitching of 4TCE operates by multiplicity-exclusive pathways, enabling additional means for tailoring switch performance by manipulating spin-orbit couplings through variations in molecular structure or physical environment.

Structural control of nonadiabatic bond formation: the photochemical formation and stability of substituted 4a,4b-dihydrotriphenylenes.

Nonadiabatic photocyclization makes bonds and is the first step in the photoinduced cyclodehydrogenation of ortho-arenes to yield polycyclic aromatic hydrocarbons. How molecular structure alters potential-energy landscapes, excited-state dynamics, and stabilities of reactants and intermediates underlies the feasibility of desirable photochemistry. In order to gain insight into these structure-dynamics relationships, we have used femtosecond transient absorption spectroscopy (TAS) to examine photoinduced dynamics of 1,2,3-triphenylbenzene (TPB) and ortho-quaterphenyl (OQTP), phenyl-subsituted analogues of ortho-terphenyl (OTP). Dynamics of TPB and OTP are quite similar: TPB exhibits fast (7.4 ps) excited-state decay with concomitant formation and vibrational relaxation of 9-phenyl-dihydrotriphenylene (9-phenyl DHT). In contrast, photoexcited OQTP exhibits multistate kinetics leading to formation of 1-phenyl DHT. Excited-state calculations reveal the existence of two distinct minima on the OQTP S1 surface and, together with photophysical data, support a mechanism involving both direct cyclization by way of an asymmetric structure and indirect cyclization by way of a symmetric quinoid-like minimum. Temperature-dependent nanosecond TAS was utilized to assess the relative stabilities of intermediates, substantiating the observed trend in photochemical reactivity OTP > OQTP > TPB. In total, this work demonstrates how specific structural variations alter the course of the excited-state dynamics and photoproduct stability that underlies desired photochemistry.

Visible-light-triggered molecular photoswitch based on reversible E/Z isomerization of a 1,2-dicyanoethene derivative.

A designed bis(dithienyl) dicyanoethene-based, strictly E/Z photoswitch (4TCE) operates through state-selective (E and Z isomer) photoactivation with visible light. The E and Z isomers of 4TCE exhibit remarkably different spectroscopic characteristics, including a large separation (70 nm) in their absorption maxima (λ(max)) and a 2.5-fold increase in molar extinction coefficient from cis to trans. The energetically stable trans form can be completely converted to the cis form within minutes when exposed to white light, whereas the reverse isomerization occurs readily upon irradiation by blue light (λ<480 nm) or completely by thermal conversion at elevated temperatures. These features together with excellent thermal stability and photostability of both isomers make this new E/Z photoswitch a promising building block for photoswitchable materials that operate without the need for UV light.

Structural and solvent control of nonadiabatic photochemical bond formation: photocyclization of o-terphenyl in solution.

Elucidating the molecular dynamics that underlie photoinduced electrocyclization is a critical step toward controlling nonadiabatic photochemistry that enables bond formation. Here we present a comprehensive examination of the photochemical dynamics of o-terphenyl (OTP) in solution. Ultrafast transient absorption measurements demonstrate that OTP cyclizes upon 266 nm photoexcitation to form 4a,4b-dihydrotriphenylene (DHT) on a solvent-dependent time scale of 1.5-4 ps, considerably slower than the nonadiabatic cyclization of related diarylethenes. Correlations in these time scales versus bulk solvent properties reveal that mechanical rather than electrostatic solvent-solute interactions impact the excited-state relaxation rate, impeding nuclear dynamics leading toward the conical intersection for cyclization. In contrast, solvent-dependent mechanical interactions are observed to facilitate vibrational relaxation of DHT on time scales of 10-25 ps. DHT decays via thermally activated ring-opening with a lifetime of ∼46 ns in tetrahydrofuran, 12 orders of magnitude faster than dihydrophenanthrenes. We conclude that the differences in excited-state dynamics of OTP and diarylethenes and the relative stability of their cyclized products are determined by the relative strain induced by twisting the central carbon-carbon bond that bridges the terminal phenyl rings in each to enable bond formation. We relate these structure-dynamics relationships to the feasibility of photoinduced cyclodehydrogenation of o-arenes and design considerations for molecular photoswitches.

Ultrafast photo-induced nuclear relaxation of a conformationally disordered conjugated polymer probed with transient absorption and femtosecond stimulated Raman spectroscopies.

A combination of transient absorption (TAS) and femtosecond stimulated Raman (FSRS) spectroscopies were used to interrogate the photo-induced nuclear relaxation dynamics of poly(3-cyclohexyl,4-methylthiophene) (PCMT). The large difference in inter-ring dihedral angles of ground and excited-state PCMT make it an ideal candidate for studying large-amplitude vibrational relaxation associated with exciton trapping. Spectral shifting in the S1 TA spectra on sub-ps timescales (110 ± 20 and 800 ± 100 fs) is similar to spectroscopic signatures of excited-state relaxation observed with related photoexcited conjugated polymers and which have been attributed to exciton localization and a combination of resonant energy transfer and torsional relaxation, respectively. Measurements made with both techniques reveal fast PCMT S1 decay and triplet formation (τS1 = 25-32 ps), which is similar to the excited-state dynamics of short oligothiophenes and highly twisted polyconjugated molecules. On ultrafast timescales FSRS of S1 PCMT offers a new perspective on the nuclear dynamics that underlie localization of excitons in photoexcited conjugated polymers: Spectral dynamics in the C=C stretching region (1400-1600 cm(-1)) include a red-shift of the in-phase C=C stretching frequency, as well as a change in the relative intensity of in-phase and out-of-phase stretch intensities on a timescale of ∼100 fs. Both changes indicate an ultrafast vibrational distortion that increases the conjugation length in the region of the localized excitation and are consistent with exciton self-localization or trapping. Wavelength-dependent excited-state FSRS measurements further demonstrate that the C=C stretching frequency provides a useful spectroscopic handle for interrogating the degree of delocalization in excited conjugated polymers given the selectivity achieved via resonance enhancement.

Tuning Optical Properties of Plasmonic Aerosols through Ligand-Solvent Interactions.

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.

Exciton Transfer Between Extended Electronic States in Conjugated Inter-Polyelectrolyte Complexes.

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.

Permittivity Threshold and Thermodynamics of Integer Charge-Transfer Complexation for an Organic Donor-Acceptor Pair.

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.

Intramolecular Photoinduced Charge Transfer and Recombination Dynamics in Vinylarene Terminated Organosilanes.

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.

Multiphoton Control of 6π Photocyclization via State-Dependent Reactant-Product Correlations.

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.

Maximized Hole Trapping in a Polystyrene Transistor Dielectric from a Highly Branched Iminobis(aminoarene) Side Chain.

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.