Intramolecular Triplet Diffusion Facilitates Triplet Dissociation in a Pentacene Hexamer.

Triplet dynamics in singlet fission depend strongly on the strength of the electronic coupling. Covalent systems in solution offer precise control over such couplings. Nonetheless, efficient free triplet generation remains elusive in most systems, as the intermediate triplet pair 1 (T1 T1 ) is prone to triplet-triplet annihilation due to its spatial confinement. In the solid state, entropically driven triplet diffusion assists in the spatial separation of triplets, resulting in higher yields of free triplets. Control over electronic coupling in the solid state is, however, challenging given its sensitivity to molecular packing. We have thus developed a hexameric system (HexPnc) to enable solid-state-like triplet diffusion at the molecular scale. This system is realized by covalently tethering three pentacene dimers to a central subphthalocyanine scaffold. Transient absorption spectroscopy, complemented by theoretical structural optimizations and steady-state spectroscopy, reveals that triplet diffusion is indeed facilitated due to intramolecular cluster formation. The yield of free triplets in HexPnc is increased by a factor of up to 14 compared to the corresponding dimeric reference (DiPnc). Thus, HexPnc establishes crucial design aspects for achieving efficient triplet dissociation in strongly coupled systems by providing avenues for diffusive separation of 1 (T1 T1 ), while, concomitantly, retaining strong interchromophore coupling which preserves rapid formation of 1 (T1 T1 ).

Extended Tetrathiafulvalenes with Fluoreno[3,2-b]fluorene and Diindeno[1,2-b : 1',2'-i]anthracene Cores.

In one-dimensional polycyclic aromatic hydrocarbons (PAHs) containing five- and six-membered rings fused together, one key question is whether the structures possess a quinoidal or aromatic diradical character. Here, we generate such PAHs by reversible oxidation of PAH-extended tetrathiafulvalenes (TTFs). Extended TTFs were thus prepared and studied for their geometrical properties (crystallography), redox properties, and UV/Vis/NIR/EPR characteristics as a function of charge state. The EPR measurements of radical cations showed unique features for each PAH-TTF. The dications, formally composed of fluoreno[3,2-b]fluorene and diindeno[1,2-b:1',2'-i]anthracene cores, were experimentally found to exhibit singlet ground states. For the latter, calculations reveal the closed shell, quinoid singlet state to be isoenergetic with the open shell singlet diradical. Each charge state exhibited unique optical properties with radical cations absorbing strongly in the NIR region with signatures from π-dimers for the large core. The experimental results were paralleled and supported by detailed computations, including spin density distribution calculations, EPR simulations, and nucleus independent chemical shift (NICS) xy scans.

Synthesis and Properties of Fluorenone-Containing Cycloparaphenylenes and Their Late-Stage Transformation.

Cycloparaphenylenes (CPPs) are the smallest possible armchair carbon nanotubes, the properties of which strongly depend on their ring size. They can be further tuned by either peripheral functionalization or by replacing phenylene rings for other aromatic units. Here we show how four novel donor-acceptor chromophores were obtained by incorporating fluorenone or 2-(9H-fluoren-9-ylidene)malononitrile into the loops of two differently sized CPPs. Synthetically, we managed to perform late-stage functionalization of the fluorenone-based rings by high-yielding Knoevenagel condensations. The structures were confirmed by X-ray crystallographic analyses, which revealed that replacing a phenylene for a fused-ring-system acceptor introduces additional strain. The donor-acceptor characters of the CPPs were supported by absorption and fluorescence spectroscopic studies, electrochemical studies (displaying the CPPs as multi-redox systems undergoing reversible or quasi-reversible redox events), as well as by computations. The oligophenylene parts were found to comprise the electron donor units of the macrocycles and the fluorenone parts the acceptor units.

Synthesis of 2-(Methylthio)-1,3-dithioles from 1,3-Dithiole-2-thiones: Important Building Blocks in Tetrathiafulvalene Chemistry.

2-(Methylthio)-1,3-dithioles are important heterocyclic compounds used for the preparation of redox-active derivatives of tetrathiafulvalene as they serve as precursors for phosphonate esters that can be employed in Horner-Wadsworth-Emmons olefination reactions. Here, we present a mild and less hazardous method than previous methods for converting readily accessible 1,3-dithiole-2-thiones into 2-(methylthio)-1,3-dithioles by methylation with trimethyl orthoformate and HBF4·Et2O and a subsequent reduction with NaBH4.

Imparting Stability to Organic Photovoltaic Components through Molecular Engineering: Mitigating Reactions with Singlet Oxygen.

One key challenge in the development of viable organic photovoltaic devices is to design component molecules that do not degrade during combined exposure to oxygen and light. Such molecules should thus remain comparatively unreactive towards singlet molecular oxygen and not act as photosensitizers for the generation of this undesirable species. Here, novel redox-active chromophores that combine these two properties are presented. By functionalizing indenofluorene-extended tetrathiafulvalenes (IF-TTFs) with cyano groups at the indenofluorene core using Pd-catalyzed cyanation reactions, we find that the reactivity of the exocyclic fulvene carbon-carbon double bonds towards singlet oxygen is considerably reduced. The new cyano-functionalized IF-TTFs were tested in non-fullerene acceptor based organic photovoltaic proof-of-principle devices, revealing enhanced device stability.

Computational Evolution Of New Catalysts For The Morita-Baylis-Hillman Reaction.

We present a de novo discovery of an efficient catalyst of the Morita-Baylis-Hillman (MBH) reaction by searching chemical space for molecules that lower the estimated barrier of the rate-determining step using a genetic algorithm (GA) starting from randomly selected tertiary amines. We identify 435 candidates, virtually all of which contain an azetidine N as the catalytically active site, which is discovered by the GA. Two molecules are selected for further study based on their predicted synthetic accessibility and have predicted rate-determining barriers that are lower than that of a known catalyst. Azetidines have not been used as catalysts for the MBH reaction. One suggested azetidine is successfully synthesized and showed an eightfold increase in activity over a commonly used catalyst. We believe this is the first experimentally verified de novo discovery of an efficient catalyst using a generative model.

On the Mechanism of the Formal [2+2] Cycloaddition - Retro-electrocyclization (CA-RE) Reaction.

The [2+2] cycloaddition - retro-electrocyclization (CA-RE) reaction is a "click-like" protocol for facile synthesis of donor-acceptor chromophores from an alkyne and tetracyanoethylene. Herein we shed light on the mechanism of this reaction by detailed kinetics studies using 1 H NMR spectroscopy. By considering several experiments simultaneously, a variety of mechanistic models was evaluated. Surprisingly, a model in which the final 1,1,4,4-tetracyanobuta-1,3-diene product promoted the first step was the only one that described well the experimental data. This autocatalysis model also involved a non-concerted, stepwise formation of the cyclobutene cycloaddition adduct. By proper choice of conditions, we were able to generate the transient cyclobutene in sufficient amount to verify it as an intermediate using 13 C NMR spectroscopy. For its final retro-electrocyclization step, simple first-order kinetics was observed and only minor solvent dependence, which indicates a concerted reaction.

Dihydroazulene-Azobenzene-Dihydroazulene Triad Photoswitches.

Photoswitch triads comprising two dihydroazulene (DHA) units in conjugation with a central trans-azobenzene (AZB) unit were prepared in stepwise protocols starting from meta- and para-disubstituted azobenzenes. The para-connected triad had significantly altered optical properties and lacked the photoactivity of the separate photochromes. In contrast, for the meta-connected triad, all three photochromes could be photoisomerized to generate an isomer with two vinylheptafulvene (VHF) units and a cis-azobenzene unit. Ultrafast spectroscopy of the photoisomerizations revealed a fast DHA-to-VHF photoisomerization and a slower trans-to-cis AZB photoisomerization. This meta triad underwent thermal VHF-to-DHA back-conversion with a similar rate of all VHFs, independent of the identity of the neighboring units, and in parallel thermal cis-to-trans AZB conversion. The experimental observations were supported by computation (excitation spectra and orbital analysis of the transitions).

Triangular Rhodamine Triads and Their Intrinsic Photophysics Revealed from Gas-Phase Ion Fluorescence Experiments.

When ionic dyes are close together, the internal Coulomb interaction may affect their photophysics and the energy-transfer efficiency. To explore this, we have prepared triangular architectures of three rhodamines connected to a central triethynylbenzene unit (1,3,5-tris(buta-1,3-diyn-1-yl)benzene) based on acetylenic coupling reactions and measured fluorescence spectra of the isolated, triply charged ions in vacuo. We find from comparisons with previously reported monomer and dimer spectra that while polarization of the π-system causes redshifted emission, the separation between the rhodamines is too large for a Stark shift. This picture is supported by electrostatic calculations on model systems composed of three linear and polarizable ionic dyes in D3h configuration: The electric field that each dye experiences from the other two is too small to induce a dipole moment, both in the ground and the excited state. In the case of heterotrimers that contain either two rhodamine 575 (R575) and one R640 or one R575 and two R640, emission is almost purely from R640 although the polarization of the π-system expectedly diminishes the dipole-dipole interaction.

Fulvalene-Based Polycyclic Aromatic Hydrocarbon Ladder-Type Structures: Synthesis and Properties.

Polycyclic aromatic hydrocarbons (PAHs) have found strong interest for their electronic properties and as model systems for graphene. While PAHs have been studied intensively as single units, here PAHs were constructed in ladder-type arrangements using cross-conjugated fulvalene and dithiafulvalene motifs as connecting units and moving forward a convenient synthetic approach for dimerizing (thio)ketones into olefins by the action of Lawesson's reagent. Some of the PAHs can also be regarded as "super-extended" tetrathiafulvalenes (TTFs) with some of the largest cores ever explored, being multi-redox systems that exhibit both reversible oxidations and reductions. Concomitant absorption redshifts were observed when expanding the ladder-type structures from one to two to three indenofluorene units, and optical and electrochemical HOMO-LUMO gaps were found to correlate linearly. Various conformations (and solid-state packing arrangements) were studied by X-ray crystallography and computations.

Gas-Phase Ion Fluorescence Spectroscopy of Tailor-made Rhodamine Homo- and Heterodyads: Quenching of Electronic Communication by π-Conjugated Linkers.

While many key photophysical features are understood for electronic communication between chromophores in neutral compounds, there is limited information on the effect of charges in practically relevant ionic chromo/fluorophores. Here we have chosen positively charged rhodamines and prepared a selection of homo- and heterodimers with alkyl or π-conjugated, acetylenic bridges. Protonated molecules were transferred as isolated ions to gas phase where there is no solvent screening of charges, and fluorescence spectra were measured with a custom-made ion-trap setup. Our work reveals strong polarization of the π-spacer (induced dipole/quadrupole) when it experiences the electric field from one/ two dyes. Hence, π-spacers provide efficient shielding of charges by reducing the Coulomb interaction, whereas two dye cations polarize each other when connected by an alkyl. The screening influences the Förster resonance energy transfer efficiency that relies on the dipole-dipole interaction.

Orthogonal Photoswitching with Norbornadiene.

Orthogonal photoswitching is a convenient but challenging way of controlling multiple functions in a system by selective photoisomerization of one unit before the other in any arbitrarily chosen sequence. Here, we present this concept for the norbornadiene/quadricyclane (NBD/QC) photo/thermo-switch in the presence or absence of a coordinated metal ion. Thus, introducing two pyridyl ligands via ethyne-1,2-diyl bridges provides a system that by chelation of metal ions, such as PdII , has altered optical and switching properties. Mixing the PdII complex with its free ligand furnishes a four-state system where NBD-to-QC photoisomerizations for complexed and uncomplexed species are controlled by the irradiation wavelength and can occur orthogonally, that is, the sequence of photoisomerizations can be swapped. Studies on AgI and PbII complexes, being less stable than the PdII complex, are also presented; these exhibit like the PdII complex significantly red-shifted NBD absorptions.

Multi-Photochromic Molecules Based on Dihydroazulene Units.

Multi-photochromic systems incorporating individually addressable switching units are attractive for development of advanced data storage devices. Here, we present the synthesis and properties of a selection of such molecular systems incorporating the dihydroazulene/vinylheptafulvene (DHA/VHF) photo-/thermoswitch. The influence of the linker (meta-phenylene vs. azulene-1,3-diyl vs. thiophene-2,5-diyl) separating two DHA units on the switching properties was investigated. An azulene-1,3-diyl spacer largely inhibited both the DHA-to-VHF photoisomerizations and the thermal VHF-to-DHA back-reactions; the latter occurred ten times slower than for the related compound with a meta-phenylene spacer. A DHA trimer containing three DHA units around a central benzene ring was found to undergo stepwise DHA-to-VHF photoisomerizations, whereas the thermal back-reactions occurred at similar rates for the three VHF entities. A meta-phenylene-bridged DHA dimer was subjected to further structural modifications at position C-1 of each DHA, having strong implications for the switching events, and synthetic steps for further functionalizations at position C-7 of each DHA were investigated. Finally, the molecular structure (from X-ray crystallographic analysis) between the meta-phenylene-bridged DHA dimer and CuI is presented.

Tuning Redox Properties and Self-Assembly of Thienoacene-Extended Tetrathiafulvalenes.

Turning on and off associations between molecules by a reversible change in their redox states is a convenient way of controlling self-assembly and hence for advancing supramolecular chemistry. Here we present systematic studies on a selection of extended tetrathiafulvalenes with thienoacene spacers. By cyclic and differential pulse voltammetry and in situ EPR/UV-Vis-NIR spectroelectrochemistry, in combination with computations, we have elucidated how the number and orientations of thiophene rings in the spacer between the two dithiafulvene rings influence both the donor strength and association properties. The radical cations and their associates were found to cover a remarkable large region of the UV-Vis-NIR spectrum, but the appearance of the absorption spectrum of the radical cations as well as of the unassociated dications also depended strongly on the exact molecular structure.

Computational and Experimental Evidence of Two Competing Thermal Electrocyclization Pathways for Vinylheptafulvene.

The thermal electrocyclic ring-closure reaction of vinylheptafulvene (VHF) to form dihydroazulene (DHA) is elucidated herein by using DFT and 1 H NMR spectroscopy. Two different transition states were found computationally; one corresponds to a disrotatory pathway, which is allowed according to the Woodward-Hoffmann selection rules, whereas the other corresponds to a conrotatory pathway. The conrotatory pathway is found to be zwitterionic in the transition state, whereas the disrotatory transition state varies in zwitterionic character depending on solvent and substituents in the molecular framework. The conrotatory and disrotatory transition states are found to have similar energy and their relative stability varies with solvent polarity and functionalization at the C1 position. To support these findings, we chemically ring-opened diastereomerically pure 1-(benzothiazol-2-yl)-DHA to give the VHF form, then subsequently thermally reconverted the VHF to DHA in a range of solvents with various polarities. We found that, depending on solvent polarity, different ratios of anti- and syn-diastereoisomers of DHA were formed in a systematic manner, which supports the existence of two distinct thermal ring-closure pathways for VHF.

Luminescence Spectroscopy of Rhodamine Homodimer Dications in Vacuo Reveals Strong Dye-Dye Interactions.

Being alone or together makes a difference for the photophysics of dyes but for ionic dyes it is difficult to quantify the interactions due to solvent screening and nearby counter ions. Gas-phase luminescence experiments are desirable and now possible based on recent developments in mass spectrometry. Here we present results on tailor-made rhodamine homodimers where two dye cations are separated by methylene linkers, (CH2 )n . In solution the fluorescence is almost identical to that from the monomer whereas the emission from bare cation dimers redshifts with decreasing n. In the absence of screening, the electric field from the charge on one dye is strong enough to polarize the other dye, both in the ground state and in the excited state. An electrostatic model based on symmetric dye responses (equal induced-dipole moments in ground state) captures the underlying physics and demonstrates interaction even at large distances. Our results have possible implications for gas-phase Förster Resonance Energy Transfer.

Single-molecule detection of dihydroazulene photo-thermal reaction using break junction technique.

Charge transport by tunnelling is one of the most ubiquitous elementary processes in nature. Small structural changes in a molecular junction can lead to significant difference in the single-molecule electronic properties, offering a tremendous opportunity to examine a reaction on the single-molecule scale by monitoring the conductance changes. Here, we explore the potential of the single-molecule break junction technique in the detection of photo-thermal reaction processes of a photochromic dihydroazulene/vinylheptafulvene system. Statistical analysis of the break junction experiments provides a quantitative approach for probing the reaction kinetics and reversibility, including the occurrence of isomerization during the reaction. The product ratios observed when switching the system in the junction does not follow those observed in solution studies (both experiment and theory), suggesting that the junction environment was perturbing the process significantly. This study opens the possibility of using nano-structured environments like molecular junctions to tailor product ratios in chemical reactions.

A Study of Electrocyclic Reactions in a Molecular Junction: Mechanistic and Energetic Requirements for Switching in the Coulomb Blockade Regime.

Molecular photoswitches incorporated in molecular junctions yield the possibility of light-controlled switching of conductance due to the electronic difference of the photoisomers. Another isomerization mechanism, dark photoswitching, promoted by a voltage stimulus rather than by light, can be operative in the Coulomb blockade regime for a specific charge state of the molecule. Here we elucidate theoretically the mechanistic and thermodynamic restrictions for this dark photoswitching for donor-acceptor substituted 4n and 4n+2 π-electron open-chain oligoenes (1,3-butadiene and 1,3,5-hexatriene) by considering the molecular energies and orbitals of the molecules placed in a junction. For an electrocyclic ring closure reaction to occur for these compounds, we put forward two requirements: a) the closed stereoisomer (cis or trans form) must be of lower energy than the open form, and b) the reaction pathway must be in accordance to the orbital symmetry rules expressed by the Woodward-Hoffmann rules (when the electrodes do not significantly alter the molecular orbital appearances). We find these two requirements to be valid for the dianion of (1E,3Z,5E)-hexa-1,3,5-triene-1,6-diamine, and the Coulomb blockade diamonds were therefore modeled for this compound to elucidate how a dark photoswitching event would manifest itself in the stability plot. From this modeling of conductance as a function of gate and bias potentials, we predict a collapse in Coulomb diamond size, that is, a decrease in the height of the island of zero conductance.

Tracking molecular resonance forms of donor-acceptor push-pull molecules by single-molecule conductance experiments.

The ability of molecules to change colour on account of changes in solvent polarity is known as solvatochromism and used spectroscopically to characterize charge-transfer transitions in donor-acceptor molecules. Here we report that donor-acceptor-substituted molecular wires also exhibit distinct properties in single-molecule electronics under the influence of a bias voltage, but in absence of solvent. Two oligo(phenyleneethynylene) wires with donor-acceptor substitution on the central ring (cruciform-like) exhibit remarkably broad conductance peaks measured by the mechanically controlled break-junction technique with gold contacts, in contrast to the sharp peak of simpler molecules. From a theoretical analysis, we explain this by different degrees of charge delocalization and hence cross-conjugation at the central ring. Thus, small variations in the local environment promote the quinoid resonance form (off), the linearly conjugated (on) or any form in between. This shows how the conductance of donor-acceptor cruciforms is tuned by small changes in the environment.