Optically induced charge-transfer in donor-acceptor-substituted p- and m- C2B10H12 carboranes.

Icosahedral carboranes, C2B10H12, have long been considered to be aromatic but the extent of conjugation between these clusters and their substituents is still being debated. m- and p-Carboranes are compared with m- and p-phenylenes as conjugated bridges in optical functional chromophores with a donor and an acceptor as substituents here. The absorption and fluorescence data for both carboranes from experimental techniques (including femtosecond transient absorption, time-resolved fluorescence and broadband fluorescence upconversion) show that the absorption and emission processes involve strong intramolecular charge transfer between the donor and acceptor substituents via the carborane cluster. From quantum chemical calculations on these carborane systems, the charge transfer process depends on the relative torsional angles of the donor and acceptor groups where an overlap between the two frontier orbitals exists in the bridging carborane cluster.

Structure and Photophysics of N-Tolanyl-phenochalcogenazines and their Radical Cations.

The study focuses on the structural and photophysical characteristics of neutral and oxidized forms of N-tolanyl-phenochalcogenazines PZX-tolan with X=O, S, Se, and Te. X-ray crystal structure analyses show a pseudo-equatorial (pe) structure of the tolan substituent in the O, S, and Se dyads, while the Te dyad possesses a pseudo-axial (pa) structure. DFT calculations suggest the pe structure for O and S, and the pa structure for Se and Te as stable forms. Steady-state and femtosecond-time resolved optical spectroscopy in toluene solution indicate that the O and S dyads emit from a CT state, whereas the Se and Te dyads emit from a tolan-localized state. The T1 state is tolan-localized in all cases, showing phosphorescence at 77 K. The heavy atom effect of chalcogens induces intersystem crossing from S1 to Tx, resulting in a decreasing S1 lifetime from 2.1 ns to 0.42 ps. The T1 states possess potential for singlet oxygen sensitization with a high quantum yield (ca. 40 %) for the O, S, and Se dyads. Radical cations exhibit spin density primarily localized at the heterocycle. EPR measurements and quasirelativistic DFT calculations reveal a very strong g-tensor anisotropy, supporting the pe structure for the S and Se derivatives.

The influence of hindered rotation on electron transfer and exchange interaction in triarylamine-triptycene-perylene diimide triads.

Stretched electron-donor-bridge-acceptor triads that exhibit intramolecular twisting degrees of freedom are capable of modulating exchange interaction (J) as well as electronic couplings through variable π-overlap at the linear bond links, affecting the rate constants of photoinduced charge separation and recombination. Here we present an in-depth investigation of such effects induced by methyl substituents leading to controlled steric hindrance of intramolecular twisting around biaryl axes. Starting from the parent structure, consisting of a triphenyl amine donor, a triptycene (TTC) bridge and a phenylene-perylene diimide acceptor (Me0), one of the two phenylene linkers attached to the TTC was ortho-substituted by two methyl groups (Me2, Me3), or both such phenylene linkers by two pairs of methyl groups (Me23). Photoinduced charge separation (kCS) leading to a charge-separated (CS) state was studied by fs-laser spectroscopy, charge recombination to either singlet ground state (kS) or to the first excited local triplet state of the acceptor (kT) by ns-laser spectroscopy, whereby kinetic magnetic field effects in an external magnetic field were recorded and analysed using quantum dynamic simulations of the spin dependent kinetics of the CS state. Kinetic spectra of the initial first order rate constants of charge recombination (k(B)) exhibited characteristic J-resonances progressing to lower fields in the series Me0, Me2, Me3, Me23. From the quantum simulations, the values of the parameters J, kS, kT and kSTD, the singlet/triplet dephasing constant, were obtained. They were analysed in terms of molecular dynamics simulations of the intramolecular twisting dynamics based on potentials calculated by density functional theory. Apart from kT, all of the parameters exhibit a clear correlation with the averaged cosine square products of the biaryl angles.

Push-pull [7]helicene diimide: excited-state charge transfer and solvatochromic circularly polarised luminescence.

In this communication we describe a helically chiral push-pull molecule named 9,10-dimethoxy-[7]helicene diimide, displaying fluorescence (FL) and circularly polarised luminescence (CPL) over nearly the entire visible spectrum dependent on solvent polarity. The synthesised molecule exhibits an unusual solvent polarity dependence of FL quantum yield and nonradiative rate constant, as well as remarkable gabs and glum values along with high configurational stability.

Excited states and spin-orbit coupling in chalcogen substituted perylene diimides and their radical anions.

While spin-orbit coupling does not play a decisive role in the photophysics of unsubstituted perylene diimides (PDI), this changes dramatically when two phenylselenyl or phenyltelluryl substituents were attached to the PDI bay positions. In the series of PhO-, PhS-, PhSe-, and PhTe-substituted PDIs we observed strongly decreasing fluorescence quantum yield as a consequence of strongly increasing intersystem crossing (ISC) rate, measured by transient absorption spectroscopy with fs- and ns-time resolution as well as by broadband fluorescence upconversion. Time-dependent density functional calculations suggest increasing spin-orbit coupling due to the internal heavy-atom effect as the reason for fast ISC. In case of the selenium PDI derivative we found significant singlet oxygen sensitization via the PDI triplet state. The corresponding radical anions of the chalcogen substituted PDIs were also prepared and investigated by optical and EPR spectroscopy. Here, the increasing SOC results in an increase of the g-tensor anisotropy, and of the isotropic g-value in solution, albeit quasirelativistic density functional calculations show only a relatively small fraction of the spin density to be located on the chalcogen atom.

Thermodynamic equilibrium between locally excited and charge-transfer states through thermally activated charge transfer in 1-(pyren-2'-yl)-o-carborane.

Reversible conversion between excited-states plays an important role in many photophysical phenomena. Using 1-(pyren-2'-yl)-o-carborane as a model, we studied the photoinduced reversible charge-transfer (CT) process and the thermodynamic equilibrium between the locally-excited (LE) state and CT state, by combining steady state, time-resolved, and temperature-dependent fluorescence spectroscopy, fs- and ns-transient absorption, and DFT and LR-TDDFT calculations. Our results show that the energy gaps and energy barriers between the LE, CT, and a non-emissive 'mixed' state of 1-(pyren-2'-yl)-o-carborane are very small, and all three excited states are accessible at room temperature. The internal-conversion and reverse internal-conversion between LE and CT states are significantly faster than the radiative decay, and the two states have the same lifetimes and are in thermodynamic equilibrium.

Macrocyclic Donor-Acceptor Dyads Composed of Oligothiophene Half-Cycles and Perylene Bisimides.

A series of donor-acceptor (D-A) macrocyclic dyads consisting of an electron-poor perylene bisimide (PBI) π-scaffold bridged with electron-rich α-oligothiophenes bearing four, five, six and seven thiophene units between the two phenyl-imide substituents has been synthesized and characterized by steady-state UV/Vis absorption and fluorescence spectroscopy, cyclic and differential pulse voltammetry as well as transient absorption spectroscopy. Tying the oligothiophene strands in a conformationally fixed macrocyclic arrangement leads to a more rigid π-scaffold with vibronic fine structure in the respective absorption spectra. Electrochemical analysis disclosed charged state properties in solution which are strongly dependent on the degree of rigidification within the individual macrocycle. Investigation of the excited state dynamics revealed an oligothiophene bridge size-dependent fast charge transfer process for the macrocyclic dyads upon PBI subunit excitation.

Macrocyclic Donor-Acceptor Dyads Composed of a Perylene Bisimide Dye Surrounded by Oligothiophene Bridges.

Two macrocyclic architectures comprising oligothiophene strands that connect the imide positions of a perylene bisimide (PBI) dye have been synthesized via a platinum-mediated cross-coupling strategy. The crystal structure of the double bridged PBI reveals all syn-arranged thiophene units that completely enclose the planar PBI chromophore via a 12-membered macrocycle. The target structures were characterized by steady-state UV/Vis absorption, fluorescence and transient absorption spectroscopy, as well as cyclic and differential pulse voltammetry. Both donor-acceptor dyads show ultrafast Förster Resonance Energy Transfer and photoinduced electron transfer, thereby leading to extremely low fluorescence quantum yields even in the lowest polarity cyclohexane solvent.

Large Stokes shift fluorescence activation in an RNA aptamer by intermolecular proton transfer to guanine.

Fluorogenic RNA aptamers are synthetic functional RNAs that specifically bind and activate conditional fluorophores. The Chili RNA aptamer mimics large Stokes shift fluorescent proteins and exhibits high affinity for 3,5-dimethoxy-4-hydroxybenzylidene imidazolone (DMHBI) derivatives to elicit green or red fluorescence emission. Here, we elucidate the structural and mechanistic basis of fluorescence activation by crystallography and time-resolved optical spectroscopy. Two co-crystal structures of the Chili RNA with positively charged DMHBO+ and DMHBI+ ligands revealed a G-quadruplex and a trans-sugar-sugar edge G:G base pair that immobilize the ligand by π-π stacking. A Watson-Crick G:C base pair in the fluorophore binding site establishes a short hydrogen bond between the N7 of guanine and the phenolic OH of the ligand. Ultrafast excited state proton transfer (ESPT) from the neutral chromophore to the RNA was found with a time constant of 130 fs and revealed the mode of action of the large Stokes shift fluorogenic RNA aptamer.

Ultrafast Resonance Energy Transfer in Ethylene-Bridged BODIPY Heterooligomers: From Frenkel to Förster Coupling Limit.

A series of distinct BODIPY heterooligomers (dyads, triads, and tetrads) comprising a variable number of typical green BODIPY monomers and a terminal red-emitting styryl-equipped species acting as an energy sink was prepared and subjected to computational and photophysical investigations in solvent media. An ethylene tether between the single monomeric units provides a unique foldameric system, setting the stage for a systematic study of excitation energy transfer processes (EET) on the basis of nonconjugated oscillators. The influence of stabilizing ß-ethyl substituents on conformational space and the disorder of site energies and electronic couplings was addressed. In this way both the strong (Frenkel) and the weak (Förster) coupling limit could be accessed within a single system: the Frenkel limit within the strongly coupled homooligomeric green donor subunit and the Förster limit at the terminal heterosubstituted ethylene bridge. Femtosecond transient-absorption spectroscopy combined with mixed quantum-classical dynamic simulations demonstrate the limitations of the Förster resonance energy transfer (FRET) theory and provide a consistent framework to elucidate the trend of increasing relaxation lifetimes at higher homologues, revealing one of the fastest excitation energy transfer processes detected to date with a corresponding lifetime of 39 fs.

Ultrafast Energy Transfer Dynamics in a Squaraine Heterotriad.

A squaraine heterotriad consisting of three different covalently linked squaraine chromophores was synthesized, and its absorption spectra were interpreted in terms of Kasha's exciton coupling theory. Using the exciton couplings derived from model dyads (ca. 700 cm-1) as the input, we were able to predict the exciton state energies of the heterotriad. Transient absorption spectroscopy with femtosecond time resolution showed that excitation of the highest exciton state populates a state mainly localized at one terminal squaraine chromophore, and energy transfer to the lowest exciton state localized at the other terminal squaraine occurs within 30 fs. Field-induced surface hopping dynamics simulations support the assumption of ultrafast energy transfer. Moreover, they show the close relationship between internal conversion and energy transfer in the intermediate to weak coupling regime. The latter is a consequence of excitation localization caused by molecular vibrations.

[n]Helicene Diimides (n = 5, 6, and 7): Through-Bond versus Through-Space Conjugation.

The interactions between auxochromic groups in π-conjugated functional molecules dictate their electronic properties. From the standpoint of potential applications, understanding and control of such interactions is a vital requirement for the material design. In this communication, we describe the design, synthesis, and functional properties of a novel class of helically chiral diimide molecules, namely, [n]HDI-OMe (n = 5, 6, and 7), in which two imide units are connected via an [n]helicene skeleton. The experimental results supported by quantum chemical calculations reveal that the helical backbone in these molecules offers not only through-bond but also through-space conjugation between imide groups, which leads to distinct optical and electrochemical properties when compared to the related [n]helicenes and rylene diimides.

Cyclic (Amino)(aryl)carbenes Enter the Field of Chromophore Ligands: Expanded π System Leads to Unusually Deep Red Emitting CuI Compounds.

A series of copper(I) complexes bearing a cyclic (amino)(aryl)carbene (CAArC) ligand with various complex geometries have been investigated in great detail with regard to their structural, electronic, and photophysical properties. Comparison of [CuX(CAArC)] (X = Br (1), Cbz (2), acac (3), Ph2acac (4), Cp (5), and Cp* (6)) with known CuI complexes bearing cyclic (amino)(alkyl), monoamido, or diamido carbenes (CAAC, MAC, or DAC, respectively) as chromophore ligands reveals that the expanded π-system of the CAArC leads to relatively low energy absorption maxima between 350 and 550 nm in THF with high absorption coefficients of 5-15 × 103 M-1 cm-1 for 1-6. Furthermore, 1-5 show intense deep red to near-IR emission involving their triplet excited states in the solid state and in PMMA films with λemmax = 621-784 nm. Linear [Cu(Cbz)(DippCAArC)] (2) has been found to be an exceptional deep red (λmax = 621 nm, ϕ = 0.32, τav = 366 ns) thermally activated delayed fluorescence (TADF) emitter with a radiative rate constant kr of ca. 9 × 105 s-1, exceeding those of commercially employed IrIII- or PtII-based emitters. Time-resolved transient absorption and fluorescence upconversion experiments complemented by quantum chemical calculations employing Kohn-Sham density functional theory and multireference configuration interaction methods as well as temperature-dependent steady-state and time-resolved luminescence studies provide a detailed picture of the excited-state dynamics of 2. To demonstrate the potential applicability of this new class of low-energy emitters in future photonic applications, such as nonclassical light sources for quantum communication or quantum cryptography, we have successfully conducted single-molecule photon-correlation experiments of 2, showing distinct antibunching as required for single-photon emitters.

Luminescent Mono-, Di-, and Triradicals: Bridging Polychlorinated Triarylmethyl Radicals by Triarylamines and Triarylboranes.

Up to three polychlorinated pyridyldiphenylmethyl radicals bridged by a triphenylamine carrying electron withdrawing (CN), neutral (Me), or donating (OMe) groups were synthesized and analogous radicals bridged by tris(2,6-dimethylphenyl)borane were prepared for comparison. All compounds were as stable as common closed-shell organic compounds and showed significant fluorescence upon excitation. Electronic, magnetic, absorption, and emission properties were examined in detail, and experimental results were interpreted using DFT calculations. Oxidation potentials, absorption and emission energies could be tuned depending on the electron density of the bridges. The triphenylamine bridges mediated intramolecular weak antiferromagnetic interactions between the radical spins, and the energy difference between the high spin and low spin states was determined by temperature dependent ESR spectroscopy and DFT calculations. The fluorescent properties of all radicals were examined in detail and revealed no difference for high and low spin states which facilitates application of these dyes in two-photon absorption spectroscopy and OLED devices.

Ultra-High to Ultra-Low Drug-Loaded Micelles: Probing Host-Guest Interactions by Fluorescence Spectroscopy.

Polymer micelles are an attractive means to solubilize water insoluble compounds such as drugs. Drug loading, formulations stability and control over drug release are crucial factors for drug-loaded polymer micelles. The interactions between the polymeric host and the guest molecules are considered critical to control these factors but typically barely understood. Here, we compare two isomeric polymer micelles, one of which enables ultra-high curcumin loading exceeding 50 wt.%, while the other allows a drug loading of only 25 wt.%. In the low capacity micelles, steady-state fluorescence revealed a very unusual feature of curcumin fluorescence, a high energy emission at 510 nm. Time-resolved fluorescence upconversion showed that the fluorescence life time of the corresponding species is too short in the high-capacity micelles, preventing an observable emission in steady-state. Therefore, contrary to common perception, stronger interactions between host and guest can be detrimental to the drug loading in polymer micelles.

Dynamic exciton localisation in a pyrene-BODIPY-pyrene dye conjugate.

The photophysics of a molecular triad consisting of a BODIPY dye and two pyrene chromophores attached in 2-position are investigated by steady state and fs-time resolved transient absorption spectroscopy as well as by field induced surface hopping (FISH) simulations. While the steady state measurements indicate moderate chromophore interactions within the triad, the time resolved measurements show upon pyrene excitation a delocalised excited state which localises onto the BODIPY chromophore with a time constant of 0.12 ps. This could either be interpreted as an internal conversion process within the excitonically coupled chromophores or as an energy transfer from the pyrenes to the BODIPY dye. The analysis of FISH-trajectories reveals an oscillatory behaviour where the excitation hops between the pyrene units and the BODIPY dye several times until finally they become localised on the BODIPY chromophore within 100 fs. This is accompanied by an ultrafast nonradiative relaxation within the excitonic manifold mediated by the nonadiabatic coupling. Averaging over an ensemble of trajectories allowed us to simulate the electronic state population dynamics and determine the time constants for the nonradiative transitions that mediate the ultrafast energy transfer and exciton localisation on BODIPY.

Fullerene-Filled Liquid-Crystal Stars: A Supramolecular Click Mechanism for the Generation of Tailored Donor-Acceptor Assemblies.

Two shape-persistent star mesogens with oligo(phenylene ethenylene) arms and a phthalocyanine core-one providing free space (2) and one sterically encumbered by four fullerenes attached through spacers (3)-have been successfully synthesized. In contrast to the smaller discotic derivative 1, mesogen 2 forms a columnar liquid crystal (LC), which can only be partially aligned without π-stacking, while 3 is not an LC. Exceptionally, the 1:1 mixture of 2 and 3 forms an alignable columnar LC with strong π-stacking and quadruply helically organized fullerenes by an unprecedented click process that is similar to a ball detent mechanism. The C60 units also interconnect different columns. This is driven by nanosegregation and space-filling of the voids with fullerenes. Photophysical studies confirm the presence of a light-collecting system that generates charge-separated states in solution and in the solid state, which makes such highly organized materials attractive for the study of future photovoltaic devices.

Magnetic field effects in rigidly linked D-A dyads: Extreme on-resonance quantum coherence effect on charge recombination.

Charge recombination in the photoinduced charge separated (CS) state of a rigidly linked donor/bridge/acceptor triad with a triarylamine (TAA) donor, a 1,3-diethynyl-2,5-dimethoxy benzene bridge (OMe), and a perylenediimide (PDI) unit as an acceptor, represents a spin chemical paradigm case of a rigid radical ion pair formed with singlet spin and recombining almost exclusively to the locally excited PDI triplet state (3PDI). The magnetic field dependence of the CS state decay and 3PDI formation kinetics are investigated from 0 to 1800 mT by nanosecond laser flash spectroscopy. The time-resolved magnetic field affected reaction yields spectra of the CS state population and 3PDI population exhibit a sharp and deep resonance at 18.9 mT, indicating level crossing of the S and T+ levels separated by an exchange interaction of J = 18.9/2 mT at zero field. The kinetics are biexponential around the resonance field and monoexponential outside that range. The monoexponential behavior can be simulated by a classical kinetic model assuming a single field dependent double Lorentzian function for the energy gap dependence of all spin conversion processes. The full field dependence of the kinetics has been simulated quantum theoretically. It has been shown that incoherent and coherent hyperfine coupling contribute to S/T+ spin conversion at all fields and that the biexponentiality of the kinetics at resonance is due to a partitioning of the overall kinetics into 2/3 of the singlet hyperfine states exhibiting strong isotropic coupling to T+ and 1/3 of the singlet hyperfine states that do not or only weakly couple isotropically to T+.

Fine tuning of electron transfer and spin chemistry parameters in triarylamine-bridge-naphthalene diimide dyads by bridge substituents.

The photoinduced charge separation and charge recombination in a set of four molecular dyads consisting of a triarylamine donor and a naphthalene diimide acceptor were investigated by time resolved transient absorption spectroscopy with fs and ns time resolution. In these dyads the donor and acceptor are bridged by a meta-conjugated diethynylbenzene bridge whose electronic nature was tuned by small electron donating (OMe, Me) or electron withdrawing (Cl, CN) substituents. While the formation of the transient charge separated states is complete within tens of ps, charge recombination is biphasic with a shorter component of several hundred ns and a longer component of several microseconds. This behaviour could be rationalized by assuming an equilibrium of singlet and triplet charge separated states. Magnetic field dependent measurements showed a strong influence on the biphasic decay kinetics and also a pronounced level crossing effect in the magnetic field affected reaction yield (MARY) spectra caused by a significant exchange coupling. An analysis of the observed kinetics using classical kinetic rate equations yields rate constants for charge separation and charge recombination as well as the exchange interaction splitting in the radical ion pair, all of them showing a delicate dependence on the bridge substituents.

Photoinduced Dynamics of Bis-dipyrrinato-palladium(II) and Porphodimethenato-palladium(II) Complexes: Governing Near Infrared Phosphorescence by Structural Restriction.

Although superficially similar, the bis-dipyrrinato-palladium(II) complex 1 and the bridged porphodimethenato-palladium(II) complex 2 possess dramatically different structures in the ground state (proved by X-ray structure analysis) and in the singlet and triplet excited states (calculated by density functional theory methods). While complex 2 is rather rigid, complex 1 undergoes a major structural reorganization in the excited state to yield a disphenoidal (seesaw) triplet state. The dynamics of the excited states were probed by transient absorption spectroscopy with femtosecond and nanosecond time resolution and with fluorescence upconversion and yield intersystem crossing rate constants of ca. (13-16 ps)-1. The observation of significant near infrared phosphorescence in complex 2 but the absence of any emission in complex 1 in fluid solution could be rationalized by the structural reorganization of 1 which results in a nonemissive triplet metal centered state.