Tuning Electron Transfer Coupling and Exchange Interaction in Bis-triarylamine Radical Cations and Dications by Bridge Electron Density.

The influence of the electron density of a bridge connecting two redox centers on both the intervalence hole transfer and the magnetic superexchange was investigated in a series of bridged bis-triarylamine mono- and dications. In this series, the bridge was 2,7-fluorenyl, where the bridge electron density was modified by substituents at the 9-position. For the mixed-valence monocations, the observation of both an intervalence charge transfer (IVCT) band and an absorption band associated with an electron transfer from the bridging fluorene to the triarylamine radical cation centers allowed determination of the electron transfer couplings in the framework of the three-state generalized Mulliken-Hush theory. Comparison of the derived couplings with those obtained from a classical two-state approach demonstrates an enhancement of the electronic coupling which increases with decreasing bridge state energy. For the dicationic diradical counterparts, the singlet-triplet gap (exchange interaction) was determined both experimentally and by quantum chemical methods. Hereby, an increase of antiferromagnetic coupling with a lowering of the bridge state energy by electron donating substituents was observed. Analysis of the involved molecular orbitals suggests that the ferromagnetic coupling is inversely proportional to the square of the bridge energy, which is also supported by the experimental findings. This influence of the bridge state energy on both types of interactions, electron transfer and magnetic exchange, provides a design guideline for fine-tuning the properties of electronically coupled organic redox dyads by variation of the bridge electron density.

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.

Control of electronic and exchange coupling by bridge substituents in donor acceptor triads with triptycene bridges.

A series of triads, consisting of a triarylamine electron donor and a perylene diimide electron acceptor which were attached to two different wings of a triptycene bridging unit, was investigated concerning the dynamics of photoinduced charge separation and charge recombination processes with a particular focus on the involved spin-chemical aspects. Attaching electron-donating or electron-withdrawing substituents to the third wing of the triptycene bridge allowed tuning the electron transfer processes. These processes were investigated via fs-transient absorption spectroscopy and ns-transient absorption spectroscopy in an external magnetic field. The resulting magnetic field-dependent decay dynamics were analysed and modelled using the stochastic Liouville equation which yielded rate constants for the charge recombination and the exchange energy. In combination with a diabatic rate theory and Anderson's perturbative treatment of the exchange energy, these data gave a complete set of rate constants for charge separation and charge recombination from which the diverse electronic couplings between the involved states were derived. These couplings depend linearly on the inverse energy of virtual triptycene bridge states which allows tuning the electron transfer dynamics by modifying the triptycene bridge.

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.

Chiroptical Properties of Planar Benzobisthiazole-Bridged Squaraine Dimers.

Five chiral squaraine dimers were synthesized by fusing chiral indolenine semisquaraines with three different benzobisthiazole bridges. The thereby created squaraine dimers show a strong splitting of the lowest energy absorption bands caused by exciton coupling. The intensities of the two exciton transitions and the energetic splitting depend on the angle of the two squaraine moieties within the chromophore dimer. The electric circular dichroism spectra of the dimers show intense Cotton effects whose sign depends on the used squaraine chromophores. Sizable anisotropies gabs of up to 2.6 × 10-3 could be obtained. TD-DFT calculations were used to partition the rotational strength into the three Rosenfeld terms where the electric-magnetic coupling turned out to be the dominant contribution while the exciton chirality term is much smaller. This is because the chromophore dimers are essentially planar but the angle between the electric transition dipole moment of one squaraine and the magnetic transition dipole moment of the other squaraine strongly deviates from 90°, which makes the dot product between the two moment vectors and, thus, the rotational strength substantial.

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.

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.

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.

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.

Solvent Induced Helix Folding of Defined Indolenine Squaraine Oligomers.

A protecting group strategy was employed to synthesise a series of indolenine squaraine dye oligomers up to the nonamer. The longer oligomers show a distinct solvent dependence of the absorption spectra, that is, either a strong blue shift or a strong red shift of the lowest energy bands in the near infrared spectral region. This behaviour is explained by exciton coupling theory as being due to H- or J-type coupling of transition moments. The H-type coupling is a consequence of a helix folding in solvents with a small Hansen dispersity index. DOSY NMR, small angle neutron scattering (SANS), quantum chemical and force field calculations agree upon a helix structure with an unusually large pitch and open voids that are filled with solvent molecules, thereby forming a kind of clathrate. The thermodynamic parameters of the folding process were determined by temperature dependent optical absorption spectra.

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.

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.

[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.

Synthesis, Photophysical and Electronic Properties of New Red-to-NIR Emitting Donor-Acceptor Pyrene Derivatives.

We synthesized new pyrene derivatives with strong bis(para-methoxyphenyl)amine donors at the 2,7-positions and n-azaacene acceptors at the K-region of pyrene. The compounds possess a strong intramolecular charge transfer, leading to unusual properties such as emission in the red to NIR region (700 nm), which has not been reported before for monomeric pyrenes. Detailed photophysical studies reveal very long intrinsic lifetimes of >100 ns for the new compounds, which is typical for 2,7-substituted pyrenes but not for K-region substituted pyrenes. The incorporation of strong donors and acceptors leads to very low reduction and oxidation potentials, and spectroelectrochemical studies show that the compounds are on the borderline between localized Robin-Day class-II and delocalized Robin-Day class-III species.

Chiroptical Properties of Indolenine Squaraines with a Stereogenic Center at Close Proximity.

A series of four indolenine squaraines bearing a chiral center at the 3-position of the indolenine moiety, with either an n-propyl or a phenyl group alongside a methyl group, were synthesized and obtained in a high purity of ≥98% for the desired stereoisomer. The indolenine precursors with a phenyl group attached at the chiral center were asymmetrically synthesized using a pericyclic-reaction cascade and obtained in a high ee of 98%, whereas the ones with an n-propyl group were prepared by kinetic resolution through asymmetric hydrogenation, resulting in an ee of up to 98%. X-ray crystallography revealed a slightly twisted geometry for the phenyl-substituted cisoid squaraine derivative, whereas the n-propyl-substituted derivative possessed the expected planar geometry. Variation of the substitution also influenced the optical properties, where the introduction of phenyl groups caused a progressive red-shift and reduction in squared transition moments, as well as reduced fluorescence quantum yields, Stokes shifts, and fluorescence lifetimes. All of the investigated compounds exhibited strong ECD signals, with Δε values of up to 24 M-1 cm-1 for the HOMO-LUMO transition. DFT calculations indicated that this was due to both large electric and magnetic transition moments, although the two vectors were mutually almost orthogonal.

Exciton coupling effects on the two-photon absorption of squaraine homodimers with varying bridge units.

We explored a series of squaraine homodimers with varying π-bridging centres to probe the relationship between the chemical structure and the two-photon absorption (2PA) characteristics. To this end, we designed and synthesised six linear homodimers based on two indolenine squaraine dyes with transoid configuration (SQA) which are connected by diverse bridges. In this regard, we investigated the effect of exciton coupling in these dimeric systems where the variation of the bridging units affects the magnitude of exciton coupling and leads to an alteration of their linear optical properties. Using two-photon absorption induced fluorescence measurements we determined the two-photon absorption cross section in this series of homodimers and found sizable values up to 5700 GM at ca. 11 000 cm-1 and 12 000 GM at 12 500 cm-1. The 2PA strength roughly follows the exciton coupling interaction between the squaraine chromophores which therefore may be used as design criteria to achieve high 2PA cross sections. The results were substantiated by polarization dependent linear and nonlinear optical measurements and by density functional theory calculations based on time dependent and quadratic response theory.

Giant magnetic field effects in donor-acceptor triads: On the charge separation and recombination dynamics in triarylamine-naphthalenediimide triads with bis-diyprrinato-palladium(II), porphodimethenato-palladium(II), and palladium(II)-porphyrin photosensitizers.

A series of triads consisting of a triarylamine donor, a naphthalenediimide acceptor, and a palladium photosensitizer bridge was investigated for the photoinduced electron transfer processes and the spin chemistry involved. In this series, the ligand in the palladium photosensitizer was varied from bis-dipyrrinato to porphodimethenato and to a porphyrin. With the porphyrin photosensitizer, no charge separated state could be reached. This is caused by the direct relaxation of the excited photosensitizer to the ground state by intersystem crossing. The bis-dipyrrinato-palladium photosensitizer gave only a little yield (7%) of the charge separated state, which is due to the population of a metal centered triplet state and a concomitant geometrical rearrangement to a disphenoidal coordination sphere. This state relaxes rapidly to the ground state. In contrast, in the porphodimethenato-palladium triads, a long lived (µs to ms) charge separated state could be generated in high quantum yields (66%-74%) because, here, the population of a triplet metal centered state is inhibited by geometrical constraints. The magnetic field dependent transient absorption measurement of one of the porphodimethenato triads revealed a giant magnetic field effect by a factor of 26 on the signal amplitude of the charge separated state. This is the consequence of a magnetic field dependent triplet-singlet interconversion that inhibits the fast decay of the charge separated triplet state through the singlet recombination channel. A systematic comparative analysis of the spin-dependent kinetics in terms of three classical and one fully quantum theoretical methods is provided, shedding light on the pros and cons of each of them.

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.

fs-ps Exciton dynamics in a stretched tetraphenylsquaraine polymer.

A tetraphenylsquaraine was synthesized whose structure was elucidated by single crystal X-ray structure analysis. Unlike all known indolenine squaraines, the tetraphenylsquaraine shows an unusual nonplanar structure with the four phenyl groups pointing away from the squaric acid core in order to avoid steric congestion. This tetraphenylsquaraine was polymerized by a Yamamoto coupling to form a conjugated polymer with Xn = 38. The absorption spectra of this polymer are red-shifted compared to that of the monomer and show a J-type absorption band due to exciton coupling. Transient absorption spectra with fs-time resolution display a strong ground state bleaching signal with a peak on the red side rising concurrently with the decay of a peak on the blue side of an isosbestic point at 12 000 cm-1. This behavior is caused by energy transfer between two slightly different sections of the polymer with time constants of 0.3 and 2.6 ps. According to semiempirical calculations these different sections are stretched and slightly bent conformations of the polymer strand. Power dependent transient absorption measurements indicate exciton annihilation which also proves the excitons to be very mobile.

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.