Long-Lived Singlet and Triplet Charge Separated States in Small Cyclophane-Bridged Triarylamine–Naphthalene Diimide Dyads

Two different dyads containing a triarylamine (TAA) donor and a naphthalene-1,8:4,5-bis­(dicarboximide) (NDI) acceptor bridged by either a [2.2]- or a [3.3]­paracyclophane (CP) were synthesized. These dyads show a high population of long-lived charge separated (CS) singlet and triplet states. The lifetimes of these different spin states vary only by 1 order of magnitude. This unique situation is a consequence of both a large electronic coupling <i>V</i> and a large exchange coupling 2<i>J</i>. The population of the different CS spin states and therefore the charge recombination (CR) and intersystem crossing (ISC) kinetics were monitored by standard ns-transient absorption spectroscopy. Together with fs-transient absorption spectroscopy supported by electrochemistry, steady state fluorescence and steady state absorption spectroscopy a detailed model of the photoinduced processes was derived

Exciton Coupling Enhancement in the Relaxed Excited State

The steady-state and photoinduced dynamical optical properties of two squaraine-bodipy dye conjugates are the focus of this work. While the squared absorption transition moments of the dye conjugates can be traced back in an additive way to the constituents of the conjugates, this is not possible for the squared fluorescence transition moments. We suggest an enhancement of electronic coupling in the relaxed excited state to be responsible for this observation. Transient absorption and fluorescence upconversion experiments with femtosecond-time resolution give insight into the relaxation phenomena of the dye conjugates, in particular concerning the relaxation within the exciton manifold

Coupled Oscillators for Tuning Fluorescence Properties of Squaraine Dyes

Combining a squaraine (S) and a BODIPY (B) chromophore in a heterodimer (SB) and two heterotrimers (BSB and SBS) by alkyne bridges leads to the formation of coupled oscillators whose fluorescence properties are superior compared to the parent squaraine chromophore. The lowest energy absorption and emission properties of these superchromophores are mainly governed by the squaraine part and are shifted by more than 1000 cm^–1 to the red by excitonic interaction between the squaraine and the BODIPY dye. Employing polarization-dependent transient absorption and fluorescence upconversion measurements, we could prove that the lowest energy absorption in SB and BSB is caused by a single excitonic state but by two for SBS. Despite the spectral red-shift of their lowest absorption band, the fluorescence quantum yields increase for SB and BSB compared to the parent squaraine chromophore SQA. This is caused by intensity borrowing from the BODIPY states, which increases the squared transition moments of the lowest energy band dramatically by 29% for SB and 63% for BSB compared to SQA. Thereby, exciton coupling leads to a substantial enhancement of fluorescence quantum yield by 26% for SB and by 46% for BSB and shifts the emission from the red into the near-infrared. In this way, the BODIPY-squaraine conjugates combine the best properties of each class of dye. Thus, exciton coupling in heterodimers and -trimers is a valuable alternative to tuning fluorescence properties by, e.g., attaching substituents to chromophores

Solvent Controlled Energy Transfer Processes in Triarylamine-Triazole Based Dendrimers

Fluorescence upconversion measurements of three different dendrimers <b>G1</b><b>–G3</b> based on triarylamines connected by triazole linkers show a strong and fast initial decay of fluorescence anisotropy for <i>t</i> < 2 ps followed by anisotropy decay on a much longer time scale (10–100 ps). At the same time, a pronounced solvent relaxation takes place. Comparison of the decay data in different solvents revealed that the initial decay of fluorescence anisotropy is governed by a competition of solvent relaxation and incoherent hopping of energy between the different dendrimer branches. Thus, it is decisive to discriminate between energy transfer processes in the Franck–Condon state or in the solvent relaxed state. We demonstrate that even for charge transfer chromophores, where a large Stokes shift leads to very weak spectral overlap of donor fluorescence and acceptor absorption, rapid homotransfer is possible if there is sufficient spectral overlap with the time zero fluorescence spectrum

Singlet–Singlet Exciton Annihilation in an Exciton-Coupled Squaraine-Squaraine Copolymer: A Model toward Hetero-J-Aggregates

Low-band-gap polymers with broad spectral absorption are highly sought after for application in organic photovoltaic cells and other optoelectronic devices. Thus, a conjugated copolymer based on two different indolenine squaraine dyes SQA and SQB was synthesized by Suzuki coupling, and its steady-state and time-resolved optical properties were investigated in detail. In CHCl₃ the copolymer [SQA-SQB]_<i>n</i> shows a strongly broadened and red-shifted absorption compared to that of its monomers, which was explained by exciton coupling of localized transition moments. The theoretical background of exciton coupling theory for copolymers was worked out in detail. In toluene, [SQA-SQB]_<i>n</i> displays a spectral narrowing of the lowest excitation band which resembles the exchange narrowing effect found in cyanine J-aggregates. In this way [SQA-SQB]_<i>n</i> behaves like a one-dimensional covalently bound hetero-J-aggregate. The photoinduced dynamics of the copolymer was investigated by transient absorption pump–probe spectroscopy with femtosecond resolution. Because of the unusually high exciton diffusion constant, singlet–singlet annihilation is the rate-limiting step for deactivation of the copolymer in solution at high laser fluencies. This is unlike the situation for many conjugated polymers in the solid state, where diffusion-limited annihilation is usually found. Thus, the [SQA-SQB]_<i>n</i> copolymer is a unique model system which combines the excitonic features of J-aggregates with the chemical robustness of a polymer

Measuring Charge-Separation Dynamics via Oligomer Length Variation

We study the optically induced charge-transfer dynamics in donor–acceptor oligomers of different chain lengths. The combination of systematic synthesis, electrochemical measurements, and ultrafast transient absorption spectroscopy allows us to determine the charge-transfer properties and dynamics in donor–acceptor systems of confined lengths. Calculations within Marcus and Jortner electron-transfer theory explain the different charge-recombination times. For compounds in which complete charge separation can occur we deduce fast equilibration between different charge-transfer configurations prior to charge recombination. Thus, monoexponential charge-recombination kinetics are observed, as only the smallest-barrier configuration leads to relaxation to the ground state. The systematic oligomer length variation along with time-resolved spectroscopy allows us to determine how far apart charges can be separated in multichromophore donor–acceptor systems. Such information is relevant for understanding on a microscopic level the charge carrier mobility in materials for organic electronics and photovoltaics

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 <b>1</b> and the bridged porphodimethenato-palladium­(II) complex <b>2</b> 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 <b>2</b> is rather rigid, complex <b>1</b> 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 <b>2</b> but the absence of any emission in complex <b>1</b> in fluid solution could be rationalized by the structural reorganization of <b>1</b> which results in a nonemissive triplet metal centered state

Photoinduced Electron Transfer Dynamics in Triarylamine–Naphthalene Diimide Cascades

A series of dyads and triads of the A-D and A-D1-D2 type, respectively, containing triarylamine (TAA) donors and naphthalene diimide (NDI) acceptors, which are linked via triazole (Tz) heterocycles, were synthesized by Cu­(I)-catalyzed azide alkyne cycloaddition (CuAAC). Upon photoexcitation, these systems undergo charge separation leading to long-lived charge-separated (CS) states. The population of these CS states was monitored using femtosecond and nanosecond transient absorption spectroscopy. The transient signals of the CS states of all triads and dyads feature biexponential decays in the nanosecond time regime with a short and a long component. These biexponential decays are the result of an ISC from the primarily populated ¹CS state into the ³CS, from which charge recombination to the S₀ state is forbidden by spin conservation rules. The existence of ³CS states in the triads was confirmed by strong magnetic field dependent transient absorption kinetics, while for the dyads no effect could be observed due to a much larger singlet–triplet splitting. Thus, although charge recombination from the ¹CS state in the triads is slowed down compared to the dyads, the lifetime of the ³CS states is clearly longer in the dyads. This is the result of the larger singlet–triplet splitting in the dyads which leads to lifetimes of several microseconds

Energy Transfer Between Squaraine Polymer Sections: From <i>Helix</i> to <i>Zigzag</i> and All the Way Back

We provide a joint experimental and theoretical study of squaraine polymers in solution. The absorption spectra show evidence that two different conformations are present in the polymer: a <i>helix</i> and a <i>zigzag</i> structure. This unique situation allows investigating ultrafast energy-transfer processes between different structural segments within a single polymer chain in solution. The understanding of the underlying dynamics is of fundamental importance for the development of novel materials for light-harvesting and optoelectronic applications. Here, we combine femtosecond transient absorption spectroscopy with time-resolved 2D electronic spectroscopy in order to demonstrate that ultrafast energy transfer within the squaraine polymer chains proceeds from initially excited <i>helix</i> segments to <i>zigzag</i> segments or vice versa, depending on the solvent as well as on the excitation wavenumber. These observations contrast other conjugated polymers such as MEH-PPV where much slower intrachain energy transfer was reported. The reason for the very fast energy transfer in squaraine polymers is most likely a close matching of the density of states between donor and acceptor polymer segments because of the very small reorganization energy in these cyanine-like chromophores