Quantitative prediction of excited-state decay rates for radical anion photocatalysts.

We present a computational approach for predicting key properties of organic radical anions, including excited-state lifetimes and redox potentials. The approach shows good agreement with experimental data and has potential for in silico screening to facilitate the rational design of photocatalysts.

Exciton Migration in Multistranded Perylene Bisimide J-Aggregates.

Exciton migration in self-assembled supramolecular ensembles of dye molecules is controlled by the electronic coupling between adjacent sites, the delocalization of the excitation and thereby by the packing arrangement. Here, we put emphasis on the packing structure and analyze the exciton migration in two perylene bisimide-based J-aggregates composed of almost identical molecular building blocks but forming double-strand versus quadruple-strand slip-stacked supramolecular architectures. Analyzing ultrafast transient absorption spectra in dependence on the exciton density by a kinetic model for exciton-exciton annihilation based on incoherent transfer demonstrates that the migration is quasi one-dimensional. The migration distance is enhanced by a beneficial geometrical structure. We find a factor of more than two between the diffusion lengths of 188 and 77 nm for the double- and quadruple-stranded system. The supramolecular design efficiently influences the exciton mobility and minor structural changes have a pronounced influence on functional properties of dye aggregates.

Defined Merocyanine Dye Stacks from a Dimer up to an Octamer by Spacer-Encoded Self-Assembly Approach.

A series of well-defined chromophore stacks is obtained upon self-assembly of merocyanine and bis(merocyanine) dyes in nonpolar solvents. Careful design of the spacer moieties linking the dipolar chromophores within the bis(merocyanine) dyes allows one to direct the dipole-dipole interaction driven aggregation into stacks of desired size from dimer up to octamer. The spacer-encoded self-assembly process was investigated by UV/vis absorption spectroscopy showing an increase of the hypsochromic shift with increasing stack size. The structure of the largest aggregate comprising eight chromophores was analyzed by 1D and 2D nuclear magnetic resonance spectroscopic studies revealing a perfectly interdigitated centrosymmetric organization of the dipolar dyes and concomitant annihilation of the ground state dipole moment is observed in the UV/vis absorption spectra. This unprecedented series of dye stacks from dimer to octamer enabled a systematic study of the optical absorption properties in dependence of the stack size disclosing that the absorption features can be rationalized by molecular exciton theory. Our results show that the noncovalent synthesis approach based on dipolar aggregation is suitable for the design of well-defined dye aggregates of specific size, allowing in-depth studies to manifest structure-property relationships.

Pyrimidopteridine N-Oxide Organic Photoredox Catalysts: Characterization, Application and Non-Covalent Interaction in Solid State.

Herein we report the photo- and electrochemical characterization of pyrimidopteridine N-oxide-based heterocycles. The potential of their application as organic photoredox catalysts is showcased in the photomediated contra-thermodynamic E→Z isomerization of cinnamic acid derivatives and oxidative cyclization of 2-phenyl benzoic acid to benzocoumarin using molecular oxygen as a mild oxidant. Furthermore, unprecedented intermolecular non-covalent n-π-hole interactions in solid state are discussed based on crystallographic and theoretical data.

Coupled Cooperative Supramolecular Polymerization: A New Model Applied to the Competing Aggregation Pathways of an Amphiphilic aza-BODIPY Dye into Spherical and Rod-Like Aggregates.

Based on our studies on biphasic self-assembly behavior of an amphiphilic BF2 -azadipyrromethene (aza-BODIPY) dye 1, a new analytical model to quantitatively describe the thermodynamic properties of the aggregation involving two competing supramolecular polymerization processes is proposed. In this model, the formation of the metastable as well as the thermodynamically stable aggregates was considered to follow a nucleated polymerization mechanism. The numerical calculation based on the new model gives insight into the formation of different species in such complicate aggregate systems. Moreover, the aggregation of the biphasic self-assembly processes for dye 1 was investigated by concentration-dependent UV/Vis spectroscopy. The experimental data were analyzed by using the new model to evaluate the thermodynamic parameters including aggregation constants, the size of nuclei, and the cooperativity the two types of aggregates.

Direct observation of exciton-exciton interactions.

Natural light harvesting as well as optoelectronic and photovoltaic devices depend on efficient transport of energy following photoexcitation. Using common spectroscopic methods, however, it is challenging to discriminate one-exciton dynamics from multi-exciton interactions that arise when more than one excitation is present in the system. Here we introduce a coherent two-dimensional spectroscopic method that provides a signal only in case that the presence of one exciton influences the behavior of another one. Exemplarily, we monitor exciton diffusion by annihilation in a perylene bisimide-based J-aggregate. We determine quantitatively the exciton diffusion constant from exciton-exciton-interaction 2D spectra and reconstruct the annihilation-free dynamics for large pump powers. The latter enables for ultrafast spectroscopy at much higher intensities than conventionally possible and thus improves signal-to-noise ratios for multichromophore systems; the former recovers spatio-temporal dynamics for a broad range of phenomena in which exciton interactions are present.

Fluorescence quantum yields of dye aggregates: a showcase example based on self-assembled perylene bisimide dimers.

Constituting an intermediate state of matter between molecules and solid state materials, dye aggregates can give important insights into functional properties. Despite the importance of fluorescence for many applications, it turns out that the conventional methods for quantum yield determination are not applicable to dye aggregates and so far no method has been introduced for the accurate determination of the fluorescence quantum yields of dye aggregates. Here we suggest two measurement routines for the quantum yield determination of strongly absorbing dye aggregates. Both methods provide the quantum yield of the aggregated species without the necessity of reaching a fully aggregated state, which is particularly important for the most common case imparted by the low association constants of π-π-stacking interactions. The routines are experimentally validated with two perylene bisimide dyes whose self-assembly is driven by the concerted interplay of hydrogen bonding and π-π-stacking interactions, leading to structurally well defined co-facially stacked dimers at intermediate concentrations and a further growth into larger H-aggregates at a higher concentration. Compared to other H-aggregates relatively high fluorescence quantum yields of up to 28% are found for both the co-facially stacked dimers and the larger H-aggregates.

Large Stokes Shift Ionic-Liquid Dye.

We have incorporated the dye N-methyl-6-oxyquinolone [6MQz] in its protonated form as a cation into an ionic liquid (IL) and thus to synthesize an IL dye. The IL dye N-methyl-6-hydroxyquinolinium bis(trifluoromethylsulfonyl) imide [6MQc][NTf2 ] was characterized by NMR, ATR IR spectroscopy and X-ray crystallography. The fluorescence of the IL dye has a large Stokes shift of Δλ=116 nm and a quantum yield of φF =0.56 in acetonitrile. Characteristic solvent dependent shifts can be detected in the emission spectra. In other ILs, acetonitrile and THF we observe a bathochromic shift of up to 28 nm compared to the pure IL dye at 467 nm. For stronger polar solvents the fluorescence signals are strongly red-shifted to 650 nm indicating proton transfer to the solvent molecules in the excited state. This underlines the importance of the IL building block [MQc]+ as photo acid.

Anti-cooperative supramolecular polymerization: a new K2-K model applied to the self-assembly of perylene bisimide dye proceeding via well-defined hydrogen-bonded dimers.

A perylene bisimide dye bearing amide functionalities at the imide positions derived from amino acid l-alanine and a dialkoxy-substituted benzyl amine self-assembles into tightly bound dimers by π-π-stacking and hydrogen bonding in chloroform. In less polar or unpolar solvents like toluene and methylcyclohexane, and in their mixtures, these dimers further self-assemble into extended oligomeric aggregates in an anti-cooperative process in which even numbered aggregates are highly favoured. The stepwise transition from dimers into oligomers can not be properly described by conventional K2-K model, and thus a new K2-K aggregation model has been developed, which interpretes the present anti-cooperative supramolecular polymerization more appropriately. The newly developed K2-K model will be useful to describe self-assembly processes of a plethora of other π-conjugated molecules that are characterized by a favored dimer species.

Biphasic self-assembly pathways and size-dependent photophysical properties of perylene bisimide dye aggregates.

The concentration-dependent absorption and temperature-dependent fluorescence of the perylene bisimide dye PBI 1 in methylcyclohexane point to a biphasic aggregation behavior. At intermediate concentrations and temperatures, respectively, a dimer with low fluorescence yield dominates, which cannot be extended to longer aggregates. Those are formed at high concentrations and low temperatures, respectively, via a second, energetically unfavorable dimer species that acts as a nucleus. A corresponding aggregation model reproduces accurately the concentration dependence and allows extracting the equilibrium constants and spectra of the distinct species. The differences in the photophysical properties indicate H-type excitonic coupling for the favored dimer and J-type characteristics for the extended aggregates which could be related to structural models based on DFT calculations. The energetics can be understood by considering hydrogen-bonding and π-π-stacking interactions.

Long distance energy transfer in a polymer matrix doped with a perylene dye.

Exciton migration over long distances is a key issue for various applications in organic electronics. We investigate a disordered material system which has the potential for long exciton diffusion lengths in combination with a high versatility. The perylene bisimide dye Perylene Red is incorporated in a polymer matrix with a high concentration. The dye molecules represent active sites with a narrow energy distribution for the electronically excited states. Excitons can be efficiently exchanged between them by Förster resonance energy transfer (FRET). The narrow energy distribution reduces drastically the trapping probability of the excitons compared to polymers and allows for long transfer distances. To characterize the mobility of the excitons and their diffusion length the dye Oxazine 1 is added as an acceptor in low concentration and the transfer probability to the acceptor is determined by measuring the reduction of Perylene Red fluorescence. The quenched quantum yield is measured for dye concentrations varying from 0.05 M to 0.15 M for Perylene Red and from 0.3 mM to 3 mM for Oxazine 1. The experimental results are compared to a model which assumes that excitons can diffuse through the material by FRET between Perylene Red sites and are trapped at an acceptor with a final hetero FRET step. We find a quite good match between theory and experiment though the observed diffusion constant is about two times smaller than the calculated one. The exciton diffusion length extracted from the data is 30 nm for a Perylene Red concentration of 0.1 M and demonstrates that long distance energy transfer is possible in this disordered material system.