ABSTRACT
Herein, we demonstrate a working prototype of a conjugated proton crane, a reversible tautomeric switching molecule in which truly intramolecular long-range proton transfer occurs in solution at room temperature. The system consists of a benzothiazole rotor attached to a 7-hydroxy quinoline stator. According to the experimental and theoretical results, the OH proton is delivered under irradiation to the quinolyl nitrogen atom through a series of consecutive proton transfer and twisting steps. The use of a rigid rotor prevents undesired side processes that decrease the switching performance in previously described proton cranes and provides an unprecedented switching efficiency and fatigue resistance. The newly designed system confirms the theoretical concept for the application of proton transfer-initiated intramolecular twisting as the switching mechanism, developed more than 10 years ago, and provides unique insights for the further development of tautomeric molecular switches and motors, molecular logic gates, and new molecular-level energy storage systems.
ABSTRACT
Exciton migration is an important process for light harvesting with organic systems and often the bottleneck. Especially the formation of trap states hinders the mobility considerably. Although excimer excitons are often referred to as traps, their mobility has been demonstrated while their nature is still unclear. Here, we compare the mobility of singlet and excimer excitons in nanoparticles consisting of the same type of perylene bisimide molecules. By changing the preparation conditions, nanoparticles with different intermolecular coupling strengths are prepared. Femtosecond transient absorption spectroscopy reveals the formation of excimer excitons from Frenkel excitons. The mobility of both exciton types is determined by evaluating exciton-exciton annihilation processes. In the lower coupling regime, singlet mobility is observed, whereas for stronger coupling the dynamics is dominated by a 10-fold increased excimer mobility. The excimer mobility can thus even be higher than the singlet mobility and is affected by the intermolecular electronic coupling.
ABSTRACT
Non-toxicity and stability make two-dimensional (2D) bismuth halide perovskites better alternatives to lead-based ones for optoelectronic applications and catalysis. In this work, we synthesize sub-micron size colloidal quasi-2D Cs3Bi2I9 perovskite nanosheets and study their generation and relaxation of charge carriers. Steady-state absorption spectroscopy reveals an indirect bandgap of 2.07 eV, which is supported by the band structure calculated using density functional theory. The nanosheets show no detectable photoluminescence at room temperature at near bandgap excitation which is attributed to the indirect bandgap. However, cathodoluminescence spanning a broad range from 500 nm to 750 nm with an asymmetric and Stokes-shifted emission is observed, indicating the phonon- and trap-assisted recombination of charge carriers. We study the ultrafast charge carrier dynamics in Cs3Bi2I9 nanosheets using femtosecond transient absorption spectroscopy. The samples are excited with photon energies higher than their bandgap, and the results are interpreted in terms of hot carrier generation (<1 ps), thermalization with local phonons (â¼1 ps), and cooling (>30 ps). Further, a relatively slow relaxation of excitons (â³3 ns) at the band edge suggests the formation of stable polarons which decay nonradiatively by releasing phonons.
ABSTRACT
Understanding the transport mechanisms of electronic excitations in molecular systems is the basis for their application in light harvesting and opto-electronic devices. The exciton transfer properties depend pivotally on the intermolecular coupling and the latter on the supramolecular structure. In this work, organic nanoparticles of the perylene derivative Perylene Red are prepared with flash-precipitation under different conditions. We correlate their intermolecular couplings, optical spectra, quantum yields, emission lifetimes and their size and characterize their exciton dynamics upon excitation with ultrashort laser pulses by transient absorption spectroscopy. We find that the intermolecular coupling can be varied by changing the preparation conditions and thus the supramolecular structure. In contrast to the monomeric system, the generation of charge-transfer states is found after optical excitation of the nanoparticles. The time of the generation step is in the order of 100 ps and depends on the intermolecular coupling. The mobility of the originally excited excitons is determined from measurements with varying exciton density. To this end, we model the contribution of exciton-exciton annihilation to the exciton decay assuming three-dimensional incoherent diffusion. The extracted exciton diffusion constant of nanoparticles with stronger intermolecular coupling is found to be 0.17 nm2 ps-1 and thereby about ten times higher than in the particles with smaller coupling.
ABSTRACT
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.
