The use of low-intensity NIR light to operate molecular switches offers several potential advantages including enhanced penetration into bulk materials, in particular biological tissues, and reduced radiation damage due to the limited photon energies. The latter, however, pose a challenge for designing reasonably bistable systems. We have developed a general design strategy for direct one-photon NIR photoswitches based on negative photochromic dihydropyrenes carrying opposing strong donor−acceptor substituents either along the long axis of the molecule or across it. Thus, two series of 2,7-and 4,9-disubstituted dihydropyrenes were synthesized, and their photothermal properties investigated as a function of the type, strength, and position of the attached donor and acceptor substituents as well as the polarity of the environment. By shifting the excitation wavelength deep into the NIR, both NIR one-photon absorption cross-section and photoisomerization efficiency could be maximized while retaining a reasonable thermal stability of the metastable cyclophanediene isomer. Thus, the lowest optical transition was shifted beyond 900 nm, the NIR cross-section was enhanced by two orders of magnitude, and the thermal half-lives vary between milliseconds and hours. These unique features open up ample opportunities for noninvasive, optically addressable materials and material systems.
We report on the synthesis and structural characterization of novel, partially fluorinated hexabenzocoronene (HBC) derivatives. The fluorination of polycyclic aromatic hydrocarbons (PAHs) is a well-established method to enhance the stability of organic semiconductors (OSCs) and render them n-type. For HBC it has been observed that fluorination leads to a modification of the molecular packing motif from a herringbone arrangement to a parallel-packed motif. Here, we study whether this transformation of the molecular packing is also found for the partially fluorinated HBCs 2,5-difluoro-hexa-peri-hexabenzocoronene (FHBC) and 2,5,8,11-tetrafluoro-peri-hexabenzocoronene (FHBC). Combining powder diffraction and NEXAFS dichroism measurements, we reveal that indeed all partially fluorinated compounds adopt a parallel molecular packing, hence maximizing their intermolecular contact area. We identify fluorine-hydrogen bonds as the mediating driving force to specifically stabilize this molecular arrangement and direct self-assembly. Furthermore, we show that the relative orientation of the HBCs on the underlying surface can be precisely controlled by varying the substrate materials. Finally, the energetic states of the compounds are analyzed using photoelectron spectroscopy, optical spectroscopy and density functional theory to identify the effects of fluorination on these fundamental electronic characteristics.
We present as trategy to achieve highly cooperative photoswitching, where the initial switching event greatly facilitates subsequent switching of the neighboring unit. By linking donor/acceptor substituted dihydropyrenes via suitable p-conjugated bridges,t he quantum yield of the second photochemical ring-opening process could be enhanced by more than two orders of magnitude as compared to the first ringopening. As ar esult, the intermediate mixed switching state is not detected during photoisomerization although it is formed during the thermal backr eaction. Comparing the switching behavior of various dimers,both experimentally and computationally,h elped to unravel the crucial role of the bridging moiety connecting both photochromic units.T he presented dihydropyrene dimer serves as model system for longer cooperative switching chains,w hich, in principle,s hould enable efficient and directional transfer of information along am olecularly defined path. Moreover,o ur concept allows to enhance the photosensitivity in oligomeric and polymeric systems and materials thereof.
zu finden. 2020 Die Autoren. Verçffentlicht von Wiley-VCH GmbH. Dieser Open Access Beitrag steht unter den Bedingungen der Creative Commons Attribution License, die jede Nutzung des Beitrages in allen Medien gestattet, sofern der ursprüngliche Beitrag ordnungsgemäß zitiert wird.
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