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Azobenzene-based chiral dopants in cholesteric liquid crystals are of interest since the properties they induce in the liquid crystal could be tuned photochemically. Here, we use a substituted binaphthyl with a halogenated azobenzene as a chiral dopant to induce a photoswitchable cholesteric phase in the nematic 4-n-pentyl-4’-cyanobiphenyl. The azobenzene group chemically attached to the chiral dopant undergoes isomerization from trans to cis upon irradiation with green light (wavelength 535 nm), and from cis to trans upon irradiation with blue light (wavelength 450 nm). The transition between the two isomers causes helicity inversion of the cholesteric, with a left-handed trans isomer and a right-handed cis isomer. We report on the kinetics of photoisomerization of both processes (trans-to-cis and cis-to-trans) in the nematic host by following the pitch evolution over time. We show that the kinetic mechanism corresponds to a two-step process: a first-order isomerization followed by a second-order autocatalytic isomerization. This mechanism differs from the typical first-order kinetics for cis-to-trans or trans-to-cis isomerization in azobenzenes. The autocatalytic process is attributed to interactions between the chiral dopant and the nematic host.
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Excited State Charge Separation in an Azobenzene‐Bridged Perylenediimide Dimer – Effect of Photochemical Trans‐Cis Isomerization
Abstract Photoinduced charge transfer and separation events in a newly synthesized azobenzene‐bridged perylenediimide‐dimer (PDI‐dimer) are demonstrated. Trans‐to‐cis conversion (∼50 % efficiency) from the initial trans PDI‐dimer by 355 nm pulsed laser light, and its reversal, cis‐to‐trans, process by 435 nm laser light irradiation has been possible to accomplish. Efficient fluorescence quenching in the PDI‐dimer, more so for the cis isomer was witnessed, and such quenching increased with increasing solvent polarity. DFT‐calculated geometry and electronic structures helped in visualizing the charge transfer in the PDI‐dimer in both isomeric forms, and also revealed certain degree of participation of the azobenzene entity in the charge transfer events. Femtosecond transient absorption spectral studies confirmed occurrence of both charge transfer followed by charge separation in the studied PDI‐dimer in both trans and cis forms in polar solvents, and the evaluated time constants from Global target analysis revealed accelerated events in the cis PDI‐dimer due to proximity effects. The present study offers key insights on the role of the azobenzene bridge, and the dimer geometry in governing the excited state charge transfer and separation in symmetrically linked PDI dimer.
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- Award ID(s):
- 2000988
- PAR ID:
- 10449868
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Chemistry – A European Journal
- Volume:
- 27
- Issue:
- 60
- ISSN:
- 0947-6539
- Page Range / eLocation ID:
- p. 14996-15005
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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