Journal
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
Volume 143, Issue 4, Pages 2025-2036Publisher
AMER CHEMICAL SOC
DOI: 10.1021/jacs.0c12169
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Funding
- National Science Foundation [CHE-1807729, CHE-1664926]
- Indian Institute of Technology Kanpur
- Ministry of Electronics & Information Technology (MeitY), Government of India
- IISER Bhopal
- Centre for Develoment of Advanced Computing (C-DAC)
- National Supercomputing Mission (NSM), Government of India
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Excited anthracene forms supramolecular host-guest complexes with octa acid in aqueous medium, with the 2:2 complex showing intense excimer emission and the 2:1 complex exhibiting only monomer emission. The behavior of anthracene molecules in the excited state is influenced by the host-guest structure and is time-dependent, indicating a change in molecule behavior under confinement.
Excited anthracene is well-known to photodimerize and not to exhibit excimer emission in isotropic organic solvents. Anthracene (AN) forms two types of supramolecular host-guest complexes (2:1 and 2:2, H:G) with the synthetic host octa acid in aqueous medium. Excitation of the 2:2 complex results in intense excimer emission, as reported previously, while the 2:1 complex, as expected, yields only monomer emission. This study includes confirming of host-guest complexation by NMR, probing the host-guest structure by molecular dynamics simulation, following the dynamics AN molecules in the excited state by ultrafast time-resolved experiments, and mapping of the excited surface through quantum chemical calculations (QM/MM-TDDFT method). Importantly, time-resolved emission experiments revealed the excimer emission maximum to be time dependent. This observation is unique and is not in line with the textbook examples of time-independent monomer-excimer emission maxima of aromatics in solution. The presence of at least one intermediate between the monomer and the excimer is inferred from time-resolved area normalized emission spectra. Potential energy curves calculated for the ground and excited states of two adjacent anthracene molecules via the QM/MM-TDDFT method support the model proposed on the basis of time-resolved experiments. The results presented here on the excited-state behavior of a well-investigated aromatic molecule, namely the parent anthracene, establish that the behavior of a molecule drastically changes under confinement. The results presented here have implications on the behavior of molecules in biological systems.
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