title: Multiphoton harvesting metal-organic frameworks.
abstract: Multiphoton upconversion is a process where two or more photons are absorbed simultaneously to excite an electron to an excited state and, subsequently, the relaxation of electron gives rise to the emission of a photon with frequency greater than those of the absorbed photons. Materials possessing such property attracted attention due to applications in biological imaging, photodynamic therapy, three-dimensional optical data storage, frequency-upconverted lasing and optical power limiting. Here we report four-photon upconversion in metal-organic frameworks containing the ligand, trans, trans-9,10-bis(4-pyridylethenyl)anthracene. The ligand has a symmetrical acceptor-π-donor-π-acceptor structure and a singlet biradical electronic ground state, which boosted its multiphoton absorption cross-sections. We demonstrate that the upconversion efficiency can be enhanced by Förster resonance energy transfer within host-guest metal-organic frameworks consisting of encapsulated high quantum yielding guest molecules. Using these strategies, metal-organic framework materials, which can exhibit frequency-upconverted photoluminescence excited by simultaneous multiphoton absorption, can be rationally designed and synthesized.
authors.affiliation: National University of Singapore, Graduate School for Integrative Sciences and Engineering, Singapore 119077, Singapore.|Department of Chemistry, National University of Singapore, 3, Science Drive 3, Singapore 117542, Singapore.
authors.forename: Hong Sheng
authors.initials: HS
authors.lastname: Quah
authors.affiliation: Department of Physics, National University of Singapore, 3, Science Drive 3, Singapore 117542, Singapore.
authors.forename: Weiqiang
authors.initials: W
authors.lastname: Chen
authors.affiliation: Institute of Chemical and Engineering Sciences, A*STAR, Jurong Island, Singapore 627833, Singapore.
authors.forename: Martin K
authors.initials: MK
authors.lastname: Schreyer
authors.affiliation: Department of Chemistry, National University of Singapore, 3, Science Drive 3, Singapore 117542, Singapore.
authors.forename: Hui
authors.initials: H
authors.lastname: Yang
authors.affiliation: Department of Chemistry, National University of Singapore, 3, Science Drive 3, Singapore 117542, Singapore.
authors.forename: Ming Wah
authors.initials: MW
authors.lastname: Wong
authors.affiliation: Department of Physics, National University of Singapore, 3, Science Drive 3, Singapore 117542, Singapore.
authors.forename: Wei
authors.initials: W
authors.lastname: Ji
authors.affiliation: Department of Chemistry, National University of Singapore, 3, Science Drive 3, Singapore 117542, Singapore.
authors.forename: Jagadese J
authors.initials: JJ
authors.lastname: Vittal