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Excited state intramolecular proton transfer (ESIPT) and trans-cis isomerization on the triplet excited states

Weragoda, Geethika K.
http://orcid.org/0000-0002-0442-2812
http://rave.ohiolink.edu/etdc/view?acc_num=ucin1439296134

Abstract Details

2015, PhD, University of Cincinnati, Arts and Sciences: Chemistry.
The work outlined in this dissertation, we report Excited State Intramolecular Proton Transfer (ESIPT) of ortho-hydroxy acetophenone derivatives and aromatic Schiff bases occurred on their triplet-excited states. Comparatively, only a little research has shown its triplet reactivity while majority reported to undergo ESIPT on the singlet surface. Enol-forms of these compounds converted to the keto-forms after being transferred the H atom on the hydroxyl group to the electronegative atoms at the γ position (N and O) in the excited states. Moreover, we have compared the ESIPT in solution and solid-states of Schiff bases. Alkyl substituent in the aniline moiety of the aromatic Schiff bases is suggested to act as space-opener, which makes twisted geometries on their ground and excited states and hence play a significant role in controlling the reactivity of the excited species. In contrast, in non-crystalline media, molecular geometries become less predominant in determining the reaction rates as molecules experience free molecular motions compared to the crystalline media. Nanosecond laser flash photolysis of aromatic Schiff bases showed the formation of triplet 1,4-biradicals (triplet excited state of the cis keto-form) in methanol with several microseconds. Nano-crystal suspension in water produces transient absorption spectra of the triplet-excited states of enol-form with the lifetime of ~1-4 µs. Crystal structure analysis and theoretical calculations revealed that these molecules are properly aligned for ESIPT and twisted-geometry of triplet-excited state of enol-form makes them longer-lived compared to the solution. Interestingly, dual phosphorescence was observed for Schiff bases corresponding to the triplet-excited state of the enol-form (n,π*) and triplet-excited states of keto-form (π,π*) generated after ESIPT process. We extended this study to design novel sunscreens agents with broad UV absorption spectrum and high photostability. Proposed compounds, α,β unsaturated ortho-hydroxyl acetophenone derivatives, absorb high energy UV light (Sunlight) and release them as harmless heat via ESIPT and hence minimize the photodegradation. Moreover, trans-cis isomerization of novel sunscreen derivatives is found to proceed through ESIPT and orthogonal triplet 1,2-biradical was not detected. In contrast, for the compounds without any substituent attached to the ortho-position of the acetophenone moiety showed the formation of orthogonal triplet 1,2-biradicals, which aligned p-orbitals nearly perpendicular to avoid repulsions between two radicals. Furthermore, effect of methyl substituent to the trans-cis isomerization was elaborated and those of the trans-compounds with methyl substituent on the β position of the vinylic C=C showed the formation of its cis-isomer while trans a-methyl substituted compound did not form its cis-isomer. Thus the geometry of the triplet 1,2-biradical plays a key role in determining the trans-cis isomerization and triplet 1,2-biradical with the methyl group attached to the a position of the vinyl C=C bond aligned favors to the formation of trans-isomer over its cis-isomer and hence trans-isomer is detected as the only product. Nano-second Laser Flash Photolysis (LFP), phosphorescence, low-temperature and room-temperature UV-Visible studies and molecular modeling are used to elucidate the mechanistic pathway of these photoreactions.
Anna Gudmundsdottir, Ph.D. (Committee Chair)
Peng Zhang, Ph.D. (Committee Chair)
Sivaramakrishnan Muthukrishnan, Ph.D. (Committee Member)
James Mack, Ph.D. (Committee Member)
Thomas Ridgway, Ph.D. (Committee Member)
365 p.
Organic Chemistry
Laser Flash Photolysis; Molecular Modeling; ESIPT; Trans-cis isomerization; biradicals; triplet excited states

Recommended Citations

Citations

  • Weragoda, G. K. (2015). Excited state intramolecular proton transfer (ESIPT) and trans-cis isomerization on the triplet excited states [Doctoral dissertation, University of Cincinnati]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1439296134

    APA Style (7th edition)

  • Weragoda, Geethika. Excited state intramolecular proton transfer (ESIPT) and trans-cis isomerization on the triplet excited states. 2015. University of Cincinnati, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=ucin1439296134.

    MLA Style (8th edition)

  • Weragoda, Geethika. "Excited state intramolecular proton transfer (ESIPT) and trans-cis isomerization on the triplet excited states." Doctoral dissertation, University of Cincinnati, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1439296134

    Chicago Manual of Style (17th edition)

ucin1439296134
241
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This open access ETD is published by University of Cincinnati and OhioLINK.