Journal
CHEMISTRY-A EUROPEAN JOURNAL
Volume 28, Issue 8, Pages -Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/chem.202103707
Keywords
fluorescent pH indicator; lipophilic probe; two-photon absorption; two-photon cross section; two-photon excitation fluorescence
Categories
Funding
- Ministry of Education and Research, Republic of Estonia [PRG661, PRG690, PSG317, PSG400, MOBTP128]
- European Regional Development Fund [TK134, TK141]
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In this study, we successfully controlled the fluorescence characteristics and two-photon excited fluorescence (2PEF) characteristics of fluorophores by changing the environmental acidity. This change was achieved through the design of four novel two-photon excited fluorophores.
Lipophilic fluorophores are widely implemented in nonlinear microscopy; however, few existing membrane-specific probes combine the high brightness of two-photon excited fluorescence (2PEF) with pH sensitivity. Herein we describe four novel two-photon excited fluorophores, based on a coumarin 151 core structure, where lipophilicity is induced by a covalently attached phosphazene moiety. Changing the environmental acidity using trifluoromethanesulfonic (triflic) acid leads to profound changes in the linear fluorescence and 2PEF characteristics, due to chromophores' switching between neutral- and protonated forms. We characterize this dependence by measuring the two-photon absorption (2PA) spectra over the region lambda(2PA)=550-1000 nm, observing 2PA cross sections of sigma(2PA)=10-20 GM, with an associated 2PEF brightness of 10-13 GM, in neutral solutions of both acetonitrile and n-octanol. Although quantum chemical modelling and NMR measurements show that, at high chromophore concentrations, protonation may be accompanied by a dimerization process, these dimers likely do not form at the lower concentrations used in optical spectroscopy.
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