Photophysical Properties of the Prefluorescent Nitroxide Probes QT and C₃₄₃T

Abstract

The photophysical properties of the nitroxide prefluorescent probes 4-(3-hydroxy-2-methyl-4-quinolinoyloxy)-2,2,6,6-tetramethyl-piperidin-4-yl) ester free radical (QT) and 2,3,4,6,7,8-hexahydro-quinolizino [1,10,9-gh] coumarin-3-carboxylic acid (1-oxyl-2,2,6,6-tetramethyl-piperidin-4-yl) ester free radical (C343T) were evaluated as a function of pH and solvent properties. The absorbance of QT showed high pH sensitivity. The pKa values for the different ionization forms involved in the acid-base equilibrium of the quinoline chromophore were determined in the ground and excited states. The fluorescence lifetimes of QT, and N-hydroxylamine (QTH) and quinoline methyl ester (QMe) derivatives, showed that the intramolecular quenching efficiency by the nitroxide moiety is independent of the quinoline ionization form. The fluorescence and absorbance of C343T were highly sensitive to solvent polarity in agreement with a charged transfer excited state of the chromophore. However, we noted a decrease in the intramolecular fluorescence quenching efficiency by the nitroxide moiety when changing the polarity of the solvent from hexane to water. This behavior has been attributed to a suppression of an energy transfer mechanism in the nitroxide quenching process in very polar solvents. The results obtained in micelles allow us to propose QT and C343T as sensors for pH and micropolarity, respectively, in addition to their role as monitors for free radicals or hydrogen transfer from phenols.

Photochemical Insights on Intramolecular Dye‐Sensitized Free‐Radical Processes with a Quinoline Antenna

“…The absorption spectrum of 1 in CH 3 CN (Figure S1, Supporting Information) shows a band attributable to the quinoline ( Q ) chromophore at λ Abs =340 nm, while its emission spectrum (Figure S1) reveals that the fluorescence at λ Em =400 nm is largely quenched, as expected for fluorophore‐TEMPO dyads and as illustrated by the comparison between the emission spectra of Q and 1 (Figure S2). Paramagnetic nitroxides covalently linked to fluorophores are well known to quench the excited states of the latter [19,29,33–46] . In these systems, intramolecular quenching occurs through electron exchange between the two units, which causes non‐radiative relaxation of the local singlet state to the ground state of the fluorophore [19,20,42,47] .…”

“…Paramagnetic nitroxides covalently linked to fluorophores are well known to quench the excited states of the latter. [19,29,[33][34][35][36][37][38][39][40][41][42][43][44][45][46] In these systems, intramolecular quenching occurs through electron exchange between the two units, which causes non-radiative relaxation of the local singlet state to the ground state of the fluorophore. [19,20,42,47] Nevertheless, hydrogen or radical trapping by TEMPO leads to the formation of a diamagnetic product, thereby disabling the quenching pathway and restoring fluorescence.…”

Coumarin 314 Free Radical Cation: Formation, Properties, and Reactivity toward Phenolic Antioxidants

“…We have recently reported solvent effects on coumarin 314 ground and singlet excited -state characteristics. 22 Absorption and fluorescence bands of C 314 at long wavelengths were attributed to an intramolecular charge-transfer state (ICT). Effectively, the absorption and fluorescence maxima were redshifted as solvent polarity increased, fully compatible with a more stabilized ICT in high-polarity media (data not shown).…”

“…In this mechanism, the absorption of the first photon by C 314 produces the ICT, and a second photon absorption is responsible for the electron photoejection from the ICT intermediate, in eq 3 and 5, respectively. A short lifetime of the intermediate, ∼4.0 ns, 22 in comparison to the laser pulse and a lower deactivation rate of the intermediate to the ground state, shown in eq 4, than the sum rate of the two consecutive photon absorptions (eq 3 and 5) will render linear dependence of the photoejection yield with the laser intensity. Moreover, the lower threshold observed for the generation of the radical cation shown in Figure 4B is fully compatible with that proposed for electron photoejection through a two consecutive photon absorption considering a 100% efficiency on the absorption of the second photon (eq 5) following a linear dependence on the radical cation formation with the integrated light intensity.…”

From UV to NIR light, photo-triggered 1,3 dipolar cycloadditions as a modern ligation method in solution and on surface

“…The cleavage of the covalent connection between both moieties would negate the quenching performance. 128 Therefore, the connection of commercially available nitroxides and fluorophores via esterification, 122,123,[129][130][131][132][133][134] amidation, [135][136][137] or sulfonamidation [138][139][140] employed in the early stages of the PFN research are less suitable for efficient PFNs (refer to Scheme 10). The resulting compounds contain long, hydrolysis sensitive linkages.…”