Jared J. Paul

Concerted electron-proton transfer in the optical excitation of hydrogen-bonded dyes

The simultaneous, concerted transfer of electrons and protons—electron-proton transfer (EPT)—is an important mechanism utilized in chemistry and biology to avoid high energy intermediates. There are many examples of thermally activated EPT in ground-state reactions and in excited states following photoexcitation and thermal relaxation. Here we report application of ultrafast excitation with absorption and Raman monitoring to detect a photochemically driven EPT process (photo-EPT). In this process, both electrons and protons are transferred during the absorption of a photon. Photo-EPT is induced by intramolecular charge-transfer (ICT) excitation of hydrogen-bonded-base adducts with either a coumarin dye or 4-nitro-4′-biphenylphenol. Femtosecond transient absorption spectral measurements following ICT excitation reveal the appearance of two spectroscopically distinct states having different dynamical signatures. One of these states corresponds to a conventional ICT excited state in which the transferring H+ is initially associated with the proton donor. Proton transfer to the base (B) then occurs on the picosecond time scale. The other state is an ICT-EPT photoproduct. Upon excitation it forms initially in the nuclear configuration of the ground state by application of the Franck–Condon principle. However, due to the change in electronic configuration induced by the transition, excitation is accompanied by proton transfer with the protonated base formed with a highly elongated +H─B bond. Coherent Raman spectroscopy confirms the presence of a vibrational mode corresponding to the protonated base in the optically prepared state.

Colorimetric Particle Acidity Analysis of Secondary Organic Aerosol Coating on Submicron Acidic Aerosols

The particle acidity of secondary organic aerosol (SOA) created from ozonolysis of α-pinene in the presence of acidic inorganic seed aerosols was investigated using an indoor Teflon film chamber. Colorimetry integrated with a reflectance UV-Visible spectrometer was used for the first time to dynamically measure the aerosol acidity over time. An external calibration curve was produced based on the correlation between the proton mass (ng) collected on the filter, which was impregnated with metanil yellow, and the absorbance of the reflectance UV-Visible spectra for the protonated and the unprotonated metanil yellow on the filter. Using this calibration curve, the particle acidity of the submicron acidic sulfate aerosol coated with SOA was measured from the reflectance UV-Visible spectra of filter samples. The colorimetric analysis requires a short sampling time (less than 2 m) for SOA studies using the laboratory Teflon film chamber and eliminates extraction of the filter sample with water. Results show that the particle acidity of the aerosol decreased over time due to the formation of organic sulfate through the interaction of sulfuric acid with SOA products. The feasibility of this method is also demonstrated for the measurement of the acidity of ambient particles.

Base-induced phototautomerization in 7-hydroxy-4-(trifluoromethyl)coumarin.

Excited-state proton-transfer dynamics between 7-hydroxy-4-(trifluoromethyl)coumarin and 1-methylimidazole base in toluene were studied using ultrafast pump-probe and time-resolved emission methods. Charge-transfer excitation of the hydroxycoumarin shifts electron density from the hydroxyl group to the carbonyl, resulting in an excited state where proton transfer to the base is highly favored. In addition to its the photoacid characteristics, the shift in the hydroxycoumarin electronic distribution gives it characteristics of a photobase as well. The result is a tautomerization process occurring on the picosecond time scale in which the 1-methylimidazole base acts as a proton-transfer shuttle from the hydroxyl group to the carbonyl.