AmericanChemicalSociety.com

The aim of our research is to improve the effectiveness of photoinduced charge-separation between donors and acceptors. The long-lived charge separated states will then have enough time to undergo desired chemistry. At the beginning of this project, we investigated hydrogen-bonded naphthalimide-pyridine (NI-PYR) systems, with a goal to learn how to take advantage of H-bond dynamics to control the electron flow in molecular systems. To obtain a good understanding of the chromophore used in our donor-acceptor system, we started out with the investigation of excited state properties of five NI derivatives.¹ For this purpose, my group developed a femtosecond pump-probe instrument that can probe the transient species in both the visible and mid-IR range. ^1-6  One of the main findings of our work is that the excited state properties of NI derivatives are characterized by a competition between n,p* and p,p* excited states. We further studied electron transfer dynamics from excited SMe-NI to NO₂,CN-PYR.² The main finding of our work is that the fast deactivation of the excited state occurs in the H-bonded complex. Unfortunately, the fast thermal deactivation of the SMe-NI excited state in H-bonded complex made it impossible to study electron transfer dynamics in NI-PYR systems.

This initial study led us to develop a project aimed at using flavonium salt derivative for catalytic oxidation of water. The main goal of this research is to use proton-coupled electron transfer to drive the production of peroxyl radicals from a flavin-based alcohol. The further oxidation of peroxyl radicals is expected to produce oxygen and recover the flavonuim salt.

The reaction of Et-Fl⁺ with OH^- ions to produce Et-FlOH was studied using UV/VIS absorption spectroscopy. Figure below presents the absorption spectra of Et-Fl⁺ as a function of solution pH.  At low pH, the spectrum arises from Et-Fl⁺ and consists of three bands centered at 285, 432 and 565 nm. As the pH is raised, these absorption bands disappear and a new band appears at 345 nm due to the formation of Et-FlOH. Figure 1b presents the geometries of Et-Fl⁺ and Et-FlOH optimized using density-functional theory (DFT). The isoalloxazine ring of Et-Fl⁺ is fully planar, which gives rise to a strong electronic delocalization and explains the absorption spectrum that extends deeply into the visible region (up to 700 nm). On the other hand, the addition of the OH^- ion breaks one of the double bonds and perturbs the planarity of the isoalloxazine ring. The outcome of this perturbation is the blue-shift of the absorption bands. Using these UV/VIS data, we calculated the driving force for this process to be DG=-15 kcal/mol. Thus, the conversion of Et-Fl⁺ to Et-FlOH is highly spontaneous.

To investigate the behavior of Et-FlOH upon one- and two-electron oxidation, we performed cyclic voltammetry using acetonitrile as a solvent. The cyclic voltammogram of Et-Fl⁺ consists of two reversible one-electron reductions at E_1/2= + 0.13 V and E_1/2 = -0.54 V. Furthermore, Et-Fl⁺ shows an irreversible one-electron oxidation at 2.27 V. Figures 2b and c show cyclic voltammograms of Et-FlOH. The scan up to + 1.2 V reveals a reversible one-electron oxidation peak at E_1/2=+0.92 V (red curves), suggesting that the radical cation of Et-FlOH is stable over the course of the cyclic voltammetry scan (scan rate: 100 mV/s). As we scan to higher potentials (blue and black curves), we observe an irreversible oxidation peak at + 1.4 V, that corresponds to the two-electron oxidized Et-FlOH²⁺. Upon this irreversible two-electron oxidation, we observe a decrease in the current originating at +0.92 V and an increase of new peaks at +2.27 V, +0.13 and -0.54 V, all of which are signature peaks of Et-Fl⁺. These peaks are consistent with the conversion of Et-FlOH²⁺ to Et-Fl⁺. Figure 2d outlines the processes occurring at the electrode. The conversion of Et-FlOH²⁺ to Et-Fl⁺ suggests that the oxygen is most probably released in the process.

We are currently investigating the photophysical behavior of Et-FL+ and Et-FlOH using pump-probe spectroscopy. Figure below presents visible and mid-IR transient absorption spectra of Et-Fl⁺. The visible spectrum consists of excited-state absorption (600 nm), ground-state bleach (520 nm) and stimulated emission (750 nm) signals that arise from ¹S state of Et-Fl⁺. The TRIR spectra of Et-Fl⁺ show signals due to C=N and C=O vibrations of Et-Fl⁺ in its excited state.  The future set of experiments will involve photochemical oxidation of Et-FlOH in the presence of a sacrificial acceptor.

Reference:

(1)        P. Kucheryavy, G. Li, S. Vyas, C. Hadad, K. D. Glusac, " “Electronic Properties of 4-Substituted Naphthalimides", J. Phys. Chem A, 2009, 113, 6453-6461.

(2)        P. Kucheryavy, G. Li, S. Vyas, C. Hadad, K. D. Glusac, "Ultrafast Excited State Deactivation in Hydrogen-Bonded Naphthalimide-Pyridine Donor-Acceptor Systems", In Preparation.

(3)   G. Li, K. Parimal, S. Vyas, C. M. Hadad, A. H. Flood, K. D. Glusac, “Pinpointing the Extent of Electronic Delocalization in the Re(I)-to-Tetrazine Charge Separated Excited State Using Time-Resolved Infrared Spectroscopy”,  J. Am. Chem. Soc., 2009, 131, 11656-11657.

 (4)     G. Li, K. D. Glusac, “The Role of Adenine in Fast Excited State Deactivation of FAD: A Femtosecond mid-IR Transient Absorption Study”, J. Phys. Chem. B., 2009, 113, 9059-9061.

 (5)     G. Li, V. Sichula, K. D. Glusac, “The Role of Adenine in Thymine Dimer Repair by Reduced FAD”, J. Phys. Chem. B, 2008, 112, 10758-10764.

(6)         G. Li, K. D. Glusac “Light-Triggered Proton and Electron Transfer in Flavin Cofactors”, J. Phys. Chem. A, 2008, 112(20), 4573-4583.