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46807-G4
Controlled Electron Transfer in Hydrogen Bonded Systems

Ksenija D. Glusac, Bowling Green State University

The aim of our research is to improve the effectiveness of photoinduced charge-separation between donors and acceptors. Since the efficiency of this process is tightly correlated with the performance of solar cells, we expect this work to provide a basic knowledge for the improvement of photovoltaic devices. In this specific 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. The NI-PYR derivatives were designed in such a way so that the initial photoinduced electron transfer from NI to PYR triggers a proton motion along an H-bonded surface. This in turn is expected to make the charge recombination thermodynamically and/or kinetically inefficient. In other words, the H-bonded surface could act as a unidirectional gate for the electron flow.

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 The study involved synthesis and solvent-dependent absorption and emission spectra. Furthermore, the excited state dynamics of these compounds were analyzed using femtosecond optical and mid-IR pump-probe spectroscopy. While the optical pump-probe setup has already been available as a part of our shared laser facility here at BGSU, my group has built a transient absorption setup that can probe in the mid-IR range. To obtain a better understanding of our experimental results, we started collaboration with Prof. Christopher Hadad, who characterized the excited state behavior of NI derivatives using DFT.

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. As an example, the figure above presents the transient absorption spectra of SMe-NI derivative obtained at different time delays after the excitation pulse. We can see that the initially produced excited state can be characterized by an excited state absorption with a maximum at ~480 nm. We assign this state to the S₂ excited state of SMe-NI. Over the course of 40 ps, S₂ excited state converts to another state that exhibits a stimulated emission signal at ~500 nm, which we assign to S₁ excited state of SMe-NI.

The information about the character of these two states was obtained from DFT calculations. The above figure presents difference density plots of S₂ and S₁ states in SMe-NI. In the case of S₂ excited state, electronic charge is depleted from lone electron pairs of imide oxygen atoms and a charge is accumulated at the aromatic rings. This sort of difference density plot is characteristic of an n,p* excited state, which is additionally supported by the small oscillator strength for this transition (f=0.0001). The transition density for the S₁ excited state is characteristic of p,p* excited state. In addition, the charge is depleted from SMe-group, which is indicative of a charge-transfer state.

We further studied electron transfer dynamics from excited SMe-NI to NO₂,CN-PYR.² The donor-acceptor system was chosen in such a way to enable: (i) photoinduced electron transfer from NI to PYR and (ii) proton transfer from NI radical cation to PYR radical anion that are produced upon electron transfer. The main finding of our work is that the fast deactivation of the excited state occurs in the H-bonded complex. This effect can be best observed from the figure above, which presents the dynamics of bleach recovery of the SMe-NI carbonyl-group at 1695 cm^-1. As the concentration of the PYR is increased, less excited state absorption is observed at t=0, suggesting that the deactivation of the NI excited state in H-bonded complex is faster than the time-resolution of our instrument (~300 fs).

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. To overcome this problem, we initiated a preparation of covalently bound donor-acceptor system presented in the scheme above. We are currently in the last step of synthesis, and we expect this model compound to be better for electron transfer studies.

Reference:

(1) Pavel Kucheryavy, G. L., Shubham Vyas, Christopher Hadad, Ksenija D. Glusac, "Excited State Properties of 4-Substituted Naphthalimides", In Preparation.

(2) Pavel Kucheryavy, G. L., Shubham Vyas, Christopher Hadad, Ksenija D. Glusac, "Ultrafast Excited State Deactivation in Hydrogen-Bonded Naphthalimide-Pyridine Donor-Acceptor Systems", In Preparation.

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