print pageprinter friendly
Reports: UR453159-UR4: Impact of Solvent-Solute Interactions on Photochemistry of p-Aminobenzoic Acid Derivatives

Sarah J. Schmidtke Sobeck, PhD, College of Wooster

            We are interested in understanding how perturbations in the environment and nature of the donor-acceptor moieties impact the photochemistry of potential charge transfer systems.   By studying small organic model systems we may gain fundamental insight into similar processes in more complex materials or biological systems.  Our studies during the first year of ACS-PRF funding have focused upon comparing the photophysical properties of p-aminobenzoic acid (PABA) and dimethyl-p-aminobenzoic acid (DABA) in a range of solvents.  A general scheme for the intramolecular charge transfer (ICT) that may occur following photo-excitation is shown in Figure 1.   PABA exhibits emission from only the LE state and is used as the control system to understand the photochemistry in the absence of the ICT reaction.   Our work in this year has advanced the understanding of how differences in the solvent-solute interactions impact the reaction potential energy surface and the physicochemical properties of our model compounds.

Figure 1.  ICT reaction from the locally excited (LE) to zwitterionic (ZI) states for PABA (R=H) and DABA (R=CH3).

Results

            Our first goal during this granting period was to experimentally evaluate the quantum efficiencies of PABA and DABA in three solvents:  dichloromethane, methanol, and acetonitrile.   The quantum yields are used to understand deactivation pathways following photo-excitation.   PABA has greater fluorescence quantum yields than DABA for all solvents, and for DABA emission from the ZI state is more efficient than the LE state.  This is a direct result of the ICT deactivation pathway following photoexcitation.  The solvent dependences of the quantum efficiencies illustrates that the deactivation pathways vary by solvent.    Both compounds have the lowest quantum yields in methanol.  This can be attributed to strong hydrogen bonding to the solvent, which allows for more efficient non-radiative cooling through energy transfer as heat to the solvent. 


         A second purpose for evaluating the quantum yields is to gain a more accurate relationship between the fluorescence intensity and excited state population.  Fitting the fluorescence spectra for DABA to isolate contributions from the LE and ZI emission, and correcting for quantum yields, provides an estimate of the populations of the ICT reactant and product.  The equilibrium constant for the ICT was evaluated in this way over a range of temperatures.  From this data a Stevens-Ban plot was made for each of the solvents to evaluate the ICT reaction thermodynamics, resulting in the data presented in Figure 2. 



Figure 2.  Stevens-Ban plot for DABA in methanol (blue circles), acetonitrile (red squares), and dichloromethane (green triangles). 


         The thermodynamics for the ICT reaction show a strong solvent dependence.  In methanol the activation thermodynamics are measured with a barrier to the reaction (positive enthalpy change) and increased entropy as the reaction proceeds through the transition state.  This is attributed to the redistribution of the solvent environment as the ICT reaction proceeds.  In contrast a barrierless process is observed in other solvents.   The reaction is most enthalpically favorable in dichloromethane, with the least specific solvent-solute interactions.  The overall reaction shows a decrease in entropy that is attributed to stronger intermolecular interactions between the charge-separated species and the solvent. The variation in the potential energy surfaces (PES) for the different solvents is illustrated in Figure 3.

Figure 3.  Potential energy surface for the ICT reaction showing the absorbance (blue) and emission (green, red) processes (I).  Panel II illustrates solvent effects on the relative positioning of the LE and ZI is shown. The solid black potential represents an initial ZI well relative to the LE state (blue), the dashed line a relative stabilization of the ZI relative to the LE state, and dotted line greater solvent reorganization accompanying the ICT.

            Solvent-solute interactions were further probed using infrared spectroscopy.  Experimentally infrared spectra were measured for the pure solid in the presence and absence of solvent. Computationally the solute was optimized alone and complexed with a methanol molecule, as illustrated in Figure 4.   Both computational and experimental analyses indicate an increased frequency of the carbonyl-stretching mode in the presence of methanol. There were no observable shifts in other IR-active modes or for solvents that do not form strong intermolecular interactions.   These results support our assignment of differences in the ICT thermodynamics in methanol to hydrogen bonding interactions.  Using the solvent to alter reaction energetics could be exploited in the design of charge transfer compounds for material applications, and fundamentally the impact of the solvent advances our understanding of this class of chemical reaction in biological systems that may or may not have included water at an active site.

Figure 4. Vector picture of normal mode of interest for the free (I) and solvent complexed (II) PABA molecule. 

Ongoing Plans

            An article summarizing this work, with three student co-authors, is in preparation.  Jacob Boroff, a senior thesis student in my lab during the grant period, carried out the majority of the work described above.  This work was presented at the Inter-American Photochemistry Society Meetings (I-APS) in January of 2014, co-authored by Jacob.  This academic year (2014-15) I am on sabbatical allowing time for continued research and manuscript preparation.  This coming summer (2015) I plan to hire three students to work on the following:  (1) synthesis of PABA and DABA derivatives with modifications at the electron acceptor moiety, and (2) analysis of the excited state dynamics of PABA and DABA through transient absorbance measurements using resources at the Ohio State University.  These studies will advance our understanding of how structure of the ICT compound can impact reactivity and assess the real-time dynamics of the ICT process.  The PI and her group are very thankful to the ACS PRF for supporting this project, and allowing the opportunity for undergraduates to fully engage in the research process.   The funding is particularly valuable for supporting travel to the labs of our collaborators, attendance at conferences, and student summer research.