Ongoing Research Projects

Ongoing Research Projects in the Modarelli Group

Organic Photovoltaics: Harnessing Solar Energy with Polymers

Photovoltaic devices (PDs) are designed to convert solar energy into electricity. Inorganic PDs are currently the industry standard, but are plagued by high costs of construction. In addition, these materials are rigid and therefore difficult to mold into shapes other than flat panels. Organic PDs are more desirable because of their low cost and the flexible nature of the organic polymers used in their construction that allow for flexible device structures.

In our group we are synthesizing cofacial oligomeric and polymeric molecules (Smith, T.; Modarelli, D.A. "The Efficient Synthesis of Unsymmetrical Oligo(phenylenevinylenes)" Tetrahedron Lett. 2008, 49, 526 - 528), and using ultrafast time-resolved laser spectroscopy to study electron and energy transfer in these molecules. These molecules are designed to absorb energy, and under a positive voltage bias, undergo electron transfer leading to sustained charge separation. We measure both energy and electron transfer rate constants using femtosecond and picosecond laser spectroscopic techniques. These molecules are then refined and used in second generation organic photovoltaic devices.

We use a combination of femtosecond, picosecond and nanosecond transient absorption and time-resolved laser experiments, and time-correlated single photon counting techniques (TCSPC) to study ultrafast electron and energy transfer events. We have a wide variety of laser equipment including an ultrafast femtosecond transient absorption spectrometer (Titan regenetively amplified/multipass laser, and three TOPAS optical parametric amplifiers), a picosecond TCSPC laser system (mode-locked picosecond Nd:YLF laser that pumps a Coherent 700 dye laser, and that is capable of producing 1 ps pulses over a range of wavelengths).

Monitoring Organic Catalysis: Time-Resolved Fluorescence Anisotropy of Chiral Dendrimers

We are interested in monitoring, in real time, the progress of catalytic reactions. The specific reactions in question are those involving dendrtitic catalysts prepared by Profs. Jon Parquette and T.V. RajanBabu from Ohio State. These dendrimers are chiral, and have been shown to have increasing ee values with increasing dendrimer generations. We use single molecule fluorescence and femtosecond pump-probe time-resolved fluorescence anisotropy measurements to examine the time dependent changes in dendrimer structure occurring as catalysis takes place. We anticipate this research will have far-reaching implications in both catalysis as well as biological chemistry. This collaboration is funded by NSF (a Center for Research Grant).

Photosynthetic Mimics Based Upon Dendritic Architectures

Photosynthesis is an elegant and complex series of efficient photo-initiated events involving energy and electron transport over long distances (>10-50Å). The net result is the conversion of light into chemically useful energy. One of the main areas of research in our group is the study of nanoscale molecules that mimic photosynthesis. These mimics are designed to absorb energy, and through a series of energy- and electron-transfer events, translate that energy into useful chemical potential. We measure both energy- and electron-transfer rate constants using femtosecond and picosecond laser spectroscopic techniques.

Recent efforts on this project have included:

(1) The synthesis and characterization of a variety of rigid and flexible dendrimers containing porphyrin core and electron-acceptor end-groups. Picosecond-resolved laser experiments were performed to measure electron-transfer rate constants. See, for example: Capitosti, G.J.; Guerrero, C.D.; Binkley, D.E., Jr.; Rajesh, C.S.; Modarelli, D.A. J. Org. Chem. 2003, 68, 247-261.; Rajesh, C.S.; Capitosti, G.J.; Cramer, S.C.; Modarelli, D.A. J. Phys. Chem. B, 2001, 105, 10175-10188, and Capitosti, G.J.; Cramer, S.J.; Rajesh, C.S.; Modarelli, D.A. Organic Letters 2001, 3, 1645-1648).

(2) Photophysical (i.e., steady state and time-resolved fluorescence and pump-probe femtosecond absorption) experiments on N-confused porphyrins, octaphryns, corroles and other unusual porphyrinoid molecules. These experiments are carried out in our group (Rajesh, C.S.; Alemán, E.A.; Ziegler, C.S.; Modarelli, D.A. J. Phys. Chem. A, 2006, 110, 8605-8612.; Wolff, S.A.; Alemán, E.A.; Banerjee, D.; Rinaldi, P.L.; Modarelli, D.A. J. Org. Chem. 2004, 69, 4571-4576.) and sometimes involve collaboration with Prof. Chris Ziegler at The University of Akron and Prof. Rick Geier from Colgate University (i.e, Shaw, J.L.; Wolff, S.A.; Aleman, E.A.; Ziegler, C.J.; Modarelli, D.A. J. Org. Chem. 2004, 69, 7423-7427.; Belair, J.P.; Ziegler, C.S.; Rajesh, C.S.; Modarelli, D.A. J. Phys. Chem. A 2002, 106, 6445-6451. Finally, we perform computational experiments in collaboration with Prof. Christopher Hadad at Ohio State University on a variety of interesting porphyrins (Vyas, S.; Hadad, C.; Modarelli, D.A. "TDDFT Calculations of the Structure and Absorption Spectra of Free-Base N-Confused Porphyrin and N-Confused Tetraphenylporphyrin" J. Phys. Chem. A 2008, 112, 6533–6549.; Vyas, S.; Alemán, E.A.; Davis, M.; Hadad, C.; Modarelli, D.A. J. Phys. Chem. A, 2009, To be submitted).

(3) Incorporating N-confused porphyrins into photonic arrays (see Wolff, S.A.; Alemán, E.A.; Banerjee, D.; Rinaldi, P.L.; Modarelli, D.A. J. Org. Chem. 2004, 69, 4571-4576 and Garrison, S.A.; Alemán, E.A.; Modarelli, D.A. J. Am. Chem. Soc. 2009, To be Submitted).

(4) Synthesizing dendrimers containing Ru(bpy)₃²⁺ and Os(tpy)₂²⁺groups, capable of electron-transfer (see Shriener, C.D.; Alemán, E.A.; Modarelli, D.A. J. Phys. Chem. A 2008, submitted for publication).

(5) Utilizing structural changes in macromolecules upon photoexcitation to promote rapid forward electron-transfer, and using these same changes to reduce reverse electron-transfer (Watchara Anannarukan, W.; Tantayanon, S.; Dong Zhang, D.; Alemán, E.A.; Modarelli, D.A.; Harris, F.W. Polymer 2006, 47, 4936-4945).

These macromolecules are studied by femtosecond, picosecond and nanosecond transient absorption and time-resolved laser experiments, and time-correlated single photon counting techniques (TCSPC). We have a wide variety of laser equipment including a new regenetively amplified femtosecond laser system consisting of a Coherent Vitesse oscillator, a Nd:YLF DQE pump laser, a Titan regenetively amplified/multipass laser, and three TOPAS optical parametric amplifiers. This laser system is used for transient absorption and fluorescence up-conversion experiments in the femtosecond regime. Our laser facilities also include, a Coherent Antares 76 mode-locked picosecond Nd:YAG laser that pumps a Coherent 700 dye laser, and that is capable of producing 1 ps pulses over a range of wavelengths. This laser system is used for performing TCSPC measurements. The mode-locked YAG is also used for seeding a regeneratively amplified dye laser (RGA-67, PTA-60) that produces ultrashort, high energy (<2 ps, 1 mJ) pulses for picosecond pump-probe absorption spectroscopic measurements. The TCSPC system has a time resolution, with reconvolution, of ~5 ps, while the pump-probe system has a resolution of ~40 ps. We also have a Spectra-Physics PRO-190 Nd:YAG pumped Lambda Physik Scanmate 2E dye laser and a Spectra-Physics GCR-150 Nd:YAG pumped Lambda Physik Scanmate 2E dye laser for tunable nanosecond laser spectroscopy.

Characterization of dendrimers is performed by MALDI-MS, GPC and NMR techniques. We also use molecular dynamics calculations in collaboration with Prof. Virginia B. Pett (The College of Wooster) to help confirm molecular structures and to aid with our understanding of the photochemistry of these macromolecules. We are also collaborating with Prof. Peter L. Rinaldi and his group (The University of Akron) to determine the three-dimensional structure of these dendrimers using a variety of multi-dimensional NMR techniques developed by Prof. Rinaldi and his group.

Photophysics of Aromatic Molecules

(With Goodyear Professor of Chemistry Edward C. Lim)

A second area of interest to us is the photophysics of excited molecules in condensed phases. In particular we study:

(1) Intramolecular charge-transfer reactions from the singlet excited state in solution (Rajesh, C.S.; Lee, J.-K.; Alemán, E.A.; Modarelli, D.A.; Lim, E.C. J. Am. Chem. Soc. 2005, submitted.).

(2) Intermolecular triplet excimers formed by laser excitation of concentrated solutions of aromatics such as dibenzothiophene and naphthalene (see, for example: Wang, X.; Kofron, W.G.; Kong, S.; Modarelli, D.A.; Lim, E.C. J. Phys. Chem. 2000, 104, 1461-1465).

(3) Intramolecular charge-transfer states formed by excitation to a triplet excimer, followed by excitation of the excimer with a second photon of different energy to generate the charge-transfer state. The charge-transfer state is probed with a pulsed Xe arc-lamp. We are also performing two-color two-photon laser experiments of this type in both the condensed and the gas phase, on a variety of theoretically interesting species.

Click HERE to read more about Aromatic Photophysics in the Modarelli Group.

Photophysics of Unusual Porphyrins

A third area of interest to us is the photophysics of excited porphyrins and porphyrin analogs in condensed phases. In particular we study the excited state dynamics (i.e., singlet and triplet state lifetimes, intersystem crossing rates, internal conversion rates, electron-transfer) in unusual porphyrins such as N-confused porphyrins

Photophysical (i.e., steady state and time-resolved fluorescence and pump-probe femtosecond absorption) experiments on N-confused porphyrins, octaphryns, corroles and other unusual porphyrinoid molecules. These experiments are carried out using the various laser spectroscopy tools in our group and include:

Femtosecond pump-probe absorption experiments
Time-correlated single photon counting (TCSPC) experiments
Nanosecond transient absorption experiments
Steady state fluorescence experiments

See for example: Vyas, S.; Hadad, C.; Modarelli, D.A. "TDDFT Calculations of the Structure and Absorption Spectra of Free-Base N-Confused Porphyrin and N-Confused Tetraphenylporphyrin" J. Phys. Chem. A 2008, 112, 6533–6549.; Alemán, E.A.; Rajesh, C.S.; Ziegler, C.S.; Modarelli, D.A. J. Phys. Chem. A, 2006, 110, 8605-8612..; Wolff, S.A.; Alemán, E.A.; Banerjee, D.; Rinaldi, P.L.; Modarelli, D.A. J. Org. Chem. 2004, 69, 4571-4576.; Shaw, J.L.; Wolff, S.A.; Aleman, E.A.; Ziegler, C.J.; Modarelli, D.A. J. Org. Chem. 2004, 69, 7423-7427.; Belair, J.P.; Ziegler, C.S.; Rajesh, C.S.; Modarelli, D.A. J. Phys. Chem. A 2002, 106, 6445-6451.

Click HERE to read more about Porphyrin Photophysics in the Modarelli Group.

Reactive Intermediates

(Funded by the University of Akron)

Another area with which we are involved centers on the interface between theoretical and experimental chemistry. Our research in this area involves the chemistry of short-lived, high energy reactive molecules such as carbenes , strained alkynes, radicals and biradicals. We have developed photochemical sources for the generation of alkylidenecarbenes (such as 2, 3, 5) in matrix-isolation experiments, and have recently shown 2 (R = Me) can be generated and studied in Ar matrices (Reed, S.C.; Capitosti, G.J.; Zhu, Z.; Modarelli, D.A. J. Org. Chem. 2001, 66, 287-299).

Together with Professor Richard P. Johnson (University of New Hampshire) we are using nanosecond laser flash photolysis to study the solution dynamics of these reactive intermediates using different precursors synthesized by his research group. These experiments allow us to determine lifetimes and absolute rate constants for inter- and intramolecular reactions of alkylidenecarbenes, while the matrix isolation experiments allow us to characterize alkylidenecarbenes by IR and UV-Vis spectrsocopy. To complement and support our experimental work, we use ab initio computational methods to predict the structure, spin-state, UV-visible spectral transitions and infrared absorptions of these molecules.

Click HERE to read more about Reactive Intermediates in the Modarelli Group.

Computational Chemistry

A fourth area of interest in our group involves the study of molecules using quantum mechanical computational and molecular dynamics programs. We examine the structures and energies of a variety of molecules, from reactive intermediates (see, for example: Modarelli, D.A. J. Org. Chem. 2000, 65, 7277-7283) and biologically interesting molecules and ions, to polymer monomers. Molecular dynamics is used to examine the conformational space of large macromolecules such as the dendrimers we study in our photosynthetic mimic research. Such studies are important as they allow us to help explain our photophysical data obtained from our laser experiments.

Click HERE to read more about Computational Chemistry in the Modarelli Group.

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