Prof. Kevin Belfield at the University of Central Florida, and collaborators Florencio Hernandez and Artem Masunov at UCF and Tatiana Timofeeva at New Mexico Highlands University are supported by the Collaborative Research in Chemistry Program to develop a predictive capability for the design of stable supramolecular aggregates that will exhibit specific two- and three-photon absorption properties. The work entails a multidisciplinary approach involving materials synthesis, characterization (NMR, linear and non-linear absorption, fluorescence, excitation anisotropy, and X-ray crystallography), and theory. Use of these aggregates represents one of the most promising avenues for developing practical multiphoton-based photonic materials and devices. With the knowledge and data gained in this research, an expanded multiphoton absorption-supramolecular structure-property database will be developed, leading to scaling relations, validation or modification of theory, and a predictive capability for materials design and behavior yet to be realized.
The approach taken in this program involves highly collaborative laboratories in which undergraduates, graduate students, and postdoctoral researchers receive cross-disciplinary training, positioning them to make significant scientific and technical contributions in the photonic materials fields. The PI and co-PIs actively recruit and engage students that are traditionally underrepresented in the sciences, facilitating the preparation of a highly skilled and diverse workforce. Through this collaborative research program, a "Pathway to the Ph.D." will be developed for NMHU students to pursue their Ph.D. at UCF and elsewhere. Through participation of high school chemistry teachers in the proposed research, along with partnerships with Space Florida and NHMU/Gear Up, a network of teachers will be created to help disseminate the project's findings and incorporate appropriate content into the curriculum, extending the impact to younger students.
The ultimate goal of the proposed research was the design of stable supramolecular aggregates that exhibit specific two-photon absorption (2PA) properties that will provide the tenets for developing new materials and processes for new photonic materials for a number of emerging technologies, such as 3D optical data storage, biological imaging, and disease detection. This was accomplished through a multidisciplinary approach involving materials synthesis, materials characterization (NMR, linear absorption, fluorescence, excitation anisotropy, and X-ray crystallography), nonlinear absorption, and theory, resulting in interdisciplinary training of students and postdoctoral researchers along with numerous publications in well-regarded journals. This project cultivated broad participation and interdisciplinary training from those traditionally underrepresented in the sciences, including Curtesa Arnett, Ramone Eldemire, Grace Gathagia, Brielle Lassiter-Helou (all African American), and Zach Armilo, Carolina Andrade, Marie Yezabel Colon Gomez, Jose Raul Costaneda, Jessica Desanto, Carlos Diaz, Jose Gallegos, Alma R. Morales, Carlos Ordonez, Carolina Ortiz, Luis Rodriguez, Carlos Toro, Joseph Torres, Rosmery Victoria, Fiona Zullo (all Hispanic), and over 50 publications (more details below). SUMMARY OF ACCOMPLISHMENTS 2PA Enhancement of Polymer-templated Porphyrin Dye J-Aggregates, Functional Block Copolymers, and Polyelectrolyte-modulated Aggregation of Squaraines. We reported the formation of J-aggregates of two porphyrin-based dyes, 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TSPP) and an amino tris-sulfonate analog in water using a functionalized norbornene-based homopolymer, synthesized by the ring opening metathesis polymerization technique. The polymer template enhanced J-aggregation of the porphyrin dyes was facilitated by ionic interactions of the cationic side chains (ammonium groups) of the polymer with the negatively charged sulfonate groups of the porphyrins. The J-aggregation behavior was investigated by UV-vis, fluorescence, and lifetime decay studies. The 2PA cross section was enhanced by about an order of magnitude for the J-aggregated TSPP relative to the free base TSPP and twice more for the polymer-templated system (Image 1). We investigated the formation of J-aggregates of a water-soluble cationic squaraine dye (SQ) using poly(acrylic acid) sodium salt (PAA-Na) as a template. The 2PA of the SQ J-aggregates was higher than for the unaggregated dye, with a 23-fold enhancement of the 2PA cross section at 920 nm per SQ unit. This represents first report of achieving 2PA enhancement of a squaraine dye through aggregation. Importantly, controllable J-aggregate formation provides a means to modulate the linear and nonlinear absorptivity of SQ and achieve a relatively high 2PA cross section, potentially interesting for a number of emerging applications (Image 2). Aggregate-enhanced Emission and 2PA of a Rotationally-restricted Fluorenyl Dye. We studied 2PA and one-photon (1P) and two-photon (2P) stimulated emission properties of a new class of red dye containing 2-pyran-4-ylidenemalononitrile as an electron acceptor moiety and diphenylamine as electron donor DFP. We found an aggregation-enhanced three-fold increase of the 2PA cross section (1700 GM) and two-fold aggregation-enhanced fluorescence, encouraging us to investigate aggregation-enhanced properties of this dye (DFP) encapsulated in and stabilized by silica nanoparticles for imaging Hela tumors in vivo (Image 3). Liquid Crystal-directed Supramolecular Assembly. A series of SQ dyes was synthesized and a number of properties were investigated, including phase behavior, aggregation, and X-ray diffraction providing insight into molecular aggregation and nonlinear optical properties. LC-directed assembly was achieved, forming a J-aggregate supramolecular structure whereas only H-aggregation was observed in the neat solid-state for this SQ, providing a new paradigm for supramolecular assembly of squaraines (Image 4). Two-photon Stimulated Emission Depletion, Time-resolved STED, and Superfluorescence. Time-resolved STED spectroscopic behavior of organic molecular systems is a relatively undeveloped area of nonlinear optical photophysical research. We reported pioneering work to quantitatively determine STED efficieny as a function of wavelength under one-photon and two-photon excitation and depletion, the groundwork for the current proposal. We also demonstrated superfluorescence, a phenomenon that may lead to higher contrast, brighter multiplex fluorescence imaging (Image 5). This work provides the tenets for design of superfluorescent probes and development of two-photon STED superresolution imaging. Personnel and Scholarship. Current graduate students whose dissertations are directly related to the project: J. R. Costaneda, W. Chemnasiri,* M. Daoudi, C. Diaz, G. Gathigia,* E. Jucov, B. Kim, X. Lu,* C. Ordonez, B. Sui, M. Wang,* A. Woodward, F. Zullo* [*female]. PhDs awarded: B. Moreshead (2013), I. Nayyar (2013),* Y. Zhang (2013), S. Gangopadhayay (2012),* H.-Y. Ahn* (2011), M. Qaddoura (2011), X. Wang (2011), C. Andrade* (2010), S. Biswas* (2010), C. Toro (2010), and D. Nguyen (2009) [*female]. Undergraduate participants: Z. Armijo, M. Y. Colon Gomez,* A. Crotty,* J. Desanto,* J. Gallegos, H. Haniff, K. Haugen,* K. King, B. Langdon, B. Lassiter-Helou,* F. Lu,* C. Ortiz,* S. Sullivan,* M. Tran,* J. Torres, R. Victoria,* L. Weare,* J. Yu [*female]. High school teacher participant: C. Arnett* [*female]. Postdoctoral researchers: S. Gangopadhayay,* A. R. Morales,* T. Liu, L. Rodriguez [*female]. Publications to date: Total of >80 publications in peer-reviewed journals.
