Professor Frank Spano Research Interests

Optical Excitations in Conjugated Oligomer and Polymer Aggregates and Films

Optical excitations in polymers and polymer films is a robust field, with progress and advances fueled by the promising commercial applications of polymer-based or plastic optical devices such as organic light-emitting diodes. Despite an enormous amount of primarily experimental research, there are significant gaps in our understanding of how such devices absorb and emit light. The goal of this research is to unravel the influences of intermolecular or excitonic interactions, exciton-vibrational coupling and structural defects on linearly and circularly polarized absorption, photoluminescence and electroluminescence in oligomer and polymer aggregates. The current focus is on aggregates, films and crystals of polyarenes as well as substituted and unsubstituted oligomers of poly-p-phenylene vinylene (PPV), and poly-thiophene (PT), among the best candidates for optical device applications. Theoretical techniques are being developed to treat weak to strong exciton-phonon coupling involving several electronic transitions and several intramolecular vibrational modes in two dimensional herringbone and lamellar aggregates as well as one-dimensional chiral aggregates, which contain point and structural defects. The influence of intermolecular modes (lattice phonons) are also of interest in establishing temperature dependent properties. The methods developed to date have proved successful in explaining several unusual spectral features of tetracene [1] quaterthiophene [2] and distyrylbenzene [3-7] nanoaggregates, and have predicted a "recipe" for superradiant DSB aggregates [7]. Extensions to aggregates of larger oligomers and polymers are currently in progress. Plans are also underway to treat aggregates of cyano derivatives of oligo-phenylene vinylenes which have a layered packing arrangement promoting the formation of excimers.

Molecular Orientation and Quantum State Control

The ability to align or orient molecular species prior to reaction is one of the main goals in the field of chemical reaction dynamics. In a successful collaboration with A. Marjatta Lyyra in the Physics Department, our research groups investigated electromagnetically induced transparency in diatomic lithium, leading to a novel method for measuring transition dipole moments [8]. We also developed and demonstrated a three laser excitation scheme to selectively excite a target mJ level within a rovibronic level of a diatomic molecule [9]. This is equivalent to alignment of the angular momentum vector. My group is responsible for developing the theoretical machinery needed to predict new excitation schemes. The theory is based on equations of motion which describe how the multilevel molecular density matrix evolves under the influence of two or more laser fields. The theory has successfully predicted full mJ resolution in the three laser experiment [10] and is currently being applied to more general problems involving the control and manipulation of molecular eigenstates via a strong coupling laser.

[1] Sang-Hyun Lim, Thomas G. Bjorklund, Frank C. Spano and Christopher J. Bardeen, “Exciton Delocalization and Superradiance in Tetracene Thin Films and Nanoaggregates”. Phys. Rev. Lett. (in press).

[2] H. Sun, Z. Zhao, F. C. Spano, D. Beljonne, J. Cornil, Z. Shuai and J-L. Bredas, “Absorption and Emission in Quaterthienyl Thin Films,” Advanced Materials 15, 818 (2003).

[3] F. C. Spano, "The Fundamental Photophysics of Conjugated Oligomer Herringbone Aggregates", J. Chem. Phys. 118, 981 (2003)..

[4] F. C. Spano, "Absorption and Emission oligo-Phenylene Vinylene Nanoaggregates: the Role of Disorder and Structural Defects", J. Chem. Phys. 116, 5877 (2002)..

[5] F. C. Spano, "Absorption and Emission in Pinwheel Aggregates of oligo-Phenylene Vinylene Molecules", J. Chem. Phys. 114, 5376 (2001)..

[6] F. C. Spano, "Absorption and Fluorescence in Distyrylbenzene Nanoaggregates", Syn. Met. 116, 337 (2001)..

[7] F. C. Spano, "Emission from Aggregates of Oligo-phenylene vinylenes: A Recipe for Superradiant H-Aggregates", Chem. Phys. Lett. 331, 7 (2000)

[8] J. Qi, F. C. Spano, T. Kirova, A. Lazoudis, J. Magnes, L. Li, L. M. Narducci, R. W. Field, and A. M. Lyyra, “Measurement of Transition Dipole Moments in Lithium Dimers Using Electromagnetically Induced Transparency”, Phys. Rev. Lett., 88, (2002).

[9] J. Qi, G. Lazarov, X. Wang, L. Li, L. M. Narducci, A. M. Lyyra and F. C. Spano, "Autler-Townes Splitting in Molecular Lithium: Prospects for All-Optical Alignment of Nonpolar Molecules", Phys. Rev. Lett. 83 288 (1999).

[10] F. C. Spano, "Theory of Sub Doppler Autler-Townes Splitting in Molecules: Alignment and Orientation of the Angular Momentum in Nonpolar Molecules", J. Chem. Phys. 114, 276 (2001).

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