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 project 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 pinwheel and herringbone nanoaggregates formed by 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 aggregates which contain point and structural defects. The methods developed to date have proved successful in explaining several unusual spectral features of distyrylbenzene (DSB) nanoaggregates [1-6], and have predicted a "recipe" for superradiant DSB aggregates [1,4]. 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 highly successful collaboration with A. Marjatta Lyyra in the Physics Department, our research groups have been able to develop and demonstrate a three laser excitation scheme to selectively excite a target m_J level within a rovibronic level of a diatomic molecule [7]. 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 m_J resolution in the three laser experiment [8] and is currently being applied to more general problems involving the control and manipulation of molecular eigenstates via a strong coupling laser. One example involves a technique which vastly increases triplet state production for possible use in studying reactive triplet states. Our theoretical analysis shows that it is possible for a strong laser to "push" (via the Autler-Townes effect) a singlet state into resonance with a triplet state in order to cause efficient mixing. Such a mixed state can be used as a stepping stone for two-laser excitation of high energy triplet states. Currently, the theory is being expanded to include nonperturbative excitation sequences and sequences using adiabatic following in order to maximize population in the target state. In addition, more sophisticated models for collisional dynamics are being incorporated.
 
 

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

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

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

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

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

[6] F. C. Spano and S. Siddiqui, "Exciton-Vibrational Coupling in Pinwheel Aggregates of p-Conjugated Molecules", Chem. Phys. Lett. 314, 481 (1999).
 
 

[7] 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).

[8] 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).
 
 

Other Selected References

E. S. Manas and F. C. Spano, "Absorption and Spontaneous Emission in Aggregates of Conjugated Molecules", J. Chem. Phys. 109, 8087 (1998).

Eric S. Manas, F. C. Spano and L. X. Chen, "Nonlinear Optical Response of Cofacial Phthalocyanine Dimers and Trimers", J. Chem. Phys. 107, 707 (1997).

F. C. Spano; "Theory of Strong-Field Pump-Probe Spectroscopy in J-aggregates"; Chem. Phys. Lett. 220, 365 (1994).

F. C. Spano and Z. G. Soos; "Second Hyperpolarizabilities of Huckel Rings: Analytical Results for Size and Alternation Dependences"; J. Chem. Phys. 99, 9265 (1993).

F. C. Spano and J. Knoester; "Fermions on a Frenkel Chain: Nonlinear Optical Response of Linear Molecular Aggregates"; Advances in Magnetic and Optical Resonance , Ed. W. S. Warren (Academic Press, San Diego, 1994), pg. 117.

F.C. Spano; "Fermion Excited States in One-Dimensional Molecular Aggregates with site Disorder", Phys. Rev. Lett. 67, 3424 (1991).
 
 

Publications/ Last Update 1/1/03