Cross-Conjugated Moieties as Design Motifs for a Class of Novel Electro-Optic Chromophores
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Organic electro-optic materials are at the forefront of current photonics applications that enable high speed data transmission, micro-scale sensors, and terahertz applications. Typically linearly conjugated push-pull type chromophores are the primary molecules of choice for various second and third order non-linear optical (NLO) applications. However, intra-molecular charge transfer responsible for many NLO properties can take place in molecules through other means such as σ-conjugation, cross-conjugation and omniconjugation as well. In Chapter 2 a study of electronic transitions, excited states and first hyperpolarizibility (β) of a series of molecules with a cross-conjugated bridge functionalized with a dialkylaminobenzene donor on one end and varying strength acceptors on the other end is reported. A lowered excited state dipole moment compared to the ground state dipole was indicated for an asymmetric cross-conjugated molecule with a relatively stronger acceptor confirming that the cross-conjugated bridge does in fact break the over-all molecular dipole into two constituent parts. The primary charge-transfer transition was, however still identified to occur from the donor across the cross-conjugated bridge to the acceptor. X-ray diffraction (XRD) found the molecules to be twisted significantly resulting in further isolation of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). This reduced overlap of the HOMO and the LUMO in cross-conjugated systems may in fact enable independent modulation of donor and acceptor strengths while minimizing unfavorable effects on electronic transitions and dipole moments. Comparison of hyperpolarizabilities with a control molecule found that cross-conjugation did not completely diminish β. In Chapter 3, a tricyanopyrroline (TCP) type acceptor was substituted on the cross-conjugated molecules to observe the effects of cross-conjugation on the molecular structure, the ground and excited states of the molecules and consequently β. Through XRD the cross-conjugated motif was found to be perpendicular to the conjugated TCP chromophore and the TCP acceptor was twisted out of plane from the donor despite being conjugated to the donor. Further the aromatic substituents on the cross-conjugated side group were found to interact with each other influencing crystal packing. The dihedral twist and the aromatic interactions are indicated as the cause of reduced aggregation behavior. Hyper-Raleigh scattering measurements found the compounds to have β values comparable to their fully conjugated control molecules. The β values were expected to be influenced by the cross-conjugated side groups acting as auxiliary acceptors as well as by the dihedral twist observed in these molecules resulting in larger spatial isolation between the HOMO and LUMO. In conclusion the use of a cross-conjugated motif by itself (Chapter 2) and in conjunction with a conjugated chromophore (Chapter 3) was studied and found to be useful towards affecting aggregation, solubility and hyperpolarizability of NLO chromophores.
- Chemistry