Linear, non-linear and raman spectroscopy of polydiacetylenes

Abstract

Polydiacetylenes (PDA) exhibit a subpicosecond third-order non-linear response whose coefficient ( χ (3)) in the transparent region is the largest of any material. With a view towards the design and synthesis of materials with further enhanced values of χ (3), the crystal structure-linear spectroscopy relationship for PDA is developed. The significance of the local crystal packing environment of the conjugated chain to the linear spectrum is discussed, with particular reference to the thermochromic derivative, bis-ethyl urethane of 5,7-dodecadiyne-1,12-diol (poly-ETCD). The room-temperature resonance Raman (RR) spectrum of poly-ETCD is sensitive to both the wavelength of excitation and the presence of unreacted monomer. When excited with 488.0 or 514.6 nm light, the observed RR spectra of as-polymerized poly-ETCD crystals reveal the presence of two PDA electronic states. Extraction of residual monomer from such crystals leads to a marked diminution of the lines associated with the lower-energy spectral state. The electronic states of conjugated polymers in the solid state are better discussed in terms of different crystal environments rather than different effective conjugation lengths.