Optical Spectroscopy of Long Polyenes

Abstract

We have synthesized a homologous series of soluble, linearly conjugated oligomers and related polymers using molybdenum alkylidene catalysts. We have developed HPLC procedures to isolate the oligomers according to their chain lengths and have obtained the absorption spectra of the purified oligomers in room temperature solutions and in 77 K glasses. The oligomer absorption spectra are structured and remarkably similar to those of simple polyenes with comparable numbers of conjugated double bonds (N). Furthermore, the electronic origins of the low-energy, strongly allowed 1 1 A g -f 1 1 B u + transitions follow the E(0-0) ) A + B/N behavior previously noted in simple polyenes and carotenoids. Extrapolation of data for oligomers with N ) 3-15 suggests E(0-0) ≈ 14 000 cm -1 (λ ≈ 700 nm) in the long polyene limit. The oligomer spectra exhibit modest red shifts on cooling, suggesting minimal conformational disorder in the room temperature samples. In contrast, the absorption spectrum of the longest soluble polymer (N > 100) in this series undergoes a significant red shift and sharpening upon cooling from 300 to 77 K. This indicates that the room temperature polymer is disordered due to relatively low thermal barriers for torsional motion about carbon-carbon single bonds. Unlike the longer oligomers, the low-temperature absorption of the polymer shows well-defined vibronic structure. The polymerization reactions lead to a distribution of conjugation lengths in the unpurified polymer sample. However, the vibronically resolved, red-shifted absorption at low temperature (λ(0-0) ) 630 nm) indicates that this distribution is dominated by species with very long conjugation lengths. The resolution of the low-temperature spectrum argues that the absorption is due to the superposition of almost identical 1 1 A g -f 1 1 B u + spectra and that all conjugated segments in this sample absorb near the asymptotic limit (1/N ≈ 0).

Multiple Metal–Carbon Bonds for Catalytic Metathesis Reactions (Nobel Lecture)

“…In the case of a cyclopolymerization of 1,6-heptadiynes, such as diethyldipropargylmalonate, a six-membered or a five-membered ring can form, depending upon the regiochemistry of the addition of the first alkyne to the alkylidene. Since the optical properties of such polyenes vary with the polymers structure and chain length, [61] it is highly desirable to make a polymer with a single structure and to control the molecular weight of that polymer through a polymerization with a welldefined catalyst. Although work is still ongoing in this area, it now appears possible to do so.…”

Molecular Weight Effects in the Third-Harmonic Generation Spectroscopy of Thin Films of the Conjugated Polymer MEH-PPV

“…10,11,18,51 For chain lengths larger than typically 2 nm, socalled saturation effects are observed, which means that λ max and γ per repeat unit converge to finite values that are sensitively dependent on the chemical structure and conformation of the polymer. [52][53][54][55][56][57][58][59][60] However, it was noted recently that the twophoton absorption coefficient of thin films of a polydiacetylene can still increase with M w , even if M w is significantly larger than the saturation limit of γ per repeat unit. 61 This behavior was attributed to chain ordering effects in thin films which can have strong impact on the third-order nonlinear optical properties of thin films of conjugated polymers.…”

Cation radicals of xanthophylls

“…The theory predicts that the energies of the transitions can be described by the relationship DE = A + B/N, where N is the effective number of conjugated double bonds (Kohler 1990(Kohler , 1991Christensen 1999;Christensen et al 2004). More quantitative agreements between theoretical and experimental energies require inclusion of extensive configuration interaction and consideration of solvent interactions, which has been prohibitively computationally intensive.…”