ortho-Phenylenes: Unusual Conjugated Oligomers with a Surprisingly Long Effective Conjugation Length

Abstract

ortho-Phenylenes represent a fundamental but relatively unexplored class of conjugated molecular architecture. We have developed a robust synthetic approach to monodisperse o-phenylene oligomers which we have demonstrated by synthesizing a homologous series up to the dodecamer. The o-phenylenes exhibit complex conformational behavior but are biased toward a specific 2-fold-symmetric conformation which we believe corresponds to a stacked helix. Surprisingly, the series exhibits long-range delocalization, as measured by bathochromic shifts in UV/vis spectra. Although the overall magnitude of the shifts is modest (but comparable to some other classes of conjugated materials), the effective conjugation length of the series is approximately eight repeat units. The oligomers also exhibit an unusual hypsochromic shift in their fluorescence spectra with increasing length. The origin of these trends is discussed in the context of conformational analysis and DFT calculations of the frontier molecular orbitals for the series.

Spirobifluorene Regioisomerism: A Structure–Property Relationship Study

“…[30] Indeed, it is admitted that there is a better delocalisation of π-electrons following the para/ortho/meta sequence and numbers of studies have tried to elucidate the origin of the restricted π-conjugation between para, ortho and meta substituted oligophenylenes. [30][31][32][33][34][35][36] In our case, it is clear that 3 displays a different behaviour compared to its building block meta terphenyl as it presents a relatively intense degree of conjugation between the phenyl and the fluorene. Thus, the 'linkage' effect cannot explain by itself this feature and other parameters should be invoked.…”

Contrasting the optical properties of the different isomers of oligophenylene

“…6,8,9 However in this case, use of Grimme's dispersion correction [31][32][33] to DFT (DFT-D3) is needed to accurately describe Van der Waals interactions (arene-arene p-stacking) between the phenylene units, due to their spatial proximity. o-Phenylenes show similar behaviour, again in line with literature.…”

“…This simplicity is, however, deceptive as polyphenylene can occur in three different structural isomers; poly(ortho-phenylene) (o-phenylene), poly(metaphenylene) (m-phenylene) and poly(para-phenylene) (p-phenylene) (see Scheme 1). [6][7][8][9] In contrast, m-phenylene is effectively nonconjugated 10 and its optical properties virutually independent of chain length. For example, experimentally the fluorescence spectrum of oligomers of p-phenylene is known to red shift with increasing chain length [1][2][3][4][5] while for oligomers of o-phenylene, surprisingly, it shifts to the blue.…”

Conformational Analysis of o-Phenylenes: Helical Oligomers with Frayed Ends

“…Recently, we reported the synthesis and characterization of the homologous series of o-phenylene oligomers oP n up to the dodecamer (where n indexes the number of repeat units). 16 Access to these compounds allowed us to investigate their electronic structure, revealing several interesting features. As expected, the twisting of the o-phenylene backbone leads to a modest overall effect of conjugation in the oP n series, as determined by the total shift in their UV-vis spectra with increasing length.…”