Calculated and Experimental UV and IR Spectra of Oligo-para-phenylenes

Abstract

The quantum mechanical properties of a series of oligo-para-phenylenes (2-11) were characterized using DFT B3LYP/6-311G(d,p) calculations. The global minimum among the various torsional conformers of an oligo-pphenylene is calculated to be a twist conformation. A less stable planar conformation, in which all the dihedral angles in oligo-p-phenylene are restricted to be planar, has also been calculated. The total electronic energies, normal vibrational modes, Gibbs free energies, and HOMOs and LUMOs of the two different conformations (twisted and planar) of the oligo-p-phenylenes were analyzed. The energy differences between the HOMOs and LUMOs of the substrates are in accord with the maximum absorption peaks of the experimental UV spectra of 2-6. The calculated normal vibrational modes of 2-6 were comparable with their experimental IR spectra.

Polymer Photocatalysts for Water Splitting: Insights from Computational Modeling

“…The reduction in thermodynamic driving force is, however, only one of the effects of increasing the chain length. For most polymers, including melon and PPP, the optical gap decreases with increasing chain length and thus in principle, a larger fraction of the spectrum of the light illuminating the polymer can be absorbed, and more photons converted into excitons (increased rates of photon absorption and exciton generation), subsequently generating more free charge carriers and ultimately more molecular hydrogen and oxygen. There might also be an effect of chain length on the lifetime of excitons (reduction in exciton loss rate) and free charge carriers (reduction in exciton recombination rate), although that is more difficult to explore computationally.…”

Contrasting the optical properties of the different isomers of oligophenylene

“…These three isomers differ in through which carbon atoms the phenylene units are linked and, importantly, all have significantly different optical properties. 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. [6][7][8][9] In contrast, m-phenylene is effectively nonconjugated 10 and its optical properties virutually independent of chain length.…”

“…14 Elucidating the origin of the starkly different optical properties of the isomers of such a conceptually simple polymer is clearly both an academically and practically relevant question. Not surprisingly, there is thus a large number of computational studies on the optical 3,4,6,8 and related structural 9,10 properties of oligomers of phenylene. Such studies generally focus on only one of the three isomers and attempt to correlate its structural and optical properties.…”