Theoretical Study of Spectroscopic Properties of Dimethoxy-p-Phenylene-Ethynylene Oligomers:  Planarization of the Conjugated Backbone

Abstract

The optical spectra of the dimethoxy-p-phenylene-ethynylene oligomers (up to n ) 10) are calculated by DFT and TD-DFT methods. It is found that the conformational rotations around the cylindrical triple-bonded carbon links impact significantly the optical spectrum. The effective conjugation length (ECL) of the oligomer is obtained by extrapolating the HOMO-LUMO gap to infinite chain length with an alternative exponential function. The spectral shift is mainly dependent on the high π-conjugation segment of oligomers, resulting from the planarization of the backbone. Although the rotational barrier is very low, the calculated results further indicate that rotation about the cylindrical triple bond still interrupts the conjugation of rod-like oligomers to some extent, and leads to an angle-dependent HOMO-LUMO gap. The results are helpful to interpret the conformational-dependent spectroscopic phenomena of p-phenyleneethynylene oligomers and polymers (PPEs) observed in ensemble and single molecule spectroscopy.

Localized Emitting State and Energy Transfer Properties of Quadrupolar Chromophores and (Multi)Branched Derivatives

“…13,66 Regarding chromophore 5, conjugation among the triphenylamine core and three branches through CC do exist, but it is not as strong as that in chromophore 4 because of the small twisting barrier of the CC bond. 10,58 Therefore, in chromophore 5, for low-energy ICT states, triphenylamine core is more active, charge is partially distributed overall molecule, and the CDD distribution at each branch agrees with C 3h symmetry Frenkel exciton model expressions (see Table S2e states, triphenylamine core is not that active, charge redistribution is partially restricted, and the CDD distribution is slightly different from the expressions of an ideal C 3h symmetry Frenkel exciton model. The angle between transition dipole moments from the ground state to the low-energy ICT states and those from the ground state to the high-energy ICT states is estimated about ∼19.3°.…”

Synthetic Approaches to Flexible Fluorescent Conjugated Polymers

“…[27][28][29] A successful quantitative conversion of butadiynes in the polymer chain into heterocycles will introduce heteroatoms into the polymer backbone, thus red-shifting the polymer's photophysical properties.…”

“…In addition to the efficient oxidation of palladium by benzoquinone, the PPBs' randomness obtained from two alkynes is believed to improve the solubility and molecular weight of the block copolymers. 27 The same group also demonstrated that high molecular weight PPBs (up to ~124,000 g/mol) were synthesized by using bulky iptycene monomers, minimizing interchain stacking. 28 The rigid iptycene scaffolds are also known to minimize π-π stacking in solid films, preserving the photophysical properties suitable for ultrasensitive detection of chemicals.…”

“…The delocalized π-electrons extend along the polymer backbone, giving rise to a semiconducting band, which is responsible for the inherent fluorescence of these materials. [27][28] The valence band of the semiconductor is formed from the interaction and mixing of the highest occupied molecular orbitals (HOMO) of the polymer. The empty conduction band is formed from the mixing of the lowest unoccupied molecular orbitals (LUMO).…”

“…27 Thus by keeping the amount of the flexible linker below 50% (i.e., P-50 or longer), flexible polymers can be synthesized with good solubility and photophysical properties comparable to those of a fully-conjugated PPE polymer backbone, while providing additional biodegradability and controllable aggregation properties through the flexible linkers.…”