Photophysical and Theoretical Investigations of Oligo(p-phenyleneethynylene)s:  Effect of Alkoxy Substitution and Alkyne−Aryl Bond Rotations

Abstract

The unique photophysical, conformational, and electronic properties of two model phenyleneethynylene-based rigid rod molecular systems, possessing dialkoxy substitutions, are reported in comparison with an unsubstituted system. Twisting of the phenyl rings along the carbon-carbon triple bond is almost frictionless in these systems giving rise to planar as well as several twisted ground-state conformations, and this results in broad structureless absorption in the spectral region of 250-450 nm. In the case of 1,4-bis(phenylethynyl)benzene, a broad absorption band was observed due to the HOMO-LUMO transition, whereas dialkoxy-substituted compounds possess two well-separated bands. Dialkoxy substitution in the 2,5-position of the phenyl ring in phenyleneethynylenes alters its central arene pi-orbitals through the resonance interaction with oxygen lone pairs resulting in similar orbital features for HOMO and HOMO-1/HOMO-2. Electronic transition from the low-lying HOMO-1/HOMO-2 orbital to LUMO results in the high-energy band, and the red-shifted band originates from the HOMO-LUMO transition. The first excited-state transition energies at different dihedral angles, calculated by the TDDFT method, indicate that the orthogonal conformation has the highest excitation energy with an energy difference of 15 kcal/mol higher than the low-lying planar conformation. The emission of these compounds originates preferentially from the more relaxed planar conformation resulting in well-defined vibronic features. The fluorescence spectral profile and lifetimes were found to be independent of excitation wavelengths, confirming the existence of a single emitting species.

Mode specific excited state dynamics study of bis(phenylethynyl)benzene from ultrafast Raman loss spectroscopy

“…3(b)), which indicates that the emission is probably due to a single emitting species. 27 The two distinct emission spectra and extra absorption band at 365 nm reported by Levitus et al were not observed in our experiments, indicating a higher purity of the sample. 26 …”

“…23 Alternatively, these partially structured absorption spectra may also be due to the transitions to higher electronic states, but according to James et al those states do not have a significant oscillator strength. 27 It has been reported by Levitus et al that in the ground state BPEB can exist as a mixture of rotamers because of the low rotational barrier of 0.5 kcal/mol (180 cm 1 ) which is below kT. 18,19 The low temperature absorption studies by Chu and Pang showed that at 198 • C (75.15 K) the absorption spectrum becomes sharper and the relative intensity of the 0-0 band increases.…”

“…However, in the excited state, the energy difference between the planar and the twisted conformation is believed to be much larger: 15 kcal/mol. 27 Interestingly, lowtemperature absorption and fluorescence studies show more structured vibronic bands, attributed to the progression of highfrequency vibrations of BPEB. Furthermore, it is reported that the mirror image symmetry of the low-temperature absorption and emission spectra confirms the similarity of the absorbing and emitting geometries.…”

Oligo(p‐phenyleneethynylene) (OPE) Molecular Wires: Synthesis and Length Dependence of Photoinduced Charge Transfer in OPEs with Triarylamine and Diaryloxadiazole End Groups

“…The parent OPE molecule, PhCCC 6 H 2 (OC 6 H 13 ) 2 CCPh, where the donor and acceptor groups are replaced by hydrogen groups in 2a, has absorption maxima at 307 and 367 nm in methylcyclohexane assigned by TD-DFT computations elsewhere as HOMO-1  LUMO and HOMO  LUMO LE transitions, respectively. [39] Excited state geometries on model geometries 1b-7b were optimized using TD-DFT computations at CAM-B3LYP/6-31G*. These model geometries denoted with suffix b contain no alkyl groups at the end groups to reduce computational efforts.…”

Ground‐State Chemical Reactivity under Vibrational Coupling to the Vacuum Electromagnetic Field

“…To investigate whether ground‐state chemical reactivity can indeed be influenced by vibrational strong coupling, the deprotection reaction of an alkynylsilane, 1‐phenyl‐2‐trimethylsilylacetylene (PTA), with tetra‐ n ‐butylammonium fluoride (TBAF) was chosen as a prototypical reaction (Figure 1 b) 10. This simple system presents several practical advantages for the demonstration of such an effect.…”