Single Molecule Studies of a Ladder Type Conjugated Polymer: Vibronic Spectra, Line Widths, and Energy Transfer

Abstract

Confocal fluorescence microscopy and spectroscopy are employed to investigate single poly(ladder-type pentaphenylene) (LPPentP) molecules dispersed in thin poly(methyl methacrylate) (PMMA) films at 1.2 K. Emission spectra of single chains show single as well as multi-chromophore emission indicating variegated communication along the chains. The vibronic structure in the emission spectra resembles the one found for other ladder-type polymers. Purely electronic zero-phonon lines in emission are substantially broadened, most probably due to fast spectral diffusion. By surmounting the limitations of emission spectroscopy, nonemitting donor chromophores, which transfer their excitation energy in a radiationless manner to emitting chromophores, are accessed by excitation spectroscopy. Remarkably, by comparing the data of emitting and nonemitting chromophores a contribution to the zero-phonon excitation line width has to be considered which places a lower limit on the estimated energy transfer time of several picoseconds between adjacent chromophores. Finally, the data indicate qualitatively a restricted flexibility of LPPentP compared to poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV).

Influence of the Conjugation Length on the Optical Spectra of Single Ladder-Type (p-Phenylene) Dimers and Polymers

“…The maximum of the ZPL distribution is slightly shifted by 50 cm –1 to lower energies, which can be attributed to the fact that MeLPPP is a multichromophoric system and features on average 2–3 emission lines of single chromophores per chain . In the center of the inhomogeneous band these lines are often spectrally closely spaced and in the high-energy tail the ZPLs can be less intense due to (partial) intrachain energy transfer to energetically lower-lying chromophores . Hence, in both situations the ZPLs are difficult to analyze, which gives rise to an apparent red-shift of the ZPL distribution with respect to the bulk spectrum of MeLPPP.…”

Towards Monodisperse Star‐Shaped Ladder‐Type Conjugated Systems: Design, Synthesis, Stabilized Blue Electroluminescence, and Amplified Spontaneous Emission

“…Unlike most linear rigid π‐system such as LPPP and MeLPPP, the absorption and fluorescence spectra of TrL ‐n ( n =1–3) and FL ‐m ( m =1–3) show poor mirror symmetry, which is also obviously different from other linear and star‐shaped π‐conjugated systems based on oligofluorenes . Furthermore, TrL ‐n show large Stokes shifts (12–18 nm), quite different from those rigid ladder‐type counterparts without diphenylamine end‐caps that exhibit only very small Stokes shifts –. But the fluorescence spectra of TrL ‐n are quite similar to those of the corresponding linear FL ‐m , suggesting similar emissive decays for both the star‐shaped and linear ladder‐type conjugated systems.…”

“…Chem.E ur.J.2017,23,[5448][5449][5450][5451][5452][5453][5454][5455][5456][5457][5458] www.chemeurj.org 2017 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim…”

Effects of molecular architecture on morphology and photophysics in conjugated polymers: from single molecules to bulk

“… 31 This lack of ZPL was assigned to linear electron–phonon coupling, which originated in the less rigid backbone of MEH-PPV giving rise to geometrical relaxation following electronic excitation. 30 As shown in Fig. 2B , single molecule excitation and emission spectroscopy showed multiple absorption sites with a single emitting site.…”

“…Purely electronic ZPL have been observed in 66% of single LPPentP chains at low temperature. 30 On the contrary, for MEH-PPV, a material with relatively more flexibility, only about 10% of the molecules showed ZPL. 31 This lack of ZPL was assigned to linear electron–phonon coupling, which originated in the less rigid backbone of MEH-PPV giving rise to geometrical relaxation following electronic excitation.…”