Homoconjugation in Light-Emitting Poly(phenylene methylene)s: Origin and Pressure-Enhanced Photoluminescence

Abstract

The surprising optical properties of the non--conjugated polymer poly(phenylene methylene) (PPM) and its derivatives-that is, absorption in the 350-450 nm and photoluminescence (PL) in the 400-600 nm spectral regions-have been attributed to chromophores formed by homoconjugation along the polymer chain. The enabling role of homoconjugation, however, was hitherto ascertained primarily by excluding alternative origins of luminescence. The present study offers direct evidence for homoconjugation by employing optical and vibrational spectroscopy to investigate the interplay between microstructure and solid-state optical properties of PPM and its derivative poly(2,4,6-trimethylphenylene methylene) (PTMPM). In particular, polarized Raman and PL spectroscopy of melt-drawn fibers reveal a preferentially perpendicular orientation of the phenylene rings relative to the fiber axis, while, simultaneously, a preferentially parallel orientation of the transition dipole moment. PL spectroscopy under applied hydrostatic pressure yields a nearly 4-fold increase in PL intensity at 8 GPa, together with a surprising absence of excimer emission. These characteristics, being highly atypical of conventional -conjugated polymers, highlight the different origin of the optical properties of poly(phenylene methylene)s as well as unique opportunities for applications.

Synthesis of Linear Polymers in High Molecular Weights via Reaction-Enhanced Reactivity of Intermediates Using Friedel–Crafts Polycondensation

“…Nevertheless, the produced linear polybenzyls demonstrated an interesting feature of homoconjugation fluorescence in both solutions and thin films due to the proximity of aryl rings in the backbone. 22,23 This mini-review article plans to summarize the recent employment of Friedel−Crafts acylation and hydroxyalkylation polycondensation reactions for synthesis of linear polymers using an excess amount of the RERI monomers. Depending on the reaction mechanism, these RERI monomers in the polymerizations could be nucleophilic monomers, such as biphenyls, in the acylation reaction or electrophilic monomers, such as benzaldehydes and activated ketones, in the hydroxyalkylation reaction.…”

“…For instance, Friedel–Crafts alkylation reactions have been reported for polymerization of an AB bifunctional monomer, such as benzyl chloride or benzyl alcohol, to synthesize linear poly­(phenylene methylene)­s or polybenzyl, although the syntheses did not exhibit a dramatic RERI feature. Nevertheless, the produced linear polybenzyls demonstrated an interesting feature of homoconjugation fluorescence in both solutions and thin films due to the proximity of aryl rings in the backbone. , …”

Unveiling the Synthetic Potential of 1,3,5-Tri(10H-phenothiazin-10-yl)benzene-Based Optoelectronic Material: A Metal-Free and Recyclable Photocatalyst for Sequential Functionalization of C(sp2)–H Bonds

“…Next, the optical and electronic properties of 3PTZ-POP were investigated. While 3PTZ displays poor visible-light absorption (Figure S1), cross-linking of 3PTZ results in much red shift and enhanced visible-light absorption possibly due to the occurrence of homoconjugation along the chain (Figure a). , The band gap of 3PTZ-POP was calculated to be 1.95 eV from a Tauc plot. The potential of conduction band (CB) was established as −1.15 V via Mott–Schottky electrochemical measurements at three frequencies including 1000, 1500, and 2000 Hz (Figure b).…”

Nickel-Catalyzed Alkene Difunctionalization as a Method for Polymerization

“…6 We hypothesized that the bifunctional monomer could be leveraged for difunctionalization with indigenous aryl bromide and exogenously functionalized organometallic reagents, leading to a polymer chain growing along both the bromide and alkene termini (Scheme 1B). Following literature protocols for styrene 1,2-diarylation 7 and upon extensive optimization (see SI and vide infra for details), we discovered that 5 mol% NiBr 2 catalyzed the polymerization of 4bromostyrene with phenylboronic acid neopentylglycol ester (FG 1 , FG 2 = H) at 100 °C in toluene, affording a poly(phenylene α-(aryl)ethylene) in 72% yield after 8 h. The structure of the poly(phenylene (aryl)ethylene) was unambiguously confirmed using 1 H and 13 C NMR spectroscopy (see SI), wherein the dibenzylic methine and benzylic methylene peaks of the polymer at 4.04 and 3.19 ppm were compared with that of 1,1,2-triarylethane 8 (Figure 1). 9 Successful polymerization was further evidenced using size-exclusion chromatography (SEC, Figure 2) wherein the polymer apparent molecular weight was found to be 59.2 kDa.…”

“…Our PAAEs were observed to fluoresce in the presence of UV light, even at the low intensity of ambient light. This behavior is akin to poly­(phenylene methylene)­s that are known to fluoresce due to homoconjugation; , thus, we sought to explore the optical properties of the PAAE. UV–vis and emission spectroscopy revealed that the unsubstituted polymer exhibited an excitation maximum at λ = 284 nm with a concomitant emission maximum at λ = 320 nm; the combined behaviors gave rise to the blue color.…”