Pyrene as Chromophore and Electrophore: Encapsulation in a Rigid Polyphenylene Shell

Abstract

Starting from the fourfold ethynyl-substituted chromophore 1,3,6,8-tetraethynylpyrene as core, a series of polyphenylene dendrimers was prepared in high yield by combining divergent and convergent growth methods. The fluorescence quantum yields (Q(f)>0.92) of the encapsulated pyrene chromophore were independent of the size of the polyphenylene shell. Fluorescence quenching studies and temperature-dependent fluorescence spectroscopy were performed to investigate the site isolation of the core. They indicate that a second-generation dendrimer layer is needed to efficiently shield the encapsulated pyrene and prevent aggregate formation. Alkali-metal reduction of the encapsulated pyrene core was carried out to afford the corresponding pyrene radical anions, for which hampered electron transfer to the core was observed with increasing dendrimer generation, which is further proof of the site isolation due to the polyphenylene shell. To improve film formation and solubility of the material, solubilizing alkyl chains were introduced on the periphery of the spherical particles. Furthermore, highly transparent films obtained by a simple drop-casting method showed blue emission mainly from the unaggregated species. The materials presented herein combine high quantum efficiency, good solubility, and improved film-forming properties, which make them possible candidates for several applications in electronic devices.

Blue Light Emitting Polyphenylene Dendrimers with Bipolar Charge Transport Moieties

“…The band with a maximum at 319 nm can be assigned to the absorption of PBD [38], similar to that of PYPBD (a pyrene-cored polyphenylene dendrimer with PBDs in the periphery, see Scheme S1). The other band (λ max : 392 nm, ε:~50,000 L·mol −1 ·cm −1 ) is much sharper and originates from π-π* transitions of the pyrene core [24,40,41]. Both dendrimers are readily soluble (≥8 g/L) in organic solvents, e.g., DCM, THF and toluene.…”

“…The band with a maximum at 319 nm can be assigned to the absorption of PBD [38], similar to that of PYPBD (a pyrene-cored polyphenylene dendrimer with PBDs in the periphery, see Scheme S1). The other band (λmax: 392 nm, ε: ~50000 L·mol −1 ·cm −1 ) is much sharper and originates from π-π* transitions of the pyrene core [24,40,41]. The photoluminescence spectra (PL) of these dendrimers showed that the emission of the donoracceptor dendrimers, PYCAB and PYPBD were all identical (λmax: ~431 nm and a weak shoulder: ~455 nm) both for excitation of the surface and the core groups (Figure 2), revealing that the emission of the donor-acceptor dendrimers can be attributed to an efficient surface-to-core energy transfer [24,40,41].…”

“…In thin films, both donor-acceptor dendrimers and PYPBD give moderate bathochromic shifts (10-20 nm) (Table 1), assigned to the intermolecular interactions among the cores or between the cores and PCs due to inadequate protection of the cores by the polyphenylene dendrons for the first-generation dendrimers [24,40]. The photoluminescence spectra (PL) of these dendrimers showed that the emission of the donor-acceptor dendrimers, PYCAB and PYPBD were all identical (λ max :~431 nm and a weak shoulder:~455 nm) both for excitation of the surface and the core groups (Figure 2), revealing that the emission of the donor-acceptor dendrimers can be attributed to an efficient surface-to-core energy transfer [24,40,41]. In thin films, both donor-acceptor dendrimers and PYPBD give moderate bathochromic shifts (10-20 nm) (Table 1), assigned to the intermolecular interactions among the cores or between the cores and PCs due to inadequate protection of the cores by the polyphenylene dendrons for the first-generation dendrimers [24,40].…”

“…In addition, a TPA-and PBD-functionalized dendrimer D2 was prepared for comparison. The emission of CAB [37], PBD [38] and TPA [39] show good overlaps with the absorption of the core, i.e., 1,3,6,8-tetraphenylpyrene [40,41], for efficient surface-to-core energy transfers. For this purpose, shape-persistent polyphenylene dendrimers enable generation-by-generation (core-shell-surface) functionalization and OLEDs utilizing efficient surface-to-core energy transfers of pyrene-cored polyphenylene dendrimers have been reported [22][23][24].…”

“…In addition, a TPA-and PBD-functionalized dendrimer D2 was prepared for comparison. The emission of CAB [37], PBD [38] and TPA [39] show good overlaps with the absorption of the core, i.e., 1,3,6,8-tetraphenylpyrene [40,41], for efficient surface-to-core energy transfers. To summarize, in this contribution a new method to make polyphenylene dendrimers with donor-acceptor periphery is introduced.…”