The first novel polyfluorene copolymers containing siloxane bridges were synthesized by Ni(0)-mediated copolymerization between 2,7-dibromo-9,9’-dihexylfluorene and a bridged fluorene monomer containing siloxane linkages. Two such bridged copolymers were prepared: PSiloBg1 (containing 1 mol.% of siloxane-bridged fluorene unit) and PSiloBg_3 (containing 3 mol.% of siloxane-bridged fluorene unit). PSiloBg1 and PSiloBg3 showed good solubility in common organic solvents, thermal stability up to 420 ℃, and facile film formation. The glass transition temperatures of the bridged polymers (106 and 110℃ respectively) were higher than that of the homo poly(dihexylfluorene) (PDHF). In particular, PSiloBg3 exhibited a higher polymerization yield (96%) and molecular weight (M.W. = 185,000) compared to PDHF (polymerization yield = 62%, M.W. = 82,000). Interestingly, after the bridged polymers had been annealed at 150 ℃ for 4 h in air, their PL spectra showed no significant increase in vibronic structures at 450 nm and 475 nm and no evidence of aggregation formations and excimers at wavelengths over 500 nm. In addition, the full width at half maximum (fwhm) of the bridged polymers was very small (fwhm = 51-52 nm) compared to that of PDHF (fwhm = 85 nm). Collectively, the results show that the polyfluorenes with siloxane bridges exhibit thermally stable almost pure blue emission, making these polymers promising candidate materials for device applications.
Novel blue polyfluorenes with polyhedral oligomeric silsesquioxanes(POSSs) were successfully synthesized by Ni(0)-mediated polymerization. The photoluminescence spectra of the copolyfluorenes containing POSSs at small contents exhibited no excimer and aggreation formations, and therefore resulted in thermally stable pure blue emission even after being annealed at 150℃ for 5h in air. Furthermore, PL quantum efficiencies of the polyfluorenes containing POSSs were highly enhanced compared to PDHF(polydihexylfluorene) in solid states. The electroluminescence spectra of the polymers also showed no sign of long wavelength tails by excimer formations and aggregate formations. As the molar fraction of the POSS-substituted monomer of the polymers is increased, the CIE coordinates of the polymers move from greenish blue region to deep blue region. The CIE coordinates of PDHF is (0.25, 0.29), and the CIE coordinates of PF-POSS05, PF-POSS10 and PF-POSS20 are (0.20, 0.17), (0.19, 0.16), and (0.18, 0.13) respectively, corresponding to blue emissions. Of all the polymers PF-POSS20 gives a deep blue-light emission.. The threshold voltage of the polymers is 4.0V for all the polymers containing POSSs unit. Maximum luminance values of PF-POSS05, PF-POSS10, and PF-POSS20 are $263 cd/ ㎡$ at $10V, 463 cd/ ㎡$ at 10V, and $420 cd/ ㎡ $at 8.5V, respectively. The introduction of POSSs into polyfluorenes is the good solution to obtain a deep blue-emission light in EL devices fabricated with polyfluorene derivatives.
Novel polyfluorene copolymers with siloxane linkages on the polymer main-chains were synthesized by Ni(0)-mediated Yamamoto coupling polymerization. These polymers, P2Silo05, P2Silo10, P2Silo15, P8Silo05, P8Silo10, and P8Silo15 were prepared by copolymerization between 2,7-dibromo-9,9’-fluorene and Silo2(bromobenzene-terminated disiloxane monomer) or Silo8(bromobenzene-terminated heptaasiloxane monomer). The molecular structures of polymers were successfully confirmed by FT-NMR, FT-IR, and elemental analysis. All the polymers showed good solubility in common organic solvents like chloroform, toluene, and p-xylenes. The glass transition temperatures of the polymers were in the ranges of 72 - 102℃, and the decomposition temperatures for 5% weight loss $(T_d)$ of all the polymers were above 430℃, showing good thermal stability. The molecular weight $(M_w)$ of the polymers were ranging from $3.6 \times 10^4$ to $8.1 × 10^4$. The polymers with higher molar % of siloxane linkages showed more blue-shifted maximum UV-vis absorption compared to those of homo polydihexylfluorene(PDHF). But the polymers showed the similar photoluminescence spectra with those of PDHF in onsets and patterns. The single layer light-emitting diodes were fabricated in a configuration of ITO/PEDOT/polymer/Ca/Al. The electroluminescence maximum emission wavelengths of the polymers were in the range of 422-449nm, corresponding to the pure blue emissions. And CIE coordinates of the polyfluorenes containing siloxane linkages were in the range of (0.20, 0.16) to (0.17, 0.10). The maximum luminance of the polymers were in the range of $120-236cd/ ㎡$. To examine the energy transfer in polyfluorene blends, UV-vis absorption spectra and PL spectra of mixtures of P2Silo10 and PFTT(Poly(9,9’-dioctylfluorene-alt-thieno[3,2-b]-thiophene)) were measured. The photoluminescence emission wavelength (448nm) of P2Silo10 correlates with the absorption wavelength (447.5nm) of PFTT well. The polymer blends exhibit a strong emissive peak at 483nm and a shoulder at 510nm. The fluorene-based polymer blends at the ratio of 20 : 1 of P2Silo10 and PFTT exhibited the enhanced PL quantum efficiency of about four times compared to PFTT. To examine EL quantum efficiency in polymer blends, we fabricated the single layer light-emitting diodes were fabricated as a configuration of ITO/PEDOT/polymer/LiF/Al. The fluorene-based polymer blend at the ratio of 20 : 1 of P2Silo10 and PFTT exhibited the enhanced EL quantum efficiency of about 3.4 times compared to P2Silo10.
Polyfluorene copolymers with distilbene kink moieties on the polymer main-chains were synthesized by Ni(0)-mediated Yamamoto coupling polymerization. Kink-structured polymers, PF-M02, PF-M05, PF-M10, PF-P02, PF-P05, and PF-P10 were prepared by copolymerization between 2,7-dibromo-9,9’-fluorene and MEHDiSt(a m-distilbene unit) or PEHDiSt(a p-distilbene unit). The molecular structures of polymers were successfully confirmed by $^1H-NMR$, FT-IR, and elemenal analysis. All the polymers showed good solubility in common organic solvents like chloroform, toluene, and p-xylenes. The decomposition temperatures for 5% weight loss $(T_d)$ of all the polymers were above 420℃, showing good thermal stability. The molecular weight $(M_w)$ of the polymers were ranging from $2.9 × 10^4$ to $8.8 × 10^4$. The polymers with higher molar % of distilbene kink linkages showed more blue-shifted maximum UV-vis absorption compared to those of homo polydihexylfluorene(PDHF). The polymers showed the similar photoluminescence (PL) spectra with those of PDHF on shapes in solutions and in solid states, but the maximum PL wavelengths of the polymers were blue-shifted on a scale of 2-3nm compared to PDHF. The single layer light-emitting diodes were fabricated in a configuration of ITO/PEDOT/polymer/Ca/Al. The electroluminescence maximum emission of the polymers were in the range of 421-450nm, corresponding to the blue emissions. The polyfluorenes with distilbene kink moieties showed more blue-shifted EL emissions than PDHF (polydihexylfluorenes). In particular, the polyfluorenes with the distilbene moieties at 5 mol.% (PF-M05 and PF-P05) showed deep blue emissions, and CIE 1931 chromaticity coordinates of PF-M05 and PF-P05 are (0.19, 0.13) and (0.18, 0.13) respectively (PDHF; 0.25, 0.29). The threshold of the polymers were in the range of 4.0-5.0V and the maximum brightness of the polymers were in the range of $62-285cd/ ㎡$ in the EL devices.
Novel blue germanium-containing poly(p-phenylenevinylene) derivatives with well-defined conjugation lengths were synthesized by wittig-condensation polymerization. The polymers were characterized by $^1H-NMR$ and FT-IR spectroscopy. The polymers were color-tuned by introduction of different chromophores. The photoluminescence (PL) spectra of the polymers, GePVK, GeMEH, and GePTH showed blue, greenish blue and geen emissions respectively. The polymer, GePTH with phenothiazine moiety showed red-shifted emssion compared to the polymers with carvazole moiety or dialkoxybenzene moiety, and the polymer, GePVK with carvazole moiety showed almost blue emission in solution PL and solid state PL. The electroluminescence (EL) spectra of GeMEH, and GePTH showed yellowy green color and yellow green respectively. And interestingly, GePVK which contains a carbazole unit showed white-emission in EL devices.
실록산 가교를 가지는 폴리플루오렌 공중합체(PSiloBg1, PSiloBg3)를 2,7-dibromo-9,9-fluorene과 실록산 가교를 가지는 플루오렌 단량체의 니켈촉매 중합법에 의해서 합성하였다. 본 연구에서 소개하는 PSiloBg1과 PSiloBg3은 일반적인 유기용매에 대해서 매우 높은 용해도를 보였으며, 420℃ 까지 열적인 안정성을 보였다. 다리구조를 가지는 고분자는 Homo PDHF(polydihexylfluorene) 에 비해서 유리전이온도가 높고, 분자량이 컸다. 실록산 가교를 가지는 폴리플루오렌은 상온에서 뿐 아니라, 150℃에서 4시간 동안 가열처리하여도 500nm 이상에서 엑사이머 빛발광현상이 거의 관찰되지 않았다. 특히, 실록산 가교의 함량이 많을수록 fwhm 값이 작았으며, 효과적인 청색 발광 고분자 성능을 나타내었다. 이러한 청색발광특성의 개선은 가교에 참여하는 실록산기가 고분자 사슬의 재배열을 효과적으로 막으며 또한 실록산기 자신의 일정한 부피가 플루오렌 고분자 사슬간의 간섭을 막기 때문으로 생각된다.
플루오렌의 9번 탄소에 비대칭적으로 POSSs (polyhedral oligomeric silsesquioxanes)를 가진 폴리플루오렌 공중합체(PF-POSS05, PF-POSS10 및 PF-POSS20)가 니켈촉매 중합법에 의해서 합성되었다. POSSs를 가진 폴리플루오렌은 상온에서 뿐 아니라, 150℃에서 5시간 동안 가열처리하여도 엑사이머 빛발광현상이 거의 관찰되지 않았으며, 녹색 전기적 발광을 거의 나타내지 않아서 안정하고 순수한 청색 전기 발광을 나타내었다. POSSs 작용기의 함량이 많을수록 순수한 청색에 더 가까워졌으며, POSSs 작용기가 결합된 플루오렌 단량체의 몰함량이 20%인 공중합체는 가장 순수한 청색 전기 발광을 보였다(CIE coordinate; 0.18, 0.13). 유기 전기 발광 다이오드에서의 문턱에너지는 모든 POSSs 를 가진 폴리플루오렌에 대해서 4V였으며, PF-POSS05, PF-POSS10 및 PF-POSS20은 각각 $263cd/ ㎡$ (10V), $463cd/ ㎡$ (10V), $420cd/ ㎡$ (8.5V)의 빛밝기을 보였다. POSSs기가 도입된 폴리플루오렌의 청색발광특성(특히 색순도)의 개선은 1-2nm 크기의 POSSs기가 고분자 사슬간의 간섭을 막기에 충분한 크기이기 때문으로 생각된다.
폴리플루오렌 주쇄에 실록산 작용기를 도입한 새로운 형태의 폴리플루오렌(P2Silo05, P2Silo10, P2Silo15, P8Silo05, P8Silo10, P8Silo15))을 합성하였다. 합성된 고분자는 열적으로 안정했으며, 실록산 작용기의 함량이 많을수록 최대 자외선 흡광파장은 단파장 쪽으로 이동했지만, 최대 빛발광 파장은 Homo PDHF와 거의 차이를 보이지 않았다. 유기 전기 발광 다이오드에서 실록산 작용기의 함량이 많을수록 순수한 청색 발광으로의 이동이 일어났으며, P2Silo15 [ CIE 1931 coordinate 값(0.17, 0.10) ]는 현재까지 보고된 발광 고분자 중에서 가장 순수하고 안정한 청색발광 성질을 나타내었다. 이렇게 폴리플루오렌의 주쇄에 위치한 실록산 작용기는 고분자사슬내의 conjugation길이를 조절하는 기능을 함으로써 엑사이머 발광현상을 거의 차단하는 것처럼 보여지며, 실록산기의 길이가 너무 긴 경우에는 분자사슬간의 재배열이 쉽게 일어날 수 있으므로, aggregation 형성이 다시 쉽게 일어날 수 있는 문제점을 가질 수 있다. 실록산 작용기를 주쇄에 가지는 폴리플루오렌(P2Silo10)과 PFTT(녹색발광 플루오렌계 고분자물질)간의 에너지 전이 특성에 대한 연구를 하였다. P2Silo10의 최대 발광파장(448nm)과 PFTT의 최대 흡수파장(447.5nm)이 잘 일치하여 효율적인 에너지 전이현상이 관찰되었으며, P2Silo10과 PFTT를 20대 1로 혼합한 고분자블랜드는 Homo P2Silo10에 비해서 유기 전기 발광 다이오드에서 3배 이상의 양자효율 향상을 보였다.
폴리플루오렌 주쇄에 distilbene 작용기를 도입한 새로운 형태의 폴리플루오렌(PF-M02, PF-M05, PF-M10, PF-P02, PF-P05, PF-P10)을 합성하였다. 합성된 고분자는 kink구조의 distilbene 작용기로 인해서 고분자 주쇄간의 aggregation 및 엑사이머 발광이 억제되어 순수한 청색발광 특성을 나타내었다. 특히 유기 전기 발광 다이오드에서 PF-M05와 PF-P05가 합성된 고분자 중에서 가장 순수한 청색 발광특성을 나타내었으며, PF-M05와 PF-P05의 CIE 1931 coordinate 값은 각각 (0.19, 0.13), (0.18, 0.13)이었다. 고분자 주쇄내에서 distilbene 작용기의 함량이 일정 수준(5몰 %)이상으로 높아지면 오히려 distilbene 작용기가 chromophore로 작용을 하여 고분자의 청색발광특성이 떨어지는 것으로 나타났다.
게르마늄이 주쇄에 포함된 poly(p-phenylenevinylene)계의 고분자물질이 wittig 반응법에 의해서 합성되었다. Poly(p-phenylenevinylene) 유도체에 도입된 chromophore로는 carbazole기, dialkyloxybenzene기, phenothiazine기 등이 있으며, 각 고분자물질은 각각 청색, 청자색, 녹색의 빛발광특성과 흰색, 연한 연두색, 진한 연두색의 전기발광특성을 보였다.
