Synthesis, Characterization, and Optical Properties of Monodisperse Chiral Oligofluorenes

Abstract

The first series of monodisperse chiral oligofluorenes was synthesized and characterized. Chain length was found to play an important role in solid morphology. Whereas dimer through tetramer are amorphous, pentamer through hexadecamer all show cholesteric mesomorphism with varying degrees of morphological stability against crystallization. Pristine spin-cast films, approximately 90 nm in thickness, are amorphous but exhibit pronounced circular dichroism and highly efficient circularly polarized fluorescence, suggesting the presence of chiral assemblies that remain to be experimentally characterized. A nonamer with two sets of the 2S-methylbutyl group replaced by the 3S,7-dimethyloctyl group was prepared and shown to be capable of forming a monodomain, glassy cholesteric film with thermal treatment. The cholesteric film is responsible for an order-of-magnitude increase in circular dichroism and a handedness reversal in circularly polarized fluorescence as compared to the amorphous pristine film. Molecular dynamics simulation furnished new insight into the molecular origin of the observed chiral optical properties in neat films.

Structure, photophysics, and the order-disorder transition to theβphase in poly(9,9-(di-n,n-octyl)fluorene)

“…Mesomorphic behavior is seen in many other conjugated polymer families containing alkyl side chain substituents 22,23,24 although the exact details are both backbone and side chain specific. It also extends to oligomers and oligofluorenes bearing chiral side chain substituents 25,26,27 and these show very rich phase behavior. The mesomorphic behavior of the PF8 polymer correlates with the existence of two distinct backbone conformations 15,28 referred to as the "alpha" or "beta" phase 29 .…”

Dispositivos poliméricos eletroluminescentes

“…Essa classe de polímeros, alguns dos quais estão mostrados na Figura 2, tem despertado o interesse dos estudiosos por suas propriedades elétricas e ópticas com potencial aplicação para fabricação de dispositivos emissores de luz (LEDs) [22][23][24][25][26][27][28][29][30][31][32][33] . Estes polímeros apresentam algumas propriedades interessantes para essa finalidade destacando-se, entre elas, grande estabilidade química, térmica e oxidativa 18,[34][35][36][37][38] ; alto rendimento quântico de fluorescência tanto em solução quanto no estado sólido 37,[39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57] ; possibilidade de formarem materiais líquido-cristalinos 46,[58][59][60][61][62][63][64]…”

“…Por ex. : as altas estabilidades química e térmica dos poli(fluorenos) refletem-se na grande estabilidade espectral, que são descritas como sendo melhores que poli(p-fenilenovinileno) (PPV) 48 ; a possibilidade de formação de fase líquido-cristalina que permite a fabricação de dispositivo eletroluminescente com emissão de luz altamente polarizada 69 , pois o ordenamento no cristal leva a uma forte anisotropia dos espectros de absorção e emissão 61,70,73 e, conseqüen- temente, esses materiais têm um grande potencial de aplicação em monitores de cristal líquido 59,71 ; a cor da emissão 37,[74][75][76] depende da estrutura do polímero, bem como das ramificações na posição 9 (Figura 2) da estrutura central 18, , sendo que muitos tipos de estruturas podem ser sintetizadas 18 .…”

Analysis of the vibronic fine structure in circularly polarized emission spectra from chiral molecular aggregates

“…[9][10][11][12][13] These molecules have a tendency to form aggregates and related superstructures. There is growing evidence that also cyanine dye molecules can be organized into chiral structures resulting in chiroptical properties.…”