PMID- 20726547
OWN - NLM
STAT- MEDLINE
DCOM- 20110118
LR  - 20100930
IS  - 1520-5207 (Electronic)
IS  - 1520-5207 (Linking)
VI  - 114
IP  - 39
DP  - 2010 Oct 7
TI  - Supramolecular liquid crystalline pi-conjugates: the role of aromatic pi-stacking 
      and van der Waals forces on the molecular self-assembly of 
      oligophenylenevinylenes.
PG  - 12508-19
LID - 10.1021/jp105839f [doi]
AB  - Here, we report a unique design strategy to trace the role of aromatic pi-stacking 
      and van der Waals interactions on the molecular self-organization of pi-conjugated 
      building blocks in a single system. A new series of bulky oligophenylenevinylenes 
      (OPVs) bearing a tricyclodecanemethylene (TCD) unit in the aromatic pi-core with 
      flexible long methylene chains (n = 0-12 and 16) in the longitudinal position 
      were designed and synthesized. The OPVs were found to be liquid crystalline, and 
      their enthalpies of phase transitions (also entropies) showed odd-even 
      oscillation with respect to the number of carbon atoms in alkyl chains. OPVs with 
      an even number of methylene units in the side chains showed higher enthalpies 
      with respect to their highly packed solid structures compared to odd-numbered 
      ones. Polarized light microscopic analysis confirmed the formation of cholesteric 
      liquid crystalline (LC) phases of fan shaped textures with focal conics in OPVs 
      with 5 </= n </= 9. OPVs with longer alkyl chains (OPV-10 to OPV-12) produced a 
      birefringence pattern consisting of dark and bright ring-banded suprastructures. 
      The melting temperature followed a sigmoidal trend, indicating the transformation 
      of molecular self-organization in OPVs from solid to ring-banded suprastructures 
      via cholesteric LC intermediates. At longer alkyl chain lengths, the van der 
      Waals interactions among the alkyl chains became predominant and translated the 
      mesogenic effect across the lamellae; as a consequence, the lamellae underwent 
      twisted self-organization along the radial growth direction of the spherulites to 
      produce bright and dark bands. Scanning electron microscope (SEM) analysis of 
      cholesteric LC and ring-banded textures strongly supported the existence of 
      twisted lamellae in the OPVs with ring-banded textures. Variable temperature 
      X-ray diffraction analysis confirmed the reversibility of the molecular 
      self-organization in the solid state and also showed the existence of the higher 
      ordered lamellar structure in ring-banded OPVs. Photophysical characterizations 
      such as excitation, emission, and time resolved fluorescence decay measurements 
      were employed to trace molecular self-organization in their liquid crystalline 
      phases. The emission spectra of the OPV samples showed odd-even oscillation in 
      their emission wavelengths with respect to the length of alkyl chains. Highly 
      packed even-OPVs showed more blue shift compared to that of less crystalline 
      odd-OPVs. Time dependent fluorescence decay of OPVs followed a biexponential fit, 
      and their lifetimes (tau(1) and tau(2) values) revealed that the decay is faster for 
      odd-OPVs compared to even-OPVs. Among all the OPVs, the tau(2) values for OPV-8 and 
      OPV-12 were found to be much higher, indicating their high luminescent 
      characteristics. In a nut shell, bulky liquid crystalline OPV chromophores were 
      cleverly utilized, for the first time, to probe the aromatic pi-stacking versus 
      van der Waals interactions on the molecular self-organization of pi-conjugated 
      system.
FAU - Goel, Mahima
AU  - Goel M
AD  - Department of Chemistry, Indian Institute of Science Education and Research 
      (IISER)-Pune, 900, NCL Innovation Park, Dr. Homi Bhabha Road, Pune -411008, 
      Maharashtra, India.
FAU - Jayakannan, M
AU  - Jayakannan M
LA  - eng
PT  - Journal Article
PT  - Research Support, Non-U.S. Gov't
PL  - United States
TA  - J Phys Chem B
JT  - The journal of physical chemistry. B
JID - 101157530
RN  - 7440-44-0 (Carbon)
SB  - IM
MH  - Carbon/chemistry
MH  - Electrons
MH  - Liquid Crystals/*chemistry
MH  - Microscopy, Electron, Scanning
MH  - Phase Transition
MH  - Thermodynamics
MH  - Transition Temperature
MH  - X-Ray Diffraction
EDAT- 2010/08/24 06:00
MHDA- 2011/01/19 06:00
CRDT- 2010/08/24 06:00
PHST- 2010/08/24 06:00 [entrez]
PHST- 2010/08/24 06:00 [pubmed]
PHST- 2011/01/19 06:00 [medline]
AID - 10.1021/jp105839f [doi]
PST - ppublish
SO  - J Phys Chem B. 2010 Oct 7;114(39):12508-19. doi: 10.1021/jp105839f.