1,2-Bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene (1a) and their derivatives, 1,2-bis(2-methyl-5-p-tolyl-3-thienyl)perfluorocyclopentene (2a) and 1,2-bis(2-methyl-5-p-tert-butylphenyl-3-thienyl)-perfluorocyclopentene (3a), were found to undergo reversible photochromic reactions in the single-crystalline phase. Upon irradiation with 366 nm light the single crystals turned blue. The blue colored crystals returned to colorless by irradiation with visible light (λ > 480 nm). The substituents at para positions of the phenyl groups did not affect the rates of photocyclization reactions both in the single-crystalline phase and in hexane. Activation energies of the photocyclization reactions were almost zero. On the other hand, activation energies as much as 5-10 kJ mol -1 were observed in the photocycloreversion reactions in the single-crystalline phase. These values were smaller than those observed in solution, ca. 16 kJ mol -1 . Slow thermal cycloreversion reaction of the closed-ring isomer (1b) was observed above 150°C. The activation energy was 139 kJ mol -1 .
Beside normal cyclization/ring-opening photochromic reactions, a side reaction to produce a photostable by-product took place when 1,2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene was irradiated in deaerated hexane with ultraviolet light.
The fluorescence quantum yields φ F (n) of oligo[2,5-bis(propyloxy)-1,4-phenylenevinylene]s (1b) reach for the trimer (n = 3) in solution a maximum of 86 %, which is almost twice as high as for the monomer (n = 1) and the octamer or undecamer (n = 8, 11). The latter represent the convergence limit of 46 %. 4-Mono-, 3,4-di-or 3,4,5-trisubstituted terminal phenyl rings in the trimers 6a-f do not lead to calamitic or phasmidic liquid crystals; however, 2,3,4-trisubstitution with
IntroductionDue to their interesting electrical, optical and optoelectronic properties, oligo-and poly(1,4-phenylenevinylene)s [OPV and PPV] are a highly attractive class of compounds in organic chemistry and materials science.[1] Already with n Ͼ 3, the parent system 1a is insoluble in organic solvents, so solubilizing alkyl or better alkoxy side chains have to be introduced. Some time ago we prepared the series 1b (n = 1-8, 11, 15) with 2,5-dipropoxy-substituted phenyl rings (Scheme 1). [2][3][4] Increasing numbers n of repeating units in conjugated oligomers lead for various properties P to limiting values which can be matched by the corresponding polymer. This approach can be monotonous, as for the λ max values of absorption and fluorescence of 1b, [2][3][4] but the approach P(n) Ǟ P ϱ can also pass for a certain number nЈ through a maximum and/or a minimum. [1l] An important example is given for the electroluminescence efficiency of 1b which reaches a maximum for n = 3. [5] Earlier studies indicated that the photoluminescence quantum yields φ F should also have a maximum for a certain repeat number n of 1b since φ F = 0.82 for the dimer n = 2 measured in CH 2 Cl 2 is much higher than φ F of a long oligomer chain (φ F ≈ 0.5 for n = 11). [6,7] We determined now the fluorescence quantum yields φ F (n) for the whole series 1b (n = 1-8, 11) and tried to obtain liquid crystals (LC) for those oligomers that have the highest φ F value.[
The (E,E)-3,6-Bis(2-arylethenyl)-l,2,4,5-tetrazines 8 were prepared in a synthetic sequence starting with aromatic aldehydes 1 and cinnamic acids 2, respectively (Scheme 1). The rod-like conjugated molecules represent calamitic mesogens as well as extended chromophores. Hence, we investi-
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