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Relevant academic research and scientific papers

Syntheses of donor-acceptor-functionalized dihydroazulenes

The dihydroazulene (DHA)/vinylheptafulvene (VHF) photo/thermoswitch has been of interest for use in molecular electronics and advanced materials. The switching between the two isomers has previously been found to depend strongly on the presence of donor a

Linear free-energy correlations for the vinylheptafulvene ring closure: A probe for Hammett σ values

Linear free-energy relationships, like Hammett correlations, are fundamental in physical organic chemistry for the elucidation of reaction mechanisms. In this work, we show that Hammett correlations exist for the ring closure of six different model systems of vinylheptafulvenes (VHFs) to their corresponding dihydroazulenes (DHAs). These first-order reactions were easily followed by UV/Vis absorption spectroscopy on account of the significantly different absorption characteristics between VHFs and DHAs. Opposing effects displayed by substituent groups at two different positions are conveniently accounted for by simply subtracting the two Hammett σ values of each group. The linear correlations readily allow us to obtain unknown and approximate Hammett σ values for previously uninvestigated substituents. We also show that they can provide alternative values to the standard ones. We present values for a variety of substituent groups ranging from alkynes, sulfones, sulfoxides, and different heteroaromatics. The electronic effects exerted by substituent groups on VHFs are also reflected in their absorption maxima. Thus, we have established an empirical relationship between the absorption maximum of the VHF and the Hammett σ values of its substituents. This fine-tuning of electronic properties is particularly important for the ongoing efforts of using the DHA/VHF molecular switch in molecular electronics devices. The value of Hammett: The rate of vinylheptafulvene (VHF) ring closure to a dihydroazulene (DHA) obeys the Hammett correlation: electron-donating groups in the seven-membered ring enhance the ring closure, whereas such groups at the dicyanoethylene unit retard it (see figure). When substituents are present at both these positions, a Hammett correlation is obtained based on the difference in Hammett substituent values. Copyright

Dihydroazulene/Vinylheptafulvene Photochromism: Effects of Substituents, Solvent, and Temperature in the Photorearrangement of Dihydroazulenes to Vinylheptafulvenes

1,1-Dicyano-2-phenyl-1,8a-dihydroazulene (H-D) and a series of p-phenyl-substituted derivatives (R-D, R: NO2, CN, Br, Cl, CH3, OCH3, and NH2) were investigated by time-resolved and steady-state photochemical methods.The dihydroazulenes (DHAs) undergo an efficient photoreaction to the corresponding vinylheptafulvenes (VHFs), the quantum yield (ΦD->V) at room temperature ranges from 0.1 to 0.6.The VHFs (λV = 440-470 nm) are nonemitting and photochemically nonreactive and undergo a thermal rearrangement.The activation energy for this back-reaction is 18-21 kcal/mol.The A factor is in the (0.01-5) * 1010 s-1 range; it varies with substituent and increases with increasing solvent polarity.The photoproduct is formed via a singlet pathway, 1DHA* -> VHF; triplet states are not involved in this reaction.Fluorescence from the DHAs was observed, weakly in fluid solution and most efficiently in glasses at low temperatures (e.g., λf ca. 480 nm for the parent compound).The quantum yield (Φf) at -196 deg C ranges from 0.15 to 0.9.A transient (λmax ca. 450 nm, τT D->V is strongly retarded, the triplet is observable on direct excitation with a lifetime of less than 10 μs even at -196 deg C.The increase of Φf and the reverse effect for ΦD->V with decreasing temperature indicates competition of these processes due to an activation barrier (1DHA* -> VHF pathway.Characteristic features of the ground-state and first excited singlet energy surfaces are presented.

NOVEL MULTICHROMOPHORIC DYES FOR CHEMICAL AND PHYSICAL SWITCHING

Functionalized photochromic compounds and their applications in chemical and physical switching processes are under investigation.In this report exclusive consideration is given to the dihydroazulene (DHA)/vinylheptafulvene (VHF) system 1/2.A short description of the photochromism of 1/2 is presented.Three topics of potential switching behaviour released by the photochromism of 1/2 are pointed out: (i) change of the pKa and of proton transfer properties in anilinium derivatives, (ii) change of the redox potential in compounds with electron transfer active subunits, (iii) increase of second-order nonlinear optical properties by irradiation of 1.

Light-Sensitive Dihydroazulenes: Alternative Synthesis - The Influence of Alkyl Substituents at the Five-Memeered Ring on the Photochromic Behaviour

Starting from cycloheptatrienylium tetrafluoroborate (8) the dihydroazulenes 1 were synthesized via substituted cycloheptatrienes 10 - 13 as intermediates.The dicyanovinyl derivatives 12 and 13 were transformed into vinylheptafulvenes 2 by dehydrogenation

Light-Induced and Reversible Transformations: Syntheses and Properties of Photochromic 1,1-Dicyano-1,8a-dihydroazulenes and Thermochromic 8-(2,2-Dicyanovinyl)heptafulvenes

1,1-Dicyano-2-aryl-1,8a-dihydroazulenes 6(a-g) are prepared in a two-step synthesis starting from 8-methoxyheptafulvene and benzylidenemalononitriles via the tetrahydroazulenes 5.The compounds 6(a-h) rearrange to 8-(2,2-dicyano-1-arylvinyl)heptafulvenes 7(a-h) by irradiation with visible light.The back reaction of 7 to 6 occurs under thermal conditions.Donor-substituted compounds indicate higher long term stability of the equilibrium 6 7 (6f 7f > 44 h).Spectroscopic data of 6 and 7 are given.The molecular structures of the p-bromophenyl derivative 6c and of the p-methoxyphenyl derivative 7f are revealed through crystallography.Some results for the rearrangements 6 7 in the solid state are given.