Environment-sensitive fluorescence of COT-fused perylene bisimide based on symmetry-breaking charge separation

Flexible and aromatic photofunctional system (FLAP) is composed of flapping rigid aromatic wings fused with a flexible 8π ring at the center such as cyclooctatetraene (COT). A series of FLAP have been actively studied for the interesting dynamic behaviors. Here, we synthesized a new flapping molecule bearing naphtho-perylenebisimide wings (NPBI-FLAP), in which two perylene units are arranged side by side. As a reference compound, we also prepared COT-fused NPBI (NPBI-COT) that contains only single perylene unit. In both compounds, inherent strong fluorescence of the NPBI moiety is almost quenched and the FL lifetime becomes much shortened in highly polar solvents (acetone and DMF). Through the analyses of environment-sensitive fluorescence, electrochemical reduction/oxidation, and femtosecond transient absorption, the fluorescence quenching behavior was attributed to rapid symmetry-breaking charge separation (SB-CS) for NPBI-FLAP and to intramolecular charge transfer (ICT) for NPBI-COT. Most of the excited species of these compounds decay with the bent geometry, which is in contrast with the excited-state planarization behavior of a previously reported COT-fused peryleneimides with the double-headed arrangement of the perylene moieties. These results indicate that changing the fusion manners between COT and other π skeletons offers new functional molecules with distinct dynamics.

Which isomer is it, 1,2,5,6- or 1,4,5,8-tetrasubstituted cycloocta-1,3,5,7-tetraene? Synthesis of symmetrically tetrasubstituted cycloocta-1,3,5,7-tetraene derivatives

Ni(0) catalyzed cyclotetramerization [(2 + 2 + 2 + 2) cycloaddition] of propargyl alcohol gave D2 symmetric 1,4,5,8-tetrakis(hydroxymethyl)cycloocta-1,3,5,7-tetraene (1) as the major product contrary to what was reported earlier to be the corresponding C2v symmetric isomer, namely 1,2,5,6-tetrakis(hydroxymethyl)cycloocta-1,3,5,7-tetraene. The structure of 1 has been unambiguously assigned on the basis of single crystal XRD data which showed a very interesting "densely" intermolecularly hydrogen bonded solid state supramolecular structure. Tetrabromide (3) prepared from tetra-alcohol (1) reacted with a variety of nucleophiles, namely carbon, nitrogen, sulfur and phosphorus nucleophiles, to give the corresponding derivatives in good yields. The tetraphosphonate derivative (9) underwent the Wittig-Horner reaction with aromatic aldehydes to give the corresponding tetra-styryl derivatives (10a-i). The reaction of tetrabromide with aniline, benzyl amine and R(+)-α-methylbenzyl amine gave the corresponding C2v symmetric pyrroline fused 1,2,5,6-tetra-substituted cycloocta-1,3,5,7-tetraene derivatives, while all the other derivatives belonged to D2 symmetric 1,4,5,8-tetra-substituted cycloocta-1,3,5,7-tetraene. Quantum mechanical calculations support the above conclusion based on the relative stabilities of the two isomers. The fusion of a 5-membered pyrroline ring with the tub-shaped cyclooctatetraene (COT) ring leads to flattening of the tub (shallow tub) making it easier for the ring inversion of COT through a planar intermediate to take place readily resulting in the formation of C2v symmetric pyrroline fused COT derivatives.