Unsymmetrical 7-(2′-bromoethoxycarbonyl)-7,8,8-tris­(methoxycarbonyl)-p-quinodimethane (2) underwent a quantitative intermolecular [6 + 6] photocycloaddition reaction through a single crystal-to-single crystal transformation to afford a bridge-substituted [2.2]­paracyclophane (3). The crystal structure of 2 indicates that the bromoethoxy groups conveniently form σ-halogen bonds with the carboxyl groups to yield 2-fold helical assemblies of an isolated pair of 2. 3 has a relatively long distance between bridged carbon–carbon bonds in comparison with the known ones. Such bonds caused an one-side insertion reaction of molecular oxygen in solution to afford the peroxide bridge-substituted [2.2]­paracyclophane in a quantitative yield.

We have found that the cis-conformational structures in single-polymer crystals obtained by solid-state polymerization are retained in solution. A soluble alternating copolymer was accidentally obtained during our research employing a series of 7,7,8,8-tetrakis­(alkoxycarbonyl)-p-quinodimethanes (1) with alkoxy groups such as methoxy­(1a(Me)), ethoxy­(1b(Et)), propoxy­(1c(Pr)), isopropoxy­(1d(iPr)), butoxy­(1e(Bu)), isobutoxy­(1f(iBu)), pentyloxy­(1g(Pen)), hexyloxy­(1h(Hex)), and dodecyloxy­(1i(Dod)). Thus, 1a(Me), 1b(Et), 1c(Pr), 1e(Bu), and 1g(Pen) with linear alkoxy groups afforded 1:1 charge-transfer complex crystals with 7,7,8,8-tetracyanoquinodimethane (3), while 1d(iPr), 1f(iBu), 1h(Hex), and 1i(Dod) with branching or linear long-chain alkoxy groups did not. The former crystals topochemically underwent photochemical and thermal copolymerizations via a radical mechanism to yield cis-conformational alternating copolymer crystals according to X-ray crystallography. Attractively, the rates of thermal copolymerization were found to increase roughly with an increase in the chain length of the linear alkoxy groups and/or a decrease in the reacting exomethylene carbon distances between 1 and 3. This finding led us to carry out a spontaneous copolymerization of 1g(Pen) with 3 in solution, resulting in the first synthesis of a soluble alternating copolymer during our research. Spectral analyses of the soluble copolymer of 1g(Pen) with 3, together with geometry optimization and spectroscopic simulations, proved that the cis-conformational structure was retained in solution as well as in the solid state. This provides the first spectral observation of a cis-conformational structure in solution with respect to the alternating copolymers of the p-quinodimethane derivatives.

We propose a mechanism for substituent‐responsive reactivities of p‐quinodimethane derivatives with four ester groups through their hierarchical and asymmetric assembly modes.   Four asymmetric 7,8,8‐tris(methoxycarbonyl)‐p‐quinodimethanes with a 7‐positioned ethoxycarbonyl (2a(H)), 2’‐fluoroethoxycarbonyl (2b(F)), 2’‐chloroethoxycarbonyl (2c(Cl)), or 2’‐bromoethoxycarbonyl (2d(Br)) were synthesized and crystallized.   2a(H), 2b(F) and 2d(Br) afforded only one shape crystal, while 2c(Cl) did two polymorphic 2c(Cl)‐a and 2c(Cl)‐b.   UV‐irradiation induced topochemical polymerization for 2a(H), no reactions for 2b(F) and 2c(Cl)‐a, and [6+6] photocycloaddition dimerization for 2c(Cl)‐b and 2d(Br).   Such substituent‐responsive reactivities and crystal structures were compared with those of the known symmetric 7,7,8,8‐tetrakis(alkoxycarbonyl)‐p‐quinodimethanes such as 7,7,8,8‐tetrakis(methoxycarbonyl)‐ (1a(Me)‐a and 1a(Me)‐b), 7,7,8,8‐tetrakis(ethoxycarbonyl)‐ (1b(Et)), and 7,7,8,8‐tetrakis(bromoethoxycarbonyl)‐ (1c(BrEt)).   The comparative study clarified that the reactivities and crystal structures are classified into four types that link to each other.   This linkage is understandable when we analyze the crystal structures through the following hierarchical and asymmetric assemblies; conformers, dimers, 1D‐columns, 2D‐sheets, and 3D‐stacked sheets (3D‐crystals).   This supramolecular viewpoint is supported by intermolecular interaction energies among neighbored molecules with the density functional theory (DFT) calculation.   Such research enables us to elucidate the substituent‐responsive reactivities of the crystals, and reminds us of the selection of the right path in a so‐called “maze game”.