We have systematically synthesized 1-, 3-, 6-, and 8-alkyl-substituted pyrene derivatives using the latest synthesis methods and investigated the effects of alkyl substitution on the photophysical properties of the pyrene chromophore. Like the trimethylsilyl group, which is known to enhance the fluorescence properties of some chromophores through σ*-π* conjugation, alkyl groups (primary, secondary, and tertiary) enhanced the fluorescence quantum yield of the pyrene chromophore through σ-π conjugation in most cases. While these enhancements in the fluorescence quantum yield were beyond expectations, the results were supported by absolute measurements. These results also indicate that ubiquitous alkyl groups can be used to tune the photophysical properties of the pyrene chromophore, as well as to improve the solubility or prevent aggregation. In other words, they can be used to develop new photofunctional materials.

Thermal reactions of benzodisilacyclobutene (1) and disilacyclobutene (2) with acetylene were investigated theoretically. The reactions are thought to proceed via the conventional Diels− Alder reaction of disilabutadiene, the conrotatory ring-opening product of disilacyclobutene, with acetylene. However, this mechanism is incompatible with the observed similar reactivities of 1 and 2 with acetylene and the retention of stereochemistry during the reaction. In our previous paper, we proposed an alternative [2 + 1] cycloaddition pathway that involved the direct addition of acetylene to the Si−Si σ−bond of 1 without ring opening. In this study, we extensively investigated the reaction pathways for both 1 and 2 on a theoretical basis. We found that charge transfer (CT) played a key role in the [2 + 1] cycloaddition pathway. On the basis of natural bond orbital (NBO) analysis, the interaction of the Si−Si σ-bond orbital (donor) with the π* orbital (acceptor) of the acetylene was attributed mainly to the CT process. Finally, an experiment to verify the [2 + 1] cycloaddition mechanism was proposed, in which the use of triacetylene as a terminal alkyne would allow the key intermediate in the pathway to be trapped.

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