Substituent effects on crosslike packing of 2′,7′-diaryl-spiro(cyclopropane-1,9′-fluorene) derivatives:: Synthesis and crystallographic, optical, and thermal properties

A series of 2',7'-diarylspiro(cyclopropane-1,9'-fluorene) derivatives are efficiently synthesized and characterized to determine the -reason for the green-light emission, of these compounds. These compounds exhibit bright-violet to blue photoluminescence (PL) (lambda(PL)(max) = 353-419 nm) with excellent PL quantum efficiencies (Phi(PL) = 83-100%) in solution and show high thermal stabilities (T-d = 267-474 degrees C). The variation of the optical properties of these molecules in the solid state depends on the different stacking modes of these compounds containing different substituents, which are revealed by crystallographic analysis. CH center dot center dot center dot pi hydrogen bonds instead of intermolecular pi-pi interactions act as the driving force between adjacent fluorenes, even though a very small dialkyl group (cyclopropane) is introduced at the C-9 position of fluorene. The crosslike molecular stacking efficiently reduces the energy transfer between the herring-like aggregates and therefore results in the absence of a greenlight emission tail. In order to determine the cause of the green-light emission tails, the fluorescence spectra of the films annealed in N-2 or in air are recorded. Broad green-light emission tails were observed for the films annealed in air, which might be caused by fluorenone defects generated during processing or during the course of the photophysical analysis by reaction with residual oxygen.