Analyzing structure-photophysical property relationships for isolated T8, T10, and T12 stilbenevinylsilsesquioxanes

Silsesquioxanes (SQs) are of considerable interest for hybrid electronic and photonic materials. However, to date, their photophysical properties have not been studied extensively, thus their potential remains conjecture. Here we describe the first known efforts to map structure-photophysical properties as a function of cage symmetry and size by comparing identically functionalized systems. Our focus here is on the solution photophysical properties of the title stilbenevinyl-SQs, which were characterized using single photon absorption, two-photon absorption, fluorescence emission, and fluorescence lifetime kinetics. We offer here the first detailed photophysical study of the larger pure T10 and T12 silsesquioxanes and show photophysical properties that differ as a function of size, especially in their fluorescence behavior, indicating that cage size and/or symmetry can strongly affect photophysical properties. We also find that they offer excitation-dependent emission (evidence of rare "red-edge" effects). The T10 stilbenevinyl-SQ offers up to a 10-fold increase in two-photon absorption cross section per chromophore over a free chromophore, signifying increased electronic coupling. The SQ cage compounds show "rise times" of 700-1000 fs and low anisotropy (~0.1) in fluorescence lifetime kinetic studies. These results indicate excited state energy transfer, unobserved for the free chromophores and unexpected for systems with "inert" silica cores and for 3-D hybrid molecular species. These findings provide the first detailed photophysical study of chromophore-functionalized T10 and T12 silsesquioxanes and show that SQs may be considered a separate class of compounds/materials with anticipated novel properties of value in developing new components for electronic and photonic applications.