摘要:

IntroductionThe study of size effect on the electronic properties of semiconductor nanocrystallites has long been a subject under intense investigation in the field of physical chemistry and materials chemistry.1–5It is of fundamental interest to understand how these properties vary as the crystallite size grows from the molecule to the bulk material. Calculations of this energy level based on the effective-mass approximation have been proposed.6The study of electronic properties of organic molecules in nanoscale confined spaces adds a new dimension to chemical physics, as verified by highly efficient nanovessel reactors and quantum-confined materials.7Because of their intriguing structural and chemical features, semicrystalline periodic nanoporous silicas of the M41S family are considered as one of the most attractive host materials.8,9Their intrinsic zeolite-like pore architecture with tunable pore sizes and narrow pore size distributions provides a unique platform for studying the encapsulation of highly reactive organic molecules. Encapsulation of these molecules in the nanoporous channels host may produce significant changes on the electronic properties of the guest species, which result in the variations of optical band gaps and excited-state lifetimes of the organic molecules in these complex systems in experimental studies by various groups.10–13Additionally, there have been previous nice theoretical studies on similar clusters, which give useful comparisons.14We believe that the electronic confinement theory is responsible for the variation of electronic properties of the guest species.10In this theory, the confinement produces an increase in energy of all the orbitals of guest molecules in confined spaces, but the increase is different for the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital): the HOMO has been predicted to be more sensitive than the LUMO, and the predicted effect is a reduction of the band gap of the frontier molecular orbitals. Therefore, the changes of electronic structures can be explained as produced by electronic confinement in which the electron density of the guest is constrained to mainly localize within the cavity as a result of the strong short-range repulsion with the electrons of the channel walls. It is generally accepted that the sizes of the channels and cavities are crucial for understanding the different electronic properties involved under incorporation of guest molecules into the host. So, it is rather important to study exclusively the influence of the confinement effect at various host environments in order to achieve a better understanding of the host–guest interactions in these complex systems. In a previous communication, we have presented experimental studies in support of the electronic confinement effect;10we report in this contribution the theoretical evaluations of three confined aromatic molecules and find that the confinement is truly responsible for remarkable changes, such as the increase in energies of molecular orbitals and the decrease of band gaps.

ISSN:1460-2733    

DOI:10.1039/b307440d

出版商:RSC    

年代:2003

数据来源: RSC