Excimer

About: Excimer is a research topic. Over the lifetime, 3725 publications have been published within this topic receiving 75104 citations.

Controllable multicolor switching of oligopeptide-based mechanochromic molecules: from gel phase to solid powder

Abstract: A series of molecules with pyrene and rhodamine B as color-producing mechanophores linked by different spacers were synthesized and their mechanochromic properties were studied. Interestingly, we found that the molecule with diphenylalanine as a linker (PHE-2) showed a sequential multicolored switch from deep blue to bluish green and further to a reddish color, which was associated with the phase change from gel to xerogel as the solvent evaporated, and to a solid powder triggered by grinding. However, the gel and xerogel of the molecule with the pentaphenylalanine linker (PHE-5) exhibited the same deep blue color that switched to bluish green and further to a reddish powder by virtue of continuously grinding the xerogel sample in situ. The multicolored switching of PHE-2 and PHE-5 was realized by the variation of the self-assembled structures, which induced the transition of the pyrene excimers from excimer 1 (deep blue) to excimer 2 (bluish green), and by the chemical reaction of rhodamine B from a spirolactam to a ring-opened amide (red). From the experimental results, we may conclude that the crucial point for controlling and tuning the tricolored fluorescent switch of this system is to constrict the pyrene excimer in an overlapped packing mode, which can be achieved (1) by controlling the molecular structure; and (2) by confining the excimers of pyrene in a restricted environment.

Interpretation of unusual absorption bandwidths and resonance Raman intensities in excited state mixed valence.

Abstract: Excited state mixed valence (ESMV) occurs in molecules in which the ground state has a symmetrical charge distribution but the excited state possesses two or more interchangeably equivalent sites that have different formal oxidation states. Although mixed valence excited states are relatively common in both organic and inorganic molecules, their properties have only recently been explored, primarily because their spectroscopic features are usually overlapped or obscured by other transitions in the molecule. The mixed valence excited state absorption bands of 2,3-di-p-anisyl-2,3-diazabicyclo[2.2.2]octane radical cation are well-separated from others in the absorption spectrum and are particularly well-suited for detailed analysis using the ESMV model. Excited state coupling splits the absorption band into two components. The lower energy component is broader and more intense than the higher energy component. The absorption bandwidths are caused by progressions in totally symmetric modes, and the difference in bandwidths is caused by the coordinate dependence of the excited state coupling. The Raman intensities obtained in resonance with the high and low energy components differ significantly from those expected based on the oscillator strengths of the bands. This unexpected observation is a result of the excited state coupling and is explained by both the averaging of the transition dipole moment orientation over all angles for the two types of spectroscopies and the coordinate-dependent coupling. The absorption spectrum is fit using a coupled two-state model in which both symmetric and asymmetric coordinates are included. The physical meaning of the observed resonance Raman intensity trends is discussed along with the origin of the coordinate-dependent coupling. The well-separated mixed valence excited state spectroscopic components enable detailed electronic and resonance Raman data to be obtained from which the model can be more fully developed and tested.