Optical transition energy

The solid-state polymerization of diacetylenes is an example of a lattice-controlled solid-state reaction. Polydiacetylenes are synthesized via a 1,4-addition reaction of monomer crystals of the form R-C=C-CeC-R. The polymer backbone has a planar, fully conjugated structure. The electronic structure is essentially one dimensional with a lowest-energy optical transition of typically 16 000 cm-l. The polydiacetylenes are unique among organic polymers in that they may be obtained as large-dimension single crystals. 

The lowest energy optical transition for the binudear systems is da - Q. The excitation results in formation of a formal metal-metal single bond in the excited state. The transition is metal localized and can be viewed as movement of an electron from an orbital localized on the exterior of the M2 unit (the da orbital) to an orbital localized in the interior of the dimer cage (the pa orbital). The excitation results in hole formation localized on a metal center at an open coordination site (Figure 2). The lifetime of the 3(da pa) excited state, normally in between 100 ns and 10 us, makes M2 systems attractive for bimolecular photoprocesses. 

For Raman measurements the QDs were deposited on a Si wafer from the toluene solution. Their Raman spectra were excited by a 488 nm line of an Ar laser with power of 0.5-0.8 mW. The micro-Raman spectrograph (Renishaw-1000) equipped with x20 objectives and cooled CCD cameras were used in the experiments. Each spectrum was averaged over about 20 measurements with accumulation time of 20 s. The PL backgrounds due to the lowest energy optical transition of QDs were substracted. 

In the early 1980s, the dinuclear platinum(II) pyrophosphite complex [Pt2(POP)4] was found to be highly luminescent [10]. The photochemistry of this complex is unique and completely different from that of ruthenium(II) polypyridine complexes. The emission has been assigned to arise from a metal-centered excited state strongly modified by metal-metal interaction. The lowest energy optical transition involves the promotion of an elecuon from a filled do antibonding orbital (formed by the interaction of two d j orbitals, z axis taken to... 

The quantity cUphoton is defined as the peak energy of the lowest-energy optical transition in the subgap region (Section 4.2). For DMQtT, DMQqT, and DMSxT (Uphoton is 1.08, 0.86, and 0.71 eV, respec-... 

The linear optical properties (UV-visible and Raman) of PDA crystals have been thoroughly characterized and are reasonably well-understood. The lowest energy optical transition is typically located at about 2.0 eV and is excitonic in origin. Distortion of the backbone due to deliberately induced disorder or strain caused by side group interactions shifts this transition to higher energies. Crystal strain (e.g. polymer chains in the monomer lattice) can shift the transition either way. More work/ theory and experiment, is needed to sort out and understand these effects. 

In summary the VEH technique has proven to be a powerful tool in the calculation of the electronic properties of conjugated polymers. Though intended only for application to the valence band related properties such as IP, we also find that in many cases the lowest energy optical transition is computed with good reliability. 

Figure 9.3 Shows the molecular orbitals for trans octatetraene ignoring, hydrogen-carbon bonds, and the filling of these orbitals with electrons. Both possible transitions that would result from a 1-electron model of the structure and the actual result including electron correlation effects are also shown, indicating that the lowest energy optical transitions are not from the HOMO to the LUMO states. Only a two-electron transition is allowed between these states for a photon-only process. Reprinted with permission from Bredas, Jean-Luc et al. Excited-state electronic structure of conjugated oligomers and polymers a quantum-chemical approach to optical phenomena . Accounts in Chemical Research 1999 32 267-276. [1] Copyright 1999 American Chemical Society.

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