Optoelectronic Properties Of Transition-Metal Alkynyl σ Monomer And Polymer: Quantum Chemical Studies

The study of transition-metal alkynyl σ complexes has, since themid-1980's, been an intense area of research due to their rigid-rod structure,relative stability and unique redox, magnetic, optical, and electronic properties.The introduction of a transition-metal center into the conjugatedσ-alkynylcomplexes may removes the spin-forbidden nature of the radiative relaxationof the triplet state, leading to high phosphorescence efficiencies via spin-orbitcoupling. Meanwhile, the rigid sp-hybridization alkynyl ligand may facilitatepolymerization which endow this type of materials possess the merit frompolymer and metal complexes.In this paper, the electronic structures and optoelectronic properties of d⁸,d¹⁰ transition-metal alkynyl monomer/polymer were investigated by quantumtheoretical strudies. The results suggest new theoretical basis and direction fordesign of novel organic materials. Our work will focus on three aspects:1. The geometrical and electronic structures of (Hg-TFT)n and (TFT)n(n=1-3) was studied by a first-principles density-functional method.Ï€-conjugation was preserved through the metal atom due to weakhybridization between the pÏ€-orbital of the conjugated ligand and the HgdÏ€-orbital. A stronger localized Hg-C σ bond was found between metal andconjugated ligand. Further electronic spectrum calculations confirm that theintroduction of metal center into the conjugated segments contribute more toconfine the excitons than to delocalization of the Ï€ electrons.The heavy metal atom forms some barrier to delocalization, so that thefirst singlet and triplet excited state of polymer have the character of molecularexcited states and mainly ligand-dominating excited state. Thus this kind ofrigid rod organometallic polymer can be used as good emitter layer in displaysdue to their potential achieving full color emission through modifying theelectronic structure of the conjugated ligand segment.2. A comprehensive photophysical study has been carried out on the golddiethynylfluorenyl complexes AuTFT, AuTFOT and AuTFCNT (where TFT =diethynylfluorenyl, TFOT = diethynylfluorenone and TFCNT =diethynyl-(9-(dicyanomethylene)fluorine). In order to evaluate the potentialapplication of these gold complexes to the OLED, the charge transfer ability isassessed in terms of internal reorganization energy.Based on our calculation, the reorganization energy for hole transport λi(h)is related to the HOMO and the reorganization energy for electron transportλi(e) is controlled by the LUMO. The values of λi(e) in this series of goldcomplexes vary from 0.24 to 0.29 eV, which are comparable with those ofwell-known electrontransport and luminescent material Alq3 (λi(e)=0.28eV atB3LYP/6-31G* level).25 Meanwhile, the withdrawing effect of carbonyl andcyano groups can effectively improve the electron affinity (lower the reductionpotential), rendering the material n-conducting. We may expect that AuTFOTand AuTFCNT would be used as electrontransport materials in LEDs.3. A comparative study was performed on PtTFT (1),PtTFOT (2)å’ŒPtTFCNT(3) [TFT = diethynylfluorenyl, TFOT = diethynyl fluorenone andTFCNT = diethynyl-(9-(dicyanomethylene) fluorene )]. M-C interaction wasdiscussed in terms of bonding energy and compounds decomposition analysis.The results show that the main interaction between metal and alkynyl ligand iscovalence bond derived from charge transfer from ligand to metal. When theelectron-withdrawing group was introduced into C9 of fluorenyl ring, theM→L back-donation is increased and reinforce dÏ€-pÏ€* interaction, which mayincrease the Ï€ delocalization of molecules and effective decrease the band gap.The maximum absorption wavelength lengthes show red shift on goingfrom PtTFT to PtTFOT and to PtTFCNT, consistent with the increasingelectron-withdrawing ability.