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Title: A theoretical investigation into the luminescent properties of d 8-transition-metal complexes with tetradentate Schiff base ligands
Authors: Tong, GSM
Chow, PK
To, WP
Kwok, WM 
Che, CM
Keywords: Density functional calculations
Nonradiative decay
Spin-orbit coupling
Issue Date: 2014
Publisher: Wiley-VCH
Source: Chemistry - a European journal, 2014, v. 20, no. 21, p. 6433-6443 How to cite?
Journal: Chemistry - a European journal 
Abstract: A theoretical investigation on the luminescence efficiency of a series of d8 transition-metal Schiff base complexes was undertaken. The aim was to understand the different photophysics of [M-salen]n complexes (salen=N,N'-bis(salicylidene)ethylenediamine; M=Pt, Pd (n=0); Au (n=+1)) in acetonitrile solutions at room temperature: [Pt-salen] is phosphorescent and [Au-salen]+ is fluorescent, but [Pd-salen] is nonemissive. Based on the calculation results, it was proposed that incorporation of electron-withdrawing groups at the 4-position of the Schiff base ligand should widen the 3MLCT-3MC gap (MLCT=metal-to-ligand charge transfer and MC=metal centered, that is, the dd excited state); thus permitting phosphorescence of the corresponding PdII Schiff base complex. Although it is experimentally proven that [Pd-salph-4E] (salph=N,N'- bis(salicylidene)-1,2-phenylenediamine; 4E means an electron-withdrawing substituent at the 4-position of the salicylidene) displays triplet emission, its quantum yield is low at room temperature. The corresponding PtII Schiff base complex, [Pt-salph-4E], is also much less emissive than the unsubstituted analogue, [Pt-salph]. Thus, a detailed theoretical analysis of how the substituent and central metal affected the photophysics of [M-salph-X] (X is a substituent on the salph ligand, M=Pt or Pd) was performed. Temperature effects were also investigated. The simple energy gap law underestimated the nonradiative decay rates and was insufficient to account for the temperature dependence of the nonradiative decay rates of the complexes studied herein. On the other hand, the present analysis demonstrates that inclusions of low-frequency modes and the associated frequency shifts are decisive in providing better quantitative estimates of the nonradiative decay rates and the experimentally observed temperature effects. Moreover, spin-orbit coupling, which is often considered only in the context of radiative decay rate, has a significant role in determining the nonradiative rate as well. Low, but not ignored: Low-frequency normal modes are usually not considered to be important in determining nonradiative decay rates because they are not effective energy acceptors. The importance of these low-frequency normal modes in estimating knr by using a convolution approach is highlighted (see figure).
ISSN: 0947-6539
EISSN: 1521-3765
DOI: 10.1002/chem.201304375
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