Intense Fluorescence of Metal-to-Ligand Charge Transfer in [Pt(0)(binap)2] [binap = 2,2‘-Bis(diphenylphosphino)-1,1‘-binaphthyl]

Zainul Abedin-Siddique, Takeshi Ohno, and Koichi Nozaki*
Department of Chemistry, Graduate School of Science, Osaka University, 1-16 Machikaneyama, Toyonaka, Osaka 560-0043, Japan,
Taro Tsubomura
Department of Applied Chemistry, Seikei University, Kichijoji-Kitamachi, Musashino, Tokyo 180-8633, Japan
Inorg. Chem., 2004, 43 (2), pp 663–673
DOI: 10.1021/ic034527z
Publication Date (Web): December 16, 2003
Copyright © 2004 American Chemical Society
*

 To whom correspondence should be addressed. E-mail:  nozaki@ch.wani.osaka-u.ac.jp.

Abstract

Abstract Image

[Pt(0)(binap)2] (binap = 2,2‘-bis(diphenylphosphino)-1,1‘-binaphthyl) is found to exhibit a luminescence from metal-to-ligand charge transfer state (MLCT) with a quantum yield of 0.12 and a lifetime of 1.2 μs in toluene at an ambient temperature. Prompt fluorescence with a quantum yield of 1.6 × 10-4 is observed by means of a picosecond time-correlated single photon counting technique. The spectrum of the steady-state luminescence is almost identical to that of the prompt fluorescence, indicating that the intense luminescence is mainly delayed fluorescence from thermally activated 1MLCT. The analysis of the temperature-dependent emission indicates that the energy difference between the 1MLCT and 3MLCT is 1.15 × 103 cm-1. The lifetime of the prompt fluorescence is determined to be 3.2 ps from the decay of stimulated emission overlapped on subpicosecond transient absorption spectra. The lifetime of the 1MLCT is much longer than expected from the large spin−orbit coupling constant of 5d (Pt) electrons (4000 cm-1). Theoretical analysis based on density functional theory reveals that structural distortion in the MLCT states causes large energy splitting between HOMO and HOMO − 1, which prevents a very fast ISC induced by strong spin−orbit interactions between these orbitals. The relatively slow ISC is therefore induced by weak spin−orbit interactions (ca. 50 cm-1) between ligand-centered molecular orbitals. Theoretical calculations indicate that the phosphorescence observed at lower temperatures is due to intensity borrowing from 41B2 → GS transition. However, the large energy difference between HOMO and HOMO − 2 reduces the extent of mixing between the lowest 3MLCT and 41B2 due to spin−orbit interaction, thereby decreasing the radiative rate of the phosphorescence.

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  • Published In Issue January 26, 2004
  • Received May 16, 2003

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