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Published March 7, 2016 | Supplemental Material
Journal Article Open

Thermally Tunable Dual Emission of the d^8–d^8 Dimer [Pt_2(μ-P_2O_5(BF_2)_2)_4]^(4–)


High-resolution fluorescence, phosphorescence, as well as related excitation spectra, and, in particular, the emission decay behavior of solid [Bu_4N]_4[Pt_2(μ-P_2O_5(BF_2)_2)_4], abbreviated Pt(pop-BF_2), have been investigated over a wide temperature range, 1.3–310 K. We focus on the lowest excited states that result from dσ^*pσ (5d_z2–6p_z) excitations, i.e., the singlet state S_1 (of ^1A_2u symmetry in D_(4h)) and the lowest triplet T_1, which splits into spin–orbit substates A_(1u)(^3A_(2u)) and E_u(^3A_(2u)). After optical excitation, an unusually slow intersystem crossing (ISC) is observed. As a consequence, the compound shows efficient dual emission, consisting of blue fluorescence and green phosphorescence with an overall emission quantum yield of ∼100% over the investigated temperature range. Our investigation sheds light on this extraordinary dual emission behavior, which is unique for a heavy-atom transition metal compound. Direct ISC processes in Pt(pop-BF_2) are largely forbidden due to spin-, symmetry-, and Franck–Condon overlap-restrictions and, therefore, the ISC time is as long as 29 ns for T < 100 K. With temperature increase, two different thermally activated pathways, albeit still relatively slow, are promoted by spin-vibronic and vibronic mechanisms, respectively. Thus, distinct temperature dependence of the ISC processes results and, as a consequence, also of the fluorescence/phosphorescence intensity ratio. The phosphorescence lifetime also is temperature-dependent, reflecting the relative population of the triplet T_1 substates E_u and A_(1u). The highly resolved phosphorescence shows a ∼220 cm^(–1) red shift below 10 K, attributable to zero-field splitting of 40 cm^(–1) plus a promoting vibration of 180 cm^(–1).

Additional Information

© 2016 American Chemical Society. Received: December 7, 2015. Publication Date (Web): February 24, 2016. T.H. and H.Y. acknowledge the financial support of the German Ministry of Education and Research (BMBF) and the Bavaria California Technology Center (BaCaTeC) for a travel grant. Further financial support was provided by the Ministry of Education of the Czech Republic grants LH13015 and LD14129, the NSF CCI Solar Fuels Program (CHE-1305124), the Arnold and Mabel Beckman Foundation, and the COST Action CM1202. M.K. acknowledges the financial support of the MSMT ERC CZ project (LL1301). We thank Capano Gloria, Ivano Tavernelli, and Majed Chergui (EPFL, Switzerland) as well as Jay. R. Winkler and Harry B. Gray (Caltech) for interesting discussions on intersystem crossing mechanisms, spectroscopy, and photophysics. The authors declare no competing financial interest.

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