Ultrafast Intersystem-Crossing in Platinum Containing -Conjugated Polymers with Tunable

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Ultrafast Intersystem-Crossing in Platinum Containing -Conjugated Polymers with Tunable

Ultrafast intersystem-crossing in platinum containing -conjugated polymers with tunable spin-orbit coupling

C.-X. Sheng,1, 2 S. Singh1, A. Gambetta1, +, T. Drori1, M. Tong1,

S. Tretiak3, and Z. V. Vardeny1, *

1Department of Physics & Astronomy, University of Utah, Salt Lake City, Utah 84112, USA

2School of Electronic and Optical Engineering, Nanjing University of Science and Technology, Nanjing, Jiangsu, 210094, China.

3Theoretical Division, Center for Nonlinear Studies (CNLS), and Center for Integrated Nanotechnologies (CINT), Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA

Supplemental Information

Synthesis route of Pt-polymers with enhanced SO coupling

Transition metals such as Pt are elements that have filled or partially filled d-orbitals, and are found in a wide variety of oxidation states. Dative covalent bonds can be formed between the central metal atoms and ligands, and the bonding geometry depends on the oxidation state of the metal [S1]. Pt(II) forms square planar complexes where the five degenerate d-orbitals of the metal ion have their degeneracy lifted by the Coulomb interactions with the ligand orbitals. It was found that the conjugation could extend through the Pt, as a result of mixing between the frontier orbitals of the metal and the conjugated ligands. The extent of mixing depends on the overlap between the ligand and metal orbitals, and thus may vary from ligand to ligand.

Fig. S1: An outline of the synthesis process of Pt-containing ethynylene monomers and polymers from organic ligands with spacer group, R.

Since Pt(II) takes a square planar configuration and forms stable bonds with ethynylenes, it is possible to synthesize organometallic polymers of the structure shown in Fig. S1 [ref. S2]. The two conjugated phosphine ligands allow for solubility, whereas the two conjugated ligands that consist of the ethynylene groups bridged by a conjugated spacer, R form the backbone of the polymer. A more elaborate synthetic scheme and alternative schemes are shown in Fig. S2.

Fig. S2: Detailed synthesis routes of several Pt-containing polymers in our studies. The numbers (1-6) in parenthesis are references [S3-S9] provided in the references section; the encircled numbers stand for the different intermediate compounds.

We also synthesized Pt-polymers and monomers in which the Pt-atoms are separated by more than one moiety (namely larger distance between adjacent intrachain Pt atoms) in order to tune the SOC in the polymer, with the assumption that is larger when the adjacent intrachain Pt-atoms are closer to each orther. An example of the synthetic route that leads to polymers where adjacent Pt-atoms are separated by three moieties is shown in Fig. S3. The synthesis methods have been successfully implemented by our chemist, Dr. Leonard Wojcik.

Fig.S3: Schematic synthesis route of the Pt-3 polymer, where the Pt-atoms are separated by three benzene moieties. Scheme (1) is the synthesis of the monomer, and scheme (2) describes the Pt-3 polymer, where the Pt monomer is shown at the top. This polymer type has never been synthesized previously.

Picosecond spectroscopy of Pt- and non-Pt polymers

For comparison reasons, we measured ps transient PM spectroscopy of two additional -conjugated polymers, a Pt-containing polymer (Pt-polymer) with the molecular structure shown Fig. S4(a) inset, and a similar polymer without the Pt-containing group (non-Pt polymer) [Fig. S4(b) inset]. The 5,8-diethynyl-2,3-diphenylquinoxaline unit and its Pt-containing polymer were prepared according to published procedures [S6, S9]. The non-Pt version of this polymer was synthesized by palladiumcatalyzed polycondensation of 1,4-bis(n-octyloxy)-2,5-diiodobenzene and 5,8-diethynyl-2,3-diphenylquinoxaline in a 1:1 ratio.

Figure S4(a) shows the transient PM spectrum of the Pt-polymer film at t = 0 and t = 50 ps, respectively. The spectrum is dominated by two PA band: PA1 at ~0.55 eV and PA2 at ~1.8 eV. The bands PA1 and PA2 decay together for few picoseconds. The PA bands at short time are interpreted as due to the transitions from 1Bu excitons to higher Ag states, in analogy with other -conjugated polymers [S10]. However PA1 decays away with a time constant of ~10 ps (Fig. S4(a) right inset), which can be interpreted as due to intersystem crossing (ISC) from the singlet to triplet manifold. In contrast the second exciton band, PA2 stops decaying after ~ 15 ps (Fig. S4(a) right inset) because of an accidental spectral overlap with a triplet PA band that is shown in the cw PM spectrum [S11]. We thus conclude that there is a fast ISC process also in this Pt-polymer.

For comparison,Fig. S4(b) shows the transient PM spectrum and dynamics of the non-Pt polymer at t = 0 ps. The PM spectrum is dominated by a single PA band at ~1.1 eV, and the decay dynamics are identical in the whole spectral range. In analogy with other -conjugated polymers we interpret this PA band as due to the transitions from singlet 1Bu excitons to excited Ag states. The decay kinetics of this band is relatively slow, and does not involve a fast ISC process similar to many other -conjugated polymers that do not contain heavy metal atoms in their backbone. We thus conclude that the Pt-atoms, rather than the polymer structure or morphology are responsible for the ultrafast dynamics seen in the PM transient spectroscopy, which shows enhanced ISC rate.

Fig. S4: a, Transient photomodulation (PM) spectrum of a Pt-polymer film at t=0 and t=50 ps. The PA bands PA1 and PA2 are assigned. The left inset is the molecular structure of this polymer; whereas the right inset shows the transient decay dynamics of the two PA bands. b, Transient photomodulation (PM) spectrum of a analog non-Pt polymer at t = 0 ps. The single PA band is assigned. The lower inset is the molecular structure of this polymer; whereas the upper inset shows the transient decay dynamics at various probe energies.

References

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