Wavelength and solvent controlled energy and charge transfer in donor-acceptor substituted platinum acetylide complexes

Abstract

• Photophysics of a Donor-Acceptor substituted Pt(II) Complex are studied. • Complex displays wavelength and solvent tuned photophysics. • Ultrafast spectroscopy reveals the existence of ultrafast energy and charge transfer. An electron donor-acceptor platinum complex trans -Pt(PBu 3 ) 2 (C≡C-T 4 )(C≡C-TBT) (Pt(T 4 )(TBT)) incorporating (3,3'''-dihexyl-2,2':5',2'':5'',2'''-quaterthiophene, T 4 ) as donor and (4,7-bis(2-thienyl)-2,1,3-benzothiadiazole, TBT) as acceptor has been synthesized and fully characterized. Model complexes trans -Pt(PBu 3 ) 2 (C≡C-T 4 ) 2 and trans -Pt(PBu 3 ) 2 (C≡CTBT) 2 featuring only donor or acceptor moieties were synthesized for reference. Steady state absorption, fluorescence, and ultrafast visible-near infrared transient absorption spectroscopy were applied to evaluate intracomplex energy and charge transfer in different solvents and by using selective excitation wavelengths corresponding to T 4 or TBT units. The fluorescence spectrum of Pt(T 4 )(TBT) is red-shifted with increasing solvent polarity, consistent with the existence of a charge transfer excited state. Detailed solvent and excitation wavelength dependent ultrafast transient absorption experiments were carried out on all of the complexes. For Pt(T 4 )(TBT) in non-polar solution excitation of T 4 leads to ultrafast (∼200 fs) T 4 to TBT energy transfer. However, in more polar solvents, T 4 excitation gives rise to an ultrafast T 4 to TBT electron transfer affording a “ligand to ligand” charge transfer excited state. The results provide insight into the existence of donor-acceptor interactions between weakly coupled π-conjugated units that are separated by a metal center.