Photophysics and Nonlinear Absorption of Gold(I) and Platinum(II) Donor-Acceptor-Donor Chromophores.

Abstract

A series of Au(I) and Pt(II) acetylide complexes of a π-conjugated donor-acceptor-donor (D-A-D) chromophore were studied to develop quantitative structure-property relationships for their photophysical and nonlinear optical properties. The D-A-D chromophore consists of a "TBT" unit, where T = 3-hexyl-2,5-thienylene and BTD = 2,1,3-benzothiadiazole, capped with ethynylene groups. The D-A-D chromophore is functionalized with Au(I)PR3 (R = -Me and -Ph) and trans-Pt(II)(PR3)2-CCPh (R = -Me and -Bu) "auxochromes". All of the metal complexes were characterized by ground-state absorption, photoluminescence, nanosecond transient absorption, and two-photon absorption (2PA) spectroscopy. The experiments provided quantitative values of the photophysical parameters, including rates for radiative decay and intersystem crossing (ISC), triplet yields, and two-photon absorption cross sections. Pronounced solvatochromism in the fluorescence spectra suggests an enhanced dipole moment in the excited state of the complexes compared to the unmetalated TBT chromophore. The gold complexes feature larger fluorescence quantum yields and longer emission lifetimes compared to platinum. The Pt(II) complexes exhibit enhanced triplet-triplet absorption, reduced triplet-state lifetimes, and larger singlet oxygen quantum yields, consistent with more efficient ISC compared to the Au(I) complexes. When excited by 100 fs pulses, all of the D-A-D chromophores exhibit moderate two-photon absorption in the near-infrared between 700 and 900 nm. The 2PA cross section for the Au(I) complexes is almost the same as the unmetalated D-A-D chromophore (∼100 GM). The Pt(II) complexes exhibit significantly enhanced 2PA compared to the other chromophores, reaching 1000 GM at 750 nm. Taken together, the results indicate that the Pt(II) center is considerably more effective in inducing singlet-triplet ISC and in enhancing the 2PA cross section. This result reveals the greater promise for Pt(II) acetylides in chromophores for temporal and frequency agile nonlinear absorption.