1060
J. Phys. Chem. C 2009, 113, 1060–1066
Ultrafast Intersystem Crossing: Excited State Dynamics of Platinum Acetylide Complexes Guda Ramakrishna,†,§ Theodore Goodson III,*,† Joy E. Rogers-Haley,‡,⊥ Thomas M. Cooper,‡,| Daniel G. McLean,‡,# and Augustine Urbas‡ Department of Chemistry, UniVersity of Michigan, Ann Arbor Michigan 48109, and Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio 45433 ReceiVed: August 11, 2008; ReVised Manuscript ReceiVed: NoVember 23, 2008
Femtosecond time-resolved fluorescence and transient absorption measurements have been carried out on a series of platinum acetylide complexes to unravel the dynamics of intersystem crossing and the formation of triplet states in real time as a function of chain length. Ultrafast inter system crossing with a time constant less than 100 fs has been observed for the case of short chain length platinum acetylide complex and this time constant increases with increasing the chain length. Apart from the singlet to triplet intersystem crossing, additional triplet state relaxation has also been observed which happens in picosecond time scale. Introduction There has been considerable interest in the synthesis, spectroscopy, nonlinear optics and structure-property relationships of platinum acetylides.1-8 They are exceptional systems for investigating excited-state phenomena like excited-state absorption (ESA), intersystem crossing (ISC), triplet state absorption spectrum (TTA), two photon absorption,9 excimers,10 and phosphorescence. At the Air Force Research Laboratory, we have been investigating the relation between chemical structure and spectroscopic properties of platinum acetylide complexes having the molecular formula trans-Pt(PBu3)2L2. The ligand L is composed of a variable number of phenyl acetylene groups. Compounds with the ligand L ) H-(C6H4-CtC)n-H, n ) 1-3, named PE1-Pt, PE2-Pt, and PE3-Pt (Figure 1) have been studied in considerable detail. In all these articles, analysis of the dependence of singlet and triplet state energies on chromophore length gives evidence that the singlet exciton is delocalized through the central platinum, while the triplet exciton is confined to one ligand. Upon excitation into the singlet excited state, intersystem crossing with near unit quantum yield occurs to the triplet state. As shown in Scheme 1, the ground-state chromophores have formal D2h symmetry, so excitation to the S1 state occurs with conversion from g to u symmetry. Through conformation changes along the molecular axis, the centrosymmetric S1 state converts to the triplet state having lower C2V symmetry. Symmetry breaking could occur either while the molecule is still in the S1 state, during the intersystem crossing process or during thermal relaxation of the T1 state. Platinum acetylides can be viewed as two phenyl acetylene branches linked by a platinum center. Strong intramolecular interaction between the branches affects the localization and delocalization of both the singlet and triplet excitons. * To whom correspondence should be addressed. E-mail: tgoodson@ umich.edu. † Department of Chemistry, University of Michigan. ‡ Materials and Manufacturing Directorate, Air Force Research Laboratory. § Present address, Department of Chemistry, Western Michigan University, College of Arts and Sciences 3425 Wood Hall Western Michigan University, Kalamazoo, Michigan 49008-3842. ⊥ Present address: UES, Inc., Dayton, OH 45432. | E-mail:
[email protected]. # Present address: SAIC, Dayton, OH 45434.
We use ultrafast time-resolved fluorescence (upconversion) and absorption (pump-probe) measurements to probe the kinetics of the fast energy redistribution processes that occur in branched (and related) macromolecular structures.11-19 We applied these techniques to distinguish between the possible symmetry breaking mechanisms in platinum acetylides. We performed femtosecond time-resolved fluorescence and transient absorption measurements on three platinum acetylides PE1, PE2 and PE3. We find that intersystem crossing takes place on a femtosecond to low picosecond time scale and most of the changes in the excited-state spectra result from picosecond time scale relaxation of the triplet state. Experimental Section a. Materials. Synthesis of investigated platinum acetylide complexes has been described previously.1 All the measurements were carried out in tetrahydrofuran (THF) (Aldrich) unless stated otherwise. Typical concentrations used in time-resolved investigations were in the range 50-200 µM for different samples. b. Femtosecond Transient Absorption Measurements. Transient absorption was used to investigate the excited-state dynamics of the platinum acetylides at different excitation wavelengths and the description of the system has been provided elsewhere.12,20 Briefly, the output of amplified laser beam was split to generate pump and probe beam pulses with a beam splitter (85% and 15%). The pump beam was produced by optical parametric amplifier (OPA-800c). The pump beams used in the present investigation (340 nm for PE1-Pt, 365 nm for PE2-Pt, and 380 nm for PE3-Pt) were obtained from the fourth harmonic of the signal beam and was focused onto the sample cuvette. The probe beam was delayed with a computer controlled motion controller and then focused into a 2 mm sapphire plate to generate white light continuum. The white light was then overlapped with the pump beam in a 2 mm quartz cuvette containing the sample and the change in absorbance for the signal was collected by a CCD detector (Ocean optics). Data acquisition was controlled by the software from Ultrafast systems inc. Typical power of probe beam was
