Spectroscopic Structure-Property Relationships of a Series of Polyaromatic Platinum Acetylides

To develop a structure-spectroscopic property relationship in platinum acetylides having poly(aromatic hydrocarbon) ligands, we synthesized a series of chromophores with systematic variation in the number of fused aromatic rings (nFAR) and ligand topology (polyacene (L), polyphenanthrene (Z), or compact(C)). We measured ground-state absorption, fluorescence, and phosphorescence spectra. We also performed nanosecond and femtosecond transient absorption experiments. To extend the range of compounds in the structure-property relationship, we did DFT calculations on an expanded series of chromophores. Both the results and experiments show that the S-1 and T-1 state energies are a function of both nFAR and the ligand topology. In the L chromophores, the S-1 and T-1 state energies decrease linearly with nFAR. In contrast, the S-1 and T-1 state energies of the Z chromophores oscillate around a fixed value with increasing nFAR The C chromophores have behavior intermediate between the and Z chromophores. A parallel series of calculations on the ligands shows the same behavior. The S-1-S-n energy obtained from ultrafast time resolved spectra has a linear variation in nFAR. The rate constant for nonradiative decay, k(nr), was calculated from the S-1 state lifetime and decreases with an increasing number of pi electrons in the aromatic ring. The result is consistent with the spin-orbit coupling caused by the central platinum heavy atom decreasing with larger nFAR. The present work shows that the framework developed for the analysis of poly(aromatic hydrocarbon) properties is useful for the understanding of the corresponding platinum acetylide complexes.