Elucidation of Complex Triplet Excited State Dynamics in Pd(II) Biladiene Tetrapyrroles

Pd(II) biladienes have been developed over the last five years as non-aromatic oligotetrapyrrole complexes that support a rich triplet photochemistry. In this work, we have undertaken the first detailed photophysical interrogation of three homologous Pd(II) biladienes bearing different combinations of methyl- and phenyl-substituents on the frameworks’ sp3-hybridized meso-carbon (i.e., the 10-position of the biladiene framework). These experiments have revealed unexpected excited-state dynamics that are dependent on the wavelength of light used to excite the biladiene. More specifically, transient absorption spectrosco-py revealed that higher-energy excitations (exc ~ 350-500 nm) led to an additional lifetime (i.e., an extra photophysical pro-cess) compared to experiments carried out following excitation into the lowest-energy excited states (exc = 550 nm). Each Pd(II) biladiene complex displayed an intersystem crossing lifetime on the order of tens of ps and a triplet lifetime of ~20 s, regardless of the excitation wavelength. However, when higher-energy light is used to excite the complexes, a new life-time on the order of hundreds of ps is observed. The origin of the ‘extra’ lifetime observed upon higher energy excitation of the Pd(II) biladiene complexes was revealed by detailed computational modeling using density functional theory (DFT) and time-dependent DFT (TDDFT). These efforts demonstrated that excitation into higher-energy metal-mixed-charge-transfer excited states with high spin-orbit coupling to higher energy metal-mixed-charge-transfer triplet states leads to the additional excitation deactivation pathway. Importantly, time-resolved spectroscopy and electronic structure calculations carried out for the analogous aromatic Pd(II) meso-tetrakis(pentafluorophenyl)porphyrin (Pd[TPFPP]) demonstrated this traditional tetrapyrrole does not display the excitation-wavelength dependent photophysics observed for the Pd(II) biladienes. These experiments confirm that the unusual photophysics we observe are unique to low-symmetry biladienes and do not apply to more well-studied porphyrinoids. The results of this work demonstrate that Pd(II) biladienes support a unique triplet photo-chemistry that may be exploited for development of new photochemical schemes and applications.