an-introduction-to-processing-fitting-interpreting-transient

An Introduction to Processing, Fitting, and Interpreting Transient Absorption Data

Transient absorption (TA) spectroscopy is a powerful time-resolved spectroscopic method used to track the evolution of excited-state processes through ...

Lehigh University

Pd(II) biladienes have been developed over the last five years as  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' sp-hybridized -carbon (, 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 spectroscopy revealed that higher-energy excitation ( ∼ 350-500 nm) led to an additional lifetime (, an extra photophysical process) compared to experiments carried out following excitation into the lowest-energy excited states ( = 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 μs, regardless of the excitation wavelength. However, when higher-energy light is used to excite the complexes, a new lifetime on the order of hundreds of ps is observed. The origin of the 'extra' lifetime observed upon higher energy excitation was revealed 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. The results of this work demonstrate that Pd(II) biladienes support a unique triplet photochemistry that may be exploited for development of new photochemical schemes and applications.

Elucidation of complex triplet excited state dynamics in Pd(II) biladiene tetrapyrroles.

In this work we untangle the ultrafast deactivation of high-energy excited states in four naphthalene-based azo dyes. Through systematic photophysical and computational study, we observed a structure-property relationship in which increasing the electron donating strength of the substituent leads to longer lived excited states in these organic dyes and faster thermal reversion from the  to  configuration. In particular, azo dyes 1-3 containing less electron donating substituents show three distinct excited-state lifetimes of ∼0.7-1.5 ps, ∼3-4 ps, and 20-40 ps whereas the most electron donating dimethyl amino substituted azo 4 shows excited-state lifetimes of 0.7 ps, 4.8 ps, 17.8 ps and 40 ps. While bulk photoisomerization of all four moieties is rapid, the  to  reversion lifetimes vary by a factor of 30 with  decreasing from 276 min to 8 min with increasing electron donating strength of the substituent. In order to rationalize this change in photophysical behavior, we explored the excited-state potential energy surfaces and spin-orbit coupling constants for azo 1-4 through density functional theory. The increase in excited-state lifetime for 4 can be attributed to geometric and electronic degrees of freedom of the lowest energy singlet excited-state potential energy surface.

Article	Year
Elucidation of complex triplet excited state dynamics in Pd(II) biladiene tetrapyrroles. Physical chemistry chemical physics : PCCP\| PubMed ID: 36594369	2023
Ultra-fast excited-state dynamics of substituted -naphthalene azo moieties. Physical chemistry chemical physics : PCCP\| PubMed ID: 37222191	2023

Robert Hamburger

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