Tailoring the Intersystem Crossing and Triplet Dynamics of Free-Base Octaalkyl-β-oxo-Substituted Porphyrins: Competing Effects of Spin–Vibronic and NH Tautomerism Relaxation Channels

Abstract

We demonstrate that β-oxo-substitution provides effective fine-tuning of both steady-state and transient electronic properties of octaalkyl-β-mono-oxochlorin and all isomers of the β,β′-dioxo-substituted chromophores. The addition of a carbonyl group increases the Q y oscillator strength and red-shifts the absorption spectra. Each oxo-substitution results in a 2-fold increase in the singlet to triplet state intersystem crossing (ISC) rates, resulting in a 4-fold ISC rate increase for the dioxo-substituted chromophores. The effects of oxo-substitution on the ISC rate are thus additive. The progressive increase in the ISC rates correlates directly with the spin–vibronic channels provided by the C=O out-of-plane distortion modes, as evidenced by density functional theory (DFT) modeling. The triplet states, however, were not evenly affected by β-oxo-substitution, and reduction in the triplet lifetime seems to be influenced instead by the presence of NH tautomers in the dioxoisobacteriochlorins.

Photophysical Exploration of Two Isomers of Octaethyltrioxopyrrocorphin

“…For both compounds, TA spectra consist of the ground-state bleach (GSB) peaks superimposed on the broad positive excited-state absorption (ESA) bands. The TA signals of both isomers show similar features to those of the octaalkyl-β,β′-dioxochlorins studied previously . In case of triketone 3-O 2,7,18 , singular value decomposition (SVD) of the fs–ns kinetics (Figure b,c) reveals time constants of 2.4 ± 0.1 ns and a nondecaying component, which coincide with the 2.6 ± 0.1 ns and 66 ± 1 μs components extracted from the ns–ms probe kinetics (Figure e,f).…”

“…This shift of the transient bleach signal, which was not captured by the SVD analysis, is representative of vibrational cooling via solvent coupling. Similar shifts were previously monitored for OEP, chlorin, and dioxochlorins . It is also interesting to note that the negative spectral feature around 600 nm in Figure a, which appears “instantaneously” (i.e., during the excitation pulse duration), does not evolve in the ps range.…”

“…The emissions decay monoexponentially with lifetimes of 3.4 ± 0.1 and 2.7 ± 0.2 ns for 3-O 2,7,12 and 3-O 2,7,18 , respectively. Following the cascaded electronic relaxation scheme of free base porphyrins, chlorins, and dioxochlorins, ,, these decay times can be attributed to the singlet lifetime of S 1 (Q x ). We previously found the emission lifetimes to decrease by half with each oxo-substitution, from 11 ns for the parent OEP , to 5.5 ns for the monooxochlorin, to 1.5–3.3 ns for the dioxochlorins .…”

“…Nanosecond to millisecond TA spectroscopy has been performed in a home-built setup in the same location. Details of the methods were reported earlier . A full description of the methods has been provided in the Supporting Information.…”

“…An alternative way to modify the porphyrinic cross-conjugated double bonds is to introduce β-oxo-functionalities into β-alkylporphyrins. One simple method is their treatment with hydrogen peroxide when dissolved in concentrated sulfuric acid. − This classic, 90-year-old reaction, when applied to octaethylporphyrin ( OEP ), generates one β-oxochlorin, five isomers of the corresponding β,β’-dioxo-chromophores, as well as two out of the four possible β,β′,β″-trioxopyrrocorphins ( 3-O 2,7,12 and 3-O 2,7,18 ), in addition to other products. − The chemical, coordination, and physical properties of β-oxochlorin have long been investigated. ,− More recently, the chemistry and photophysical properties of the dioxo-derivatives were the subject of several studies, revealing the large impact of the β-oxo-substituents on the electronic properties of the porphyrinic chromophore. ,− Remarkable regiochemical influences were revealed in porphyrinic chromophores carrying multiple β-oxo-substituents, ,,,, including the meso -arylporphodilactones. − …”