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Dual Singlet Excited-State Quenching Mechanisms in an Artificial Caroteno-Phthalocyanine Light Harvesting Antenna
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DOI：10.1021/acsphyschemau.1c00008.png)
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Abstract
En 中文
Under excess illumination,photosystem II of plants dissipatesexcess energy through the quenching of chlorophyll fluorescence inthe light harvesting antenna. Various models involving chlorophyllquenching by carotenoids have been proposed, including (i) directenergy transfer from chlorophyll to the low-lying optically forbiddencarotenoid S-1 state, (ii) formation of a collective quenchedchlorophyll-carotenoid S-1 excitonic state, (iii)chlorophyll-carotenoid charge separation and recombination,and (iv) chlorophyll-chlorophyll charge separation and recombination.In previous work, the first three processes were mimicked in modelsystems: in a Zn-phthalocyanine-carotenoid dyad with an amidelinker, direct energy transfer was observed by femtosecond transientabsorption spectroscopy, whereas in a Zn-phthalocyanine-carotenoiddyad with an amine linker excitonic quenching was demonstrated. Here,we present a transient absorption spectroscopic study on a Zn-phthalocyanine-carotenoiddyad with a phenylene linker. We observe that two quenching phasesof the phthalocyanine excited state exist at 77 and 213 ps in additionto an unquenched phase at 2.7 ns. Within our instrument response of similar to 100 fs, carotenoid S-1 features rise which pointat an excitonic quenching mechanism. Strikingly, we observe an additionalrise of carotenoid S-1 features at 3.6 ps, which shows thata direct energy transfer mechanism in an inverted kinetics regimeis also in effect. We assign the 77 ps decay component to excitonicquenching and the 3.6 ps/213 ps rise and decay components to directenergy transfer. Our results indicate that dual quenching mechanismsmay be active in the same molecular system, in addition to an unquenchedfraction. Computational chemistry results indicate the presence ofmultiple conformers where one of the dihedral angles of the phenylenelinker assumes distinct values. We propose that the parallel quenchingpathways and the unquenched fraction result from such conformationalsubpopulations. Our results suggest that it is possible to switchbetween different regimes of quenching and nonquenching through aconformational change on the same molecule, offering insights intopotential mechanisms used in biological photosynthesis to adapt tolight intensity changes on fast time scales.
Keywords:
photosynthetic lightharvesting
artificial light harvestingdyad
energy transfer
excitonic coupling
nonphotochemical quenching
excess energy dissipation
carotenoid
phthalocyanine
optically forbiddenstate
ultrafast spectroscopy
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