Twisted Carotenoids Do Not Support Efficient Intramolecular Singlet Fission in the Orange Carotenoid Protein

Singlet exciton fission is the spin-allowed generationof two tripletelectronic excited states from a singlet state. Intramolecular singletfission has been suggested to occur on individual carotenoid moleculeswithin protein complexes provided that the conjugated backbone istwisted out of plane. However, this hypothesis has been forwardedonly in protein complexes containing multiple carotenoids and bacteriochlorophyllsin close contact. To test the hypothesis on twisted carotenoids ina "minimal" one-carotenoid system, we study the orangecarotenoid protein (OCP). OCP exists in two forms: in its orange form(OCPo), the single bound carotenoid is twisted, whereas in its redform (OCPr), the carotenoid is planar. To enable room-temperaturespectroscopy on canthaxanthin-binding OCPo and OCPr without laser-inducedphotoconversion, we trap them in a trehalose glass. Using transientabsorption spectroscopy, we show that there is no evidence of long-livedtriplet generation through intramolecular singlet fission despitethe canthaxanthin twist in OCPo.