X‐ray Transient Absorption and Picosecond IR Spectroscopy of Fulvalene(tetracarbonyl)diruthenium on Photoexcitation

Abstract

In the burgeoning efforts of collecting solar energy for power generation, [1] the development of rechargeable solar thermal batteries containing photochromic molecules capable of reversible photoisomerization is receiving increasing scrutiny. [2] Among them, organometallic compounds show particular promise, because of their complementary potential for steric and electronic tunability. [3] In this regard, an intriguing system with which to illustrate the concept is the robust photothermal fulvalene (Fv) diruthenium couple 1Q2 (Scheme 1). [4] Initially assumed to occur by a concerted pathway, a recent study of the heat releasing step 2 a!1 a pinpointed a stepwise trajectory. Its salient features (Figure 1, black solid line) consist of a pre-equilibrium of 2 a (20.8 kcal mol À1 ) with anti biradical B (38.8 kcal mol À1 ) by initial cyclopentadienyl (Cp) coupling (transition state, TS, C, 43.2 kcal mol À1 ), subsequent rate-determining CpRu(CO) 2 rotation (TS A, 50.5 kcal mol À1 ), and Ru-Ru bond formation to give 1 a (0.0 kcal mol À1 ). [5] The relative difficulty of anti to syn biradical rotation and the ease with which B proceeds to 2 a (DH°= 4.4 kcal mol À1 ) prompted a reconsideration of the mechanism of the photostorage step. Originally, [4a] the normally expected Ru-Ru photodissociation [6] was discounted, because added CCl 4 (1m) had no effect on the outcome of the photorearrangement, leading again to the postulate of a concerted process. However, the effortless step B!2 a makes B a viable photointermediate, provided that it Scheme 1. Photoisomerization of fulvalene(tetracarbonyl)diruthenium at ! 350 nm and its thermal reversal. Irradiation 300 nm causes decarbonylation. [4a] [*] Dr.

In Silico Design of Dihydroazulene/Vinylheptafulvene Photoswitches for Solar‐Energy Storage Guided by an All‐Around Performance Descriptor**

“…Among them, perhaps the norbornadiene/quadricyclane system, in which the former species produces the latter, highly strained species through a [2+2] cycloaddition, has been developed the furthest [7–13] . Other systems include azobenzene, [14–18] dithienylethene, [19–22] and fulvalene ruthenium [23–26] compounds. More recently, 1,2‐azaborines have been brought into consideration for MOST applications, owing to their ability to photoisomerize into strained, Dewar‐like structures [27] …”

Switching from Ru to Fe: picosecond IR spectroscopic investigation of the potential of the (fulvalene)tetracarbonyldiiron frame for molecular solar-thermal storage

“…Here, 1 photobleaches in the sun (or, optimally, hn with l Z 400 nm), quantum yield = 0.15, in preparative runs quantitatively, to 2, from which it is recovered completely by thermal reversal or catalytically (1-10% AgNO 3 on silica) with the release of B20-23 kcal mol À1 of energy (energy density 0.2 MJ kg À1 ). [7][8][9][10] The bis(1,1-dimethyltridecyl) derivatives 1c,d have been implemented in a proof-of-principle solar storage-heat release device. 10 Both steps of the cycle are now well understood on the basis of experimental and DFT data.…”

“…The latter then rapidly (barrier C = 4.4 kcal mol À1 ) rearranges to 2a. 9 Conversely, the thermal release step is characterized by preequilibrium between 2a (20.8 kcal mol À1 ) and B, followed by rate determining rotation via the transition state (TS) A (50.5 kcal mol À1 ) to (re)generate 1a. 8 In envisaging practical applications of the solar-thermal couple 1 # 2, a major drawback is the requirement for ruthenium, a very rare element (abundance in the earth's crust B1 ppb), and hence expensive [Ru 3 (CO) 12 : B$100 per g].…”