propanone singlet excited state was detected on the reaction path to the metallaketene species. This metallacyclopropanone excited state species has a lifetime of less than 100 ps and a characteristic bridging carbonyl band at 1770 cm −1 . The tungsten ketene species was also detected but in contrast to the chromium system, this forms directly from a low-lying triplet excited state. The electrochemical release of CO showed a greater efficiency for the chromium complex when compared to the tungsten.

Multimodal photo, thermal and electrochemical approaches toward CO release from the amino carbene complex [(CO) 5 CrC(NC 4 H 8 )CH 3 ] is reported. Picosecond time resolved infrared spectroscopy was used to probe the photo-induced early state dynamics leading to CO release, and DFT calculations confirmed that CO release occurs from a singlet excited state.CO releasing molecules (CORMs) are an area of current interest due to the physiological role of CO in the body. 1,2 CO is endogenously generated during oxidative heme degradation by the heme oxygenase (HO) enzymes. 3 The potential therapeutic applications of CO include vasodilation, anti-inflammatory and anti-proliferative effects. CORMs have been developed to deliver CO in a controlled manner. However, there is a major challenge in controlled CO release, be it photochemically, thermally or enzymatically induced. It is well known that chromium carbene complexes can release CO both thermally and photochemically, as the resulting tetracarbonyl reactive intermediates have been used as synthons in organic chemistry. 4 Recently, Lynam et al. reported the suitability of heteroatom (amino, methoxy and thio) stabilised chromium carbenes as thermal CORMs. 5 Rapid CO release was observed for the thio-and methoxy carbenes (t 1/2 = 313 s and 306 s, respectively, for a 60 mM solution of the complex to achieve an Mb-CO concentration of 30 mM), compared to slow release in the case of the amino analogue which required more than 2 h for a 60 mM solution of the complex to form 10 mM of carboxy-myoglobin (Mb-CO). In addition the chromium carbonyls investigated, demonstrated the fastest and most regulated CO release characteristics compared to the molybdenum or tungsten complexes studied. 5 In this communication we set out to compare the rate of CO release from [(CO) 5 CrC(NC 4 H 8 )CH 3 ] using photochemical, thermal and electrochemical stimulation. To the best of our knowledge this is first example of a CORM involving electrochemical stimulation. In the present study we report our findings on a ps-time resolved infrared (ps-TRIR) study on [(CO) 5 CrC(NC 4 H 8 )CH 3 ] in conjunction with DFT calculations to investigate the excited state responsible for the CO release. Chromium Fischer carbenes have received significant attention as photochemical synthons for a range of organic compounds. Photolysis of [(CO) 5 CrC(XR)R 0 ] in the presence of imines, olefins, aldehydes, or alcohols are known to yield b-lactams, cyclobutanones, b-lactones or amino esters respectively. 6 The proposed mechanism for these reactions involves a metal-toligand charge transfer photoexcitation resulting in the formation of a ketene intermediate which is formed by the insertion of a cis-CO ligand into the adjacent Cr-carbene bond (Fig. 1). Experimental and theoretical calculations reported by Sierra et al. support the proposal that this occurs through a MLCT triplet excited state. 7 Although a number of low temperature and time-resolved studies have investigated photo-induced CO release from chromium carbene...

The photochemistry and photophysics of three model "half-sandwich" complexes (η(6)-benzophenone)Cr(CO)3, (η(6)-styrene)Cr(CO)3, and (η(6)-allylbenzene)Cr(CO)3 were investigated using pico-second time-resolved infrared spectroscopy and time-dependent density functional theory methods. The (η(6)-benzophenone)Cr(CO)3 complex was studied using two excitation wavelengths (470 and 320 nm) while the remaining complexes were irradiated using 400 nm light. Two independent excited states were detected spectroscopically for each complex, one an unreactive excited state of metal-to-arene charge-transfer character and the other with metal-to-carbonyl charge transfer character. This second excited state leads to an arrested release of CO on the pico-second time-scale. Low-energy excitation (470 nm) of (η(6)-benzophenone)Cr(CO)3 populated only the unreactive excited state which simply relaxes to the parent complex. Higher energy irradiation (320 nm) induced CO-loss. Irradiation of (η(6)-styrene)Cr(CO)3, or (η(6)-allylbenzene)Cr(CO)3 at 400 nm provided evidence for the simultaneous population of both the reactive and unreactive excited states. The efficiency at which the unreactive excited state is populated depends on the degree of conjugation of the substituent with the arene π-system and this affects the efficiency of the CO-loss process. The quantum yield of CO-loss is 0.50 for (η(6)-allylbenzene)Cr(CO)3 and 0.43 for (η(6)-styrene)Cr(CO)3. These studies provide evidence for the existence of two photophysical routes to CO loss, a minor ultrafast route and an arrested mechanism involving the intermediate population of a reactive excited state. This reactive excited state either relaxes to reform the parent species or eject CO. Thus the quantum yield of the CO-loss is strongly dependent on the excitation wavelength. Time-dependent density functional theory calculations confirm that the state responsible for ultrafast CO-loss has significant metal-centred character while the reactive state responsible for the arrested CO-loss has significant metal-to-carbonyl charge-transfer character. The CO-loss product (η(6)-allylbenzene)Cr(CO)2 formed following irradiation of (η(6)-allylbenzene)Cr(CO)3 reacts further with the pendent alkenyl group to form the chelate product (η(6),η(2)-allylbenzene)Cr(CO)2.

Photo-activated CO-release in the amino tungsten Fischer carbene complex, [(CO)5WC(NC4H8)Me], picosecond time resolved infrared spectroscopy, timedependent density functional theory, and an antimicrobial study