1 The photochemistry of rhenium(I) tricarbonyl N-heterocyclic carbene complexes Jamila G. Vaughan, a Brodie L. Reid, a Sushil Ramchandani, a Phillip J. Wright, a Sara Muzzioli, b Brian W. Skelton, c Paolo Raiteri, a David H. Brown, a, * Stefano Stagni, b, * Massimiliano Massi a, * a Department of Chemistry – Curtin University, Kent Street, Bentley 6102 WA, Australia. b Department of Industrial Chemistry “Toso Montanari” – University of Bologna, viale del Risorgimento 4, Bologna 40126, Italy. c Centre for Microscopy, Characterisation and Analysis, University of Western Australia, Crawley 6009 WA, Australia. Corresponding Authors *E-mail: m.massi@curtin.edu.au, stefano.stagni@unibo.it , d.h.brown@curtin.edu.au Abstract The photophysical and photochemical properties of the new tricarbonyl rhenium(I) complexes bound to N-heterocyclic carbene ligands (NHC), fac-[Re(CO) 3 (N^C)X] (N^C = 1-phenyl-3-(2-pyridyl)imidazole or 1-quinolinyl-3-(2-pyridyl)imidazole; X = Cl or Br), are reported. The photophysics of these complexes highlight phosphorescent emission from triplet metal-to-ligand ( 3 MLCT) excited states, typical of tricarbonyl rhenium(I) complexes, with the pyridyl-bound species displaying a ten-fold shorter excited state lifetime attributed to thermally accessible quenching excited states. On the other hand, these pyridyl-bound species display solvent-dependent photochemical CO dissociation following what appear to be two different mechanisms, with a key step being the formation of cationic tricarbonyl solvato- complexes, being themselves photochemically active. The photochemical mechanisms are illustrated with a combination of NMR, IR, UV-Vis, emission and X-ray structural characterization techniques, clearly demonstrating that the presence of the NHC ligand is responsible for the previously unobserved photochemical behavior in other photoactive tricarbonyl rhenium(I) species. The complexes bound to the quinolinyl-NHC ligand (which possess a lower-energy 3 MLCT) are photostable, suggesting that the photoreactive excited state is not any longer thermally accessible. The photochemistry of the pyridyl complexes was investigated in acetonitrile solutions and also in the presence of triethylphosphite, showing a