PAPER www.rsc.org/pps | Photochemical & Photobiological Sciences Solvent dependent photophysics of fac-[Re(CO) 3 (11,12-X 2 dppz)(py)] + (X = H, F or Me) Joanne Dyer, a Caitriona M. Creely, b J. Carlos Penedo, b David C. Grills, a Sarah Hudson, b Pavel Matousek, c Anthony W. Parker, c Michael Towrie, c John M. Kelly* b and Michael W. George* a Received 21st December 2006, Accepted 2nd April 2007 First published as an Advance Article on the web 8th May 2007 DOI: 10.1039/b618651c The photophysical properties of [Re(CO) 3 (dppz)(py)] + (dppz = dipyrido-[3,2-a:2 ′ ,3 ′ -c] phenazine) (1) and its 11,12 substituted derivatives [Re(CO) 3 (dppzMe 2 )(py)] + (2) and [Re(CO) 3 (dppzF 2 )(py)] + (3) have been examined in organic and aqueous environments using phosphorescence and picosecond transient visible and infrared absorption spectroscopic methods. The roles of the intraligand IL(p–p*) and metal-to-ligand charge transfer MLCT(phz) excited states are evaluated and used to explain the major effect of difluoro-substitution, which is particularly remarkable in water, where the excited state of [Re(CO) 3 (dppzF 2 )(py)] + (3) is strongly quenched. Introduction It has been amply demonstrated over the last few years that the photophysical and photochemical properties of metal polypyridyl complexes make them suitable for a wide range of applica- tions, ranging from energy conversion and storage 1 to optoelec- tronic devices 2 and sensors. 3 Complexes containing dipyrido[3,2- a:2 ′ ,3 ′ -c]phenazine (dppz) and its derivatives have been widely studied particularly because of their interesting applications with DNA. The discovery that the excited state properties of [Ru(diimine) 2 (dppz)] 2+ (diimine = phen (1,10-phenanthroline) or bpy (2,2 ′ -bipyridyl)) are very different in water than in an organic environment (such as when intercalated into DNA) has stimu- lated considerable interest in understanding the water-induced switching of the excited state properties. 4 The dppz ligand can be considered as a fusion of two ring systems: a phenanthroline (phen) system and a phenazine (phz) system. 5 The current model, based on a study of the temperature-dependence of the emission, holds that there is a bright state (with the electron based on the bpy or phen moiety of the dppz ligand) and a dark state (where the electron is located mainly on the phenazine portion of the ligand). 6,7 The importance of entropic factors due to the H- bonding of water to the phenazine-N atoms has been emphasised. Evidence for the possible roles for intra-ligand p–p* (dppz) states (IL(p–p*)) has also been given from theoretical support. 8 The properties of related dppz complexes containing the Re(CO) 3 moiety have attracted much less attention than the analogous Ru and Os compounds. The vibrational signatures of fac-[Re(CO) 3 (dppz)(PPh 3 )] + (Ph = phenyl) were reported 9 and the IR spectrum of the excited states of these and related complexes showed that the lowest-lying excited state is IL(p–p*) in character. [Re(CO) 3 (dppz)(py)] + has been shown to interact with DNA and a School of Chemistry, University of Nottingham, University Park, Notting- ham, UK NG7 2RD b School of Chemistry and School of Physics, University of Dublin, Trinity College, Dublin 2, Ireland c Central Laser Facility, CCLRC Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, UK OX11 0QX. E-mail: Mike.George@nottingham. ac.uk; Fax: +44 (0)115-9513563; Tel: +44(0)115-9513512 photophysical investigations led to the assignment of the lowest- lying excited state as being IL(p–p*) in nature. 10,11 To elucidate the nature and dynamics of the excited state it is necessary to carry out ultrafast time-resolved spectroscopic studies. We have already performed a detailed investigation 12 of the photophysical properties of fac-[Re(CO) 3 (dppz)(py)] + (1) using both picosecond and nanosecond time-resolved visible and infrared absorption and resonance Raman spectroscopy where it was found that a IL(p–p*) state was initially formed followed by partial decay to a relaxed equilibrium of 3 IL(p–p*) and 3 MLCT(phz) excited states which then decayed with the same rate. The time-resolved IR measurements were particularly useful in distinguishing between MLCT states involving either phen or phenazine based orbitals since the m(CO) band positions are diagnostic of each state. Thus, the IR spectrum of the phz-based MLCT states showed these bands were shifted to higher wavenumber relative to the phen- based states reflecting the lower electron density on the metal centre in these states. Time resolved resonance Raman measurements have been used to probe fac-[Re(CO) 3 (dppz-X,Y)Cl] (dppz-X,Y = 11-X,12-Y- dppz with X,Y = Me, Me, 13 Br, H, 14 CO 2 Et, H 14 ) and have shown that the nature of the lowest excited state can vary depending upon X and Y. The emission spectra of fac-[Re(CO) 3 (dppz-X 2 )(L)] 0,+ (L = Cl, 4-ethylpyridine (4-Etpy), 4,4 ′ -bipyridine (4,4 ′ -bpy); with the dppz-X 2 = 11,12-dppz-X 2 with X = CH 3 or Cl) have been studied both in dichloroethane solution at room temperature and in a MeTHF glass. 15 From the emission spectra and room temper- ature lifetime measurements for the Etpy and 4,4 ′ -bpy complexes it was concluded that their emitting states are dppz-localized, while the appearance of broad structureless emission at room tempera- ture and greatly shortened lifetimes in fac-[Re(CO) 3 (dppz-X 2 )Cl], This journal is © The Royal Society of Chemistry and Owner Societies 2007 Photochem. Photobiol. Sci., 2007, 6, 741–748 | 741