Structural, Optical, and Photophysical Properties of Nickel(II) Alkylthioporphyrins: Insights from Experimental and DFT/TDDFT Studies Angela Rosa,* ,§ Giampaolo Ricciardi,* ,§ Evert Jan Baerends, ‡ Mikhail Zimin, ⊥ Michael A. J. Rodgers,* ,⊥ Shingo Matsumoto, † and Noboru Ono † Dipartimento di Chimica, UniVersita ` della Basilicata, Via N. Sauro 85, 85100 Potenza, Italy, Afdeling Theoretische Chemie, Vrije UniVersiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands, Center for Photochemical Sciences and Department of Chemistry, Bowling Green State UniVersity, Bowling Green, Ohio 43403, Department of Chemistry, Faculty of Science, Ehime UniVersity, Matsuyama 790 8577, Japan Received May 24, 2005 The ground- and excited-state properties of a Ni(II) porphyrin bearing peripheral alkylthio group, NiOMTP (OMTP ) 2,3,7,8,12,13,17,18-octakis methylthio porphyrinate) have been investigated by steady-state and time-resolved absorption spectrometry and DFT/TDDFT theoretical methods. Several conformations corresponding to different deformations of the porphyrin core and to different orientations of the alkylthio groups have been theoretically explored. The nearly degenerate, purely ruffled D 2d and hybrid (ruffled with a modest degree of saddling) D 2 conformations, both characterized by an up-down (ud) orientation of the vicinal methylthio groups are by far the preferred conformations in the “gas phase”. In contrast to NiOEP, it is the orientation of the peripheral substituents rather than the type and degree of distortions of the porphyrin core that determines the stability of the NiOMTP conformers. The ground-state electronic absorption spectra of NiOMTP exhibit significant changes compared to its parent NiP and -alkylated analogues, such as NiOEP, resulting in a considerable red shift of the B and the Q bands, intensification and broadening of the Q band, and additional weak absorptions in the region between the Q and B bands. These spectral changes can be understood in terms of the electronic effects of the methylthio groups with nonplanar distortions of the porphyrin ring playing a very minor role. Transient absorption measurements with sub-picosecond resolution performed in toluene and TDDFT calculations reveal that following photoexcitation, NiOMTP deactivates by the pathway 1 (π,π*) f 3 (d z 2 ,d x 2 -y 2 ) f ground state. The (d,d) state exhibits complex spectral evolution over ca. 8 ps, interpreted in terms of vibrational relaxation and cooling. The cold ligand-field excited state decays with a lifetime of 320 ps. At variance with the highly distorted nickel porphyrins but similar to the planar analogues, the (d,d) spectrum of NiOMTP has transient absorption bands immediately to the red of the bleaching of the ground-state Q and B bands. Introduction It has been well documented that the physical, chemical, and biological properties of porphyrins and metalloporphyrins can be fine-tuned or dramatically altered by introducing heteroatoms or heteroatom-based groups at the , meso, or both positions of the macrocycle. 1-16 For instance, the introduction of more than four halogens at the -pyrrole * To whom correspondence should be addressed. E-mail: rosa@unibas.it (A.R.); rg010sci@unibas.it (G.R.); rodgers@bgnet.bgsu.edu (M.A.J.R.). § Universita ` della Basilicata. ‡ Vrije Universiteit. ⊥ Bowling Green State University. † Ehime University. (1) Jaquinod, L. Heteroatom Porphyrins. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego, CA, 2000; Vol. I., p 201. (2) Kalish, H.; Camp, J. E.; Stepien, M.; Latos-Grazynski, L.; Olmstead, M. M.; Balch, A. L. Inorg. Chem. 2002, 41, 989. (3) Kalish, H.; Camp, J. E.; Stepien, M.; Latos-Grazynski, L.; Balch, A. L. J. Am. Chem. Soc. 2001, 123, 11719. (4) Sibilia, S. A.; Czernuszewicz, R. S.; Crossley, M. J.; Spiro, T. G. Inorg. Chem. 1997, 36, 6450. (5) Binstead, R. A.; Crossley, M. J.; Hush, N. S. Inorg. Chem. 1991, 30, 1259. Inorg. Chem. 2005, 44, 6609-6622 10.1021/ic050838t CCC: $30.25 © 2005 American Chemical Society Inorganic Chemistry, Vol. 44, No. 19, 2005 6609 Published on Web 08/24/2005