Photoinduced Electron Transfer in Bisporphyrin ± Diimide Complexes Lucia Flamigni,* [a] Martin R. Johnston,* [b, c] and Lingamallu Giribabu [b] Abstract: The bisporphyrin host ZnH was synthesized, and its complexation with two aromatic diimide guest mole- cules, bis(pyridyl)naphthalenediimide NIN and bis(pyridyl)phenyldiimide PIN, was investigated by 1 H NMR and UV/Vis spectroscopy. The diimide guests were complexed simultaneously withbothmetalloporphyrinsofthehost, with association constants on the order of10 8 m 1 .Theprocessesoccurringinthe complex after excitation of the porphy- rinic host were studied by steady-state and time-resolved emission and transi- ent absorption spectroscopy. Complex- ationaltersthephotophysicalproperties of the host ZnH; the luminescence bands shift to the red by 30 nm in the complexed forms, while the emission quantumyieldandthelifetimedecrease. Comparisonofacomplexbetween ZnH and a model guest unable to undergo photoinduced processes allowed us to establishthat,inthediimidecomplexes, quenching of the porphyrinic lumines- cenceoccurswitharateof1.1  10 10 s 1 . The process is identified as an electron transfer from the excited singlet of the porphyrinic host to the imide guest, which yields charge-separated states withalifetimeof710psfor ZnH  -NIN  and260psfor ZnH  -PIN  . Keywords: electron transfer ¥ host±guest systems ¥ N ligands ¥ photochemistry ¥ porphyrinoids Introduction In the last decades, the construction of artificial porphyrinic arrays for solar energy conversion by using noncovalent bonds [1] has paralleled that based on covalent linkages. [2] However,theformerapproach,whichmorecloselyresembles the natural photosynthetic apparatus, in which the photo- active components are positioned by a protein matrix by means of a combination of interactions, has the inconven- iences of low association tendency and poor control over geometry. In this respect, a great improvement has been the introductionofacooperativestrategyinwhichmorethanone noncovalent interaction gives control over the positioning of chromophores and dramatically increases the association tendency of the components. [3] An advantage in using non- covalently assembled structures is the simple and fast interchange of components within the system; this does not require the lengthy synthetic schemes that the covalent approachdoes. A further advantage of noncovalently linked systems for conversion of light energy to chemical energy is the contri- bution to the photoinduced separation of charges over long distances, which could be provided by movements in the weakly bound noncovalent array. In rigid, covalently linked systems,long-livedcharge-separatedstatesareproducedbya sequenceofvectorialelectrontransfersthattaketheelectron to an acceptor distant from the donor. A very demanding designandsynthesisofcovalentlyassembledtriads,tetrads,or even pentads bearing the donor and acceptor at the extrem- ities of long arrays is often necessary to optimize the performance of these systems. [2] When the structure is held together by noncovalent interactions, a further possibility is open in addition to the multistep electron transfer outlined above.Theintrinsiclabilityofbondscaninprincipleallow,in the presence of a photochemically induced modification that weakens the interaction, the escape [4] or the rearrangement [5] (e.g., rotation or translation) of a component that could greatly contribute to the spatial separation of charges. Great efforts in the design, synthesis, and photophysical character- ization are still needed to achieve such a goal, but this possibilitymakesnoncovalentlyboundarraysverypromising for the construction of structures able to perform efficient charge separation. The systems presented here are based on the axial coordination of the two pyridyl residues of the aromatic [a] Dr. L. Flamigni Istituto ISOF-CNR ViaP.Gobetti101 40129 Bologna (Italy) Fax:( 39)051-639-9844 E-mail:flamigni@frae.bo.cnr.it [b] Dr.M.R.Johnston,Dr.L.Giribabu Center for Molecular Architecture Central Queensland University Rockhampton, Queensland 4702 (Australia) [c] Dr.M.R.Johnston Current address: School of Chemistry, Physics and Earth Sciences FlindersUniversity,SouthAustralia5042(Australia) Fax:( 61)8-8201-2905 E-mail:martin.johnston@flinders.edu.au FULL PAPER ¹ 2002 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim 0947-6539/02/0817-3938 $ 20.00+.50/0 Chem.Eur.J. 2002, 8,No.17 3938