9 April 1999 Ž . Chemical Physics Letters 303 1999 261–267 Dynamic process in a hexameric benzo-porphyrin studied by femtosecond transient absorption G. Schweitzer a , G. De Belder a , L. Latterini a , Y. Karni a , A.E. Rowan b , R.J.M. Nolte b , F.C. De Schryver a, ) a ( ) Molecular Dynamics and Spectroscopy, Katholieke UniÕersiteit LeuÕen KUL , Celestijnenlaan 200F, 3001 HeÕerlee, Belgium b Department of Organic Chemistry, NSR Center UniÕersity of Nijmegen, ToernooiÕeld, 6525 ED, Nijmegen, The Netherlands Received 28 December 1998; in final form 9 February 1999 Abstract The fast kinetics of a hexameric benzo-porphyrin compound was investigated by femtosecond transient absorption spectroscopy. Using polychromatic detection, a sequence of spectra acquired at different delays revealed complex kinetic processes on the timescale of approximately 20 ps. The samples were then investigated at selected wavelengths by using a dual-colour femtosecond pump–probe setup. The same sequence of measurements was carried out under identical conditions on a benzo-porphyrin model sample. The comparison of the transient absorption decays recorded for both compounds revealed that only in the hexamer a 20 ps decay component is present which is attributed to an intramolecular interchromophoric excited-state process. q 1999 Elsevier Science B.V. All rights reserved. 1. Introduction Porphyrins and porphyrin-based systems have at- tracted considerable attention. In particular, por- phyrin assemblies have been prepared and investi- gated to understand better the activity of more com- w x plex biological systems 1–5 . It has recently been wx shown that the spectral properties 6 and the fluores- wx cence decay processes 7 of the porphyrin chro- mophore within a multi-chromophoric synthetic as- sembly are controlled by the spatial arrangement of the molecules. In this contribution, the dynamics of a ) Corresponding author. Fax: q32 16 32 7989; e-mail: frans.deschryver@chem.kuleuven.ac.be Ž hexameric porphyrin array BP6; see Fig. 1, insert . A in which each of the six porphyrin chromophores is covalently bound to a central benzene ring is wx reported. Previous NMR investigations 8 indicated the occurrence of interactions among the porphyrin moieties. The observation of a splitting of the Soret band at low temperatures for the hexamer and the wx absence thereof in the model compound 8 was indicative of an electronic interaction between the porphyrin chromophores. Comparison of the fluores- cence properties of the hexamer and the correspond- ing monomer in chloroform using single photon Ž . timing SPT detection with a time increment of 42 ps resulted in a decay time of 3.8 ns for BP6, which as the quantum yield of fluorescence is unchanged indicates an enhanced k for the hexamer. f 0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. Ž . PII: S0009-2614 99 00246-8