Electron, Hole, Singlet, and Triplet Energy Transfer in Photoexcited Porphyrin-Naphthalenediimide Dyads Oleksandr Yushchenko, † Rahul V. Hangarge, ‡ Sandra Mosquera-Vazquez, † Sheshanath V. Boshale, § and Eric Vauthey* ,† † Department of Physical Chemistry, University of Geneva, 30 quai Ernest-Ansermet, CH-1211 Geneva, 4, Switzerland ‡ Department of Organic Chemistry, School of Chemical Sciences, North Maharashtra University, Jalgaon, 425 001 Maharashtra, India § School of Applied Sciences, RMIT University, GPO Box 2476, Melbourne, Victoria 3001, Australia * S Supporting Information ABSTRACT: The excited-state dynamics of two molecular dyads, consisting of zinc (1) and free-base (2) porphyrin connected via a peptide linker to a core- substituted naphthalenediimide (NDI) have been investigated using optical spectroscopy. These dyads exhibit rich photophysics because of the large number of electronic excited states below 3 eV. In the case of 1 in apolar solvents, excitation energy transfer from the vibrationally hot singlet excited porphyrin to the NDI takes place with a 500 fs time constant. Electronic energy ends up in the NDI-localized triplet state, which decays to the ground state on a microsecond timescale. In polar solvents, ground-state recovery is faster by 5 orders of magnitude because of the occurrence of charge separation followed by recombination. On the other hand, excitation energy transfer in 2 takes place in the opposite direction, namely from the NDI to the porphyrin, which then undergoes intersystem crossing to the triplet state, followed by triplet energy transfer back to the NDI. Therefore, four distinct local electronic excited states are consecutively populated after excitation of the NDI unit of 2, with the energy shuttling between the two ends of the dyad. ■ INTRODUCTION Light absorption, excitation energy transfer (EET) and charge separation (CS) are crucial steps in natural photosynthesis, 1−3 which have to be optimized when designing efficient synthetic analogues. 4−11 Harnessing these processes in multichromo- phoric systems is also of the utmost importance for development in photonics, sensing and other applications based on photoactive molecular systems. 12−19 Among the vast number of multichromophoric systems that have been reported so far, a large fraction are composed of identical chromophores arranged according to different motifs to favor excitation energy hopping while inhibiting aggregation and to act as antennae. Although most are based on porphyrins, 20−32 arrays with other chromophores, such as bodipy, 33−35 perylenediimides, 6,36−39 naphthalenediimides (NDI) 40−42 or triarylamines, 43 have also been reported. In polar environments, photoinduced symmetry-breaking CS between two identical units, usually perylenediimides and core-substituted NDIs, 44−47 can also take place, giving these arrays both antenna and reaction center functionality. A significant number of systems composed of different chromophores absorbing in the visible region have also been reported, 48−54 the main motivation being to broaden the absorption spectrum of the array while creating an energy gradient to funnel the optical excitation toward a trap connected, for example, to an electron donor or acceptor. 55−61 In such cases, the CS dynamics does not depend on the excitation wavelength, as the process usually takes place with the excitation on the same reaction partner. However, dyads where both the electron donor and the acceptor act as a chromophore are much less documented, and the excitation wavelength dependence (i.e., whether the donor or the acceptor is initially excited) of the CS dynamics has not often been investigated. 62−66 We have recently reported on the excited-state dynamics of a pentad consisting of a central NDI unit decorated at the core with four zinc (ZnP) or free-base porphyrins (FbP). 67 The CS dynamics in these arrays was found to depend on whether a porphyrin unit or the NDI core was initially excited. However, both pathways resulted in the same charge-separated state, and consequently, the ensuing charge recombination (CR) dynamics was independent of the excitation wavelength. An interesting feature of these arrays was that the lowest singlet excited-state was delocalized over the whole pentad. We report here on our investigation of the excited-state dynamics of two bichromophoric systems (Chart 1) consisting of either a ZnP or FbP unit covalently bound via a peptide Special Issue: John R. Miller and Marshall D. Newton Festschrift Received: October 30, 2014 Revised: November 22, 2014 Published: November 24, 2014 Article pubs.acs.org/JPCB © 2014 American Chemical Society 7308 DOI: 10.1021/jp5108685 J. Phys. Chem. B 2015, 119, 7308−7320