Ultrafast Excited-State Dynamics and Inter-Chromophore Interactions in Self-Assembled Coordination Complexes

Chromophore-decorated self-assembled coordination complexes (SCCs) potentially
provide a synthetic analogue to the organization of pigments in the light harvesting
antenna systems of photosynthetic organisms. A clear understanding of the impact of
coordination on inter-chromophore interactions, as well as of the energy and
exciton/charge transfer mechanisms between decorating chromophores and their
interaction with the metal ions at the coordination points, is needed for further
development of artificial photosynthetic systems. To shed more light on these
interactions [1, 2] we performed a series of ultrafast optical spectroscopy experiments
on spherical [M12L24] SCCs decorated with different ratios of BODIPY and anthracene
moieties in the exo-positions.

Time-resolved photoluminescence experiments showed a slowdown of the BODIPY
emission upon increasing anthracene/BODIPY ration when exciting the anthracene
chromophore, demonstrating a chromophore-chromophore interaction leading to a
unidirectional energy transfer from the anthracene to the BODIPY chromophores.
Transient absorption experiments on SSCs either fully decorated with BODIPY or
with anthracene, evidenced novel optically induced transitions substantially below the
energy of the first excited singlet states of the corresponding chromophores. Since
this type of excited state dynamics is not observed for free-floating chromophore-
functionalized ligands, this observation indicates strong interactions between the
BODIPY/anthracene chromophores and the coordinating metal ions.

The ultrafast experiments show that the coordination complex serves as an effective
scaffold to mediate inter-chromophore interactions. At the same time, they show that
the coordination complex is more than just a passive structure and plays an active
role in the photophysics of the material. The present work provides initial steps to
obtain an understanding of the photophysics of chromophore-decorated spherical
SCCs, which contribute to the overall goal to realize artificial photosynthetic systems.

[1] A. J. Musser, P. P. Neelakandan, J. M. Richter, H. Mori, R. H. Friend, and J. R.
Nitschke, Exciton Energy Delocalization and Transfer to Guests within MII4L6 Cage
Frameworks. Journal of the American Chemical Society 2017 139(34), 12050-12059,
DOI: 10.1021/jacs.7b06709

[2] E. Maligaspe, T. Kumpulainen, H. Lemmetyinen, N. V. Tkachenko, N. K.
Subbaiyan, M. E. Zandler, and F. D'Souza, Ultrafast Singlet−Singlet Energy Transfer
in Self-Assembled via Metal−Ligand Axial Coordination of Free-Base Porphyrin−Zinc
Phthalocyanine and Free-Base Porphyrin−Zinc Naphthalocyanine Dyads. The
Journal of Physical Chemistry A 2010 114(1), 268-277, DOI: 10.1021/jp908115e