Cascades of energy and electron transfer in a panchromatic absorber

The investigation of molecular model systems is fundamental towards a deeper understanding of key photochemical steps in natural photosynthesis. Herein, we report an entirely non-covalent triad consisting of boron dipyrromethene (BDP), porphyrin (ZnP), and fullerene (C60). Non-covalent binding motifs such as an amidinium-carboxylate salt bridge as well as axial pyridyl-metal coordination offer substantial electronic coupling and establish efficient pathways for photoactivated energy and electron transfer processes along a well-tuned gradient. Experimental findings from steady-state and time-resolved spectroscopic assays, as well as (spectro-)electrochemical measurements corroborate the formation of BDP|ZnP|C60 in solution, on one hand, and significant communication in the excited states, on the other hand. BDP acts as an energy harvesting antenna towards ZnP, which eventually undergoes charge separation with C60 by electron transfer from ZnP to C60. Notably, full spectral deconvolution of the transient species was achieved, supporting the successful self-assembly as well as giving a clear view onto the occurring photophysical processes and their spectral footprints upon photoexcitation.

Synergy of Electrostatic and π-π Interactions in the Realization of Nanoscale Artificial Photosynthetic Model Systems

In the scientific race to build up photoactive electron donor-acceptor systems with increasing efficiencies, little is known about the interplay of their building blocks when integrated into supramolecular nanoscale arrays, particularly in aqueous environments. Here, we describe an aqueous donor-acceptor ensemble whose emergence as a nanoscale material renders it remarkably stable and efficient. We have focused on a tetracationic zinc phthalocyanine (ZnPc) featuring pyrenes, which shows an unprecedented mode of aggregation, driven by subtle cooperation between electrostatic and π-π interactions. Our studies demonstrate monocrystalline growth in solution and a symmetry-breaking intermolecular charge transfer between adjacent ZnPcs upon photoexcitation. Immobilizing a negatively charged fullerene (C60 ) as electron acceptor onto the monocrystalline ZnPc assemblies was found to enhance the overall stability, and to suppress the energy-wasting charge recombination found in the absence of C60 . Overall, the resulting artificial photosynthetic model system exhibits a high degree of preorganization, which facilitates efficient charge separation and subsequent charge transport.

Applications of Photoinduced Phenomena in Supramolecularly Arranged Phthalocyanine Derivatives: A Perspective

This review focuses on the description of several examples of supramolecular assemblies of phthalocyanine derivatives differently functionalized and interfaced with diverse kinds of chemical species for photo-induced phenomena applications. In fact, the role of different substituents was investigated in order to tune peculiar aggregates formation as well as, with the same aim, the possibility to interface these derivatives with other molecular species, as electron donor and acceptor, carbon allotropes, cyclodextrins, protein cages, drugs. Phthalocyanine photo-physical features are indeed really interesting and appealing but need to be preserved and optimized. Here, we highlight that the supramolecular approach is a versatile method to build up very complex and functional architectures. Further, the possibility to minimize the organization energy and to facilitate the spontaneous assembly of the molecules, in numerous examples, has been demonstrated to be more useful and performing than the covalent approach.

New Insight into an Old Problem: Analysis, Interpretation, and Theoretical Modeling of the Absorption and Magnetic Circular Dichroism Spectra of Monomeric and Dimeric Zinc Phthalocyanine Cation Radical

The chemically or spectroelectrochemically generated formation and aggregation of zinc(II) tetra-tert-butylphthalocyanine cation radical [ZnPc^tBu]^+•, which was highly soluble in common organic solvents, were investigated using UV-vis and magnetic circular dichroism (MCD) spectroscopies with an emphasis on the influence of the axial ligand on the fingerprint (∼500 nm) and NIR (720∼1000 nm) spectral envelopes. MCD spectroscopy is suggestive that the NIR band at ∼1000 nm observed for the antiferromagnetically coupled cation radical dimer, [ZnPc^tBu]₂²⁺, has no degeneracy, the monomer-dimeric equilibrium is temperature dependent, and higher degree aggregates can be formed at specific conditions. Sixteen different exchange-correlation functionals were tested to accurately predict the energies, intensities, and profiles of the UV-vis and MCD spectra of the phthalocyanine cation radical monomer and dimer. It was found that the M05 exchange-correlation functional (along with several other functionals that include 27-42% of Hartree-Fock exchange) provided an excellent agreement (∼0.1 eV for the degenerate excited states observed by MCD spectroscopy) between theory and experiment for the phthalocyanine cation-radical monomer and dimer. Not only did time-dependent density functional theory (TDDFT) calculations with M05 exchange-correlation functional correctly predict the nondegenerate NIR charge-transfer band at ∼1000 nm, all degenerate excited states, monomer and dimer energies, and oscillator strengths, but also they correctly described the nature of the experimentally observed at ∼500 nm MCD B-term (fingerprint band) detected for both the monomeric and dimeric phthalocyanine cation radicals. The TDDFT data explain the similarities in the UV-vis and MCD spectra of the monomeric and dimeric species observed between the UV and fingerprint spectral envelopes as well as correctly predicted the antiferromagnetic coupling between the two singly oxidized phthalocyanine macrocycles in the dimer.

Combining Zinc Phthalocyanines, Oligo(p-Phenylenevinylenes), and Fullerenes to Impact Reorganization Energies and Attenuation Factors

A study on electron transfer in three electron donor-acceptor complexes is reported. These architectures consist of a zinc phthalocyanine (ZnPc) as the excited-state electron donor and a fullerene (C₆₀ ) as the ground-state electron acceptor. These complexes are brought together by axial coordination at ZnPc. The key variable in our design is the length of the molecular spacer, namely, oligo-p-phenylenevinylenes. The lack of appreciable ground-state interactions is in accordance with strong excited-state interactions, as inferred from the quenching of ZnPc centered fluorescence and the presence of a short-lived fluorescence component. Full-fledged femtosecond and nanosecond transient absorption spectroscopy assays corroborated that the ZnPc ⋅ ⁺ -C₆₀  ⋅ ^- charge-separated state formation comes at the expense of excited-state interactions following ZnPc photoexcitation. At a first glance, the ZnPc ⋅ ⁺ -C₆₀  ⋅ ^- charge-separated state lifetime increased from 0.4 to 86.6 ns as the electron donor-acceptor separation increased from 8.8 to 29.1 Å. A closer look at the kinetics revealed that the changes in charge-separated state lifetime are tied to a decrease in the electronic coupling element from 132 to 1.2 cm^-1 , an increase in the reorganization energy of charge transfer from 0.43 to 0.63 eV, and a large attenuation factor of 0.27 Å^-1 .