Prashanth Kumar Poddutoori

A panchromatic hypervalent phosphorus(V) porphyrin exhibiting reversible intramolecular charge transfer (CT) is reported. The complex was constructed by coupling the electron-deficient phosphorus(V) porphyrin core with electron-rich N,N-dimethylaniline (DMA) substituents, creating an electronic disparity between the central macrocycle and peripheral donor groups. As a result, the compound displays intense charge-transfer absorption bands spanning 200–800 nm. Steady-state and transient optical spectroscopy, transient EPR spectroscopy, and DFT studies reveal the CT character of the excited states and that the emission at ambient temperature arises from both singlet and triplet CT states, with the latter being populated via spin–orbit-coupling mediated intersystem crossing. Notably, the CT process can be reversibly switched ON and OFF via protonation of the DMA nitrogen atoms. The combination of singlet and triplet CT states with appreciable lifetimes, their switching via simple protonation of peripheral DMA groups, photostability, and broad optical coverage makes these dyes highly promising chromophores for artificial photosynthesis, optoelectronics, and photocatalysis, and strengthens our fundamental understanding of these phenomena.

The charge and electron spin density distribution in the lowest excited triplet state of a cofacial Al(III) porphyrin‐P(V) porphyrin heterodimer (AlPor‐O‐PPor·PF ₆ ) has been studied at low temperature in different glass‐forming solvents using electron nuclear double resonance (ENDOR) spectroscopy and density functional theory (DFT) calculations. The ¹ H hyperfine couplings (hfcs) at specific positions of the porphyrin rings have been used to differentiate spin delocalization and charge transfer (CT) character in the heterodimer. While both effects contribute similarly to the zero‐field splitting tensor, they can be distinguished by ENDOR spectroscopy through comparing the heterodimer triplet state hfcs with those of the corresponding monomers in their triplet state, oxidized, and reduced radical forms. From ENDOR data and DFT calculations, the triplet state of AlPor‐O‐PPor·PF ₆ is estimated to have ~24% and ~35% CT character respectively, in agreement with the Mulliken partial charges. These results indicate that the CT character is modulated by solvent interactions, depending both on its polarity and coordination of the Al(III) center. The estimate of the CT character in the triplet state and its dependence on the molecular structure and the solvent is crucial to achieve efficient long‐lived triplet born charge separation in model photosynthetic systems.

High-potential hypervalent antimony(V) porphyrins have been covalently linked to electron acceptor C₆₀ molecule, resulting in the formation of antimony(V) porphyrin-fullerene dyads: SbP-C₆₀ and SbPF₃-C₆₀. In these systems, SbP consists of meso-phenyl units, while SbPF₃ includes meso-3,4,5-trifluorophenyl units. The aim of this work is to utilize the high potentials of SbP and SbPF₃ to promote electron transfer from the C₆₀ unit. Energetics suggest that electron transfer from the C₆₀ unit to the SbP and SbPF₃ moiety is feasible in these dyads. The driving force for this transfer is slightly greater in the SbPF₃-C₆₀ due to the presence of meso-3,4,5-trifluorophenyl units. However, experimental observations have not detected the predicted electron transfer from the C₆₀ unit to SbP or SbPF₃; instead, ultrafast singlet-singlet energy transfer from the porphyrin to the C₆₀ unit has been observed. The trends indicate that the rate of energy transfer is faster than that of electron transfer, which can be attributed to both thermodynamic and kinetic factors. I will discuss these results in relation to their structural, redox, and photophysical properties.

Photosynthesis is nature’s most elegantly engineered process, expertly converting sunlight to chemical energy. In the face of an exponentially escalating global energy crisis, photosynthesis offers a compelling natural blueprint for sustainable energy production. This has fostered significant efforts in developing artificial photosynthetic (AP) systems that mimic this intricate process for solar energy conversion and storage applications. Within the design of AP systems, porphyrin (Por) complexes have emerged as promising candidates, aiming to replicate the natural photosystem. These design strategies leverage the porphyrin’s strong visible region absorbance, high fluorescence, and easily tunable optical and redox properties. Further, the capabilities of these systems can be enhanced through the insertion of group 15 elements to the porphyrin cavity. Notably, antimony(Sb(V)) porphyrin systems prove to be an exciting space within AP complexes, due to their induction of intense positive redox potentials within the porphyrin. These potentials enable the design of push−pull type photoinduced intramolecular charge transfer (ICT) architectures, displaying broad absorption across the visible spectrum with charge-transfer states. In addition, Sb(V)’s elevated oxidation state facilitates hypervalent coordination environments, able to accommodate axial ligands to the porphyrin system. Exploitation of these fascinating properties produces optimal candidates for the construction of diverse AP system components, facilitating the efficient capture and storage of solar energy through a varied set of strategies.

Porphyrins play a central role in artificial photosynthesis due to their strong light harvesting capabilities and ability to participate in the electron transfer process. Sharing similar properties with chlorophyll, porphyrins are highly effective photosensitizers, and their performance can be enhanced through strategic conjugation with complementary moieties. Fullerene (C₆₀) is a widely studied electron acceptor characterized by high electron affinity and mobility, as well as an intrinsically low-lying lowest unoccupied molecular orbital (LUMO), making it a powerful component in molecular donor-acceptor (D-A) assemblies. In previous research, C₆₀ has been axially covalently linked to high-potential, hypervalent antimony (V) porphyrins to produce two antimony(V) porphyrin-fullerene conjugates. This system was originally designed to switch the typical D-A roles in the system by making the porphyrin electronegative enough to accept an electron from fullerene, becoming the electron acceptor in the system. Unfortunately, the findings of that research demonstrated that direct energy transfer was the dominant process in the synthesised antimony(V) porphyrin-C₆₀ dyads. However, some of the findings of that research could not be explained by simple energy transfer parameters, giving rise to a new idea. By adding more driving force to the porphyrin, changing the orientation of the dyad, and changing the electronic coupling, the D-A role reversal for the porphyrin-C₆₀ dyad could still be possible.

A covalently linked cofacial homodimer has been synthesized using hypervalent antimony(V) porphyrins. The two porphyrins are connected by an -OCH₂O- bridge, maintaining a distance of 5.85 Å between the Sb centers. Despite the positive charge on the porphyrin entities, the resulting homodimer is structurally stable, with minimal or no repulsive forces between them. Density functional theory (DFT)-optimized structures show that the two porphyrins are nearly coplanar with an angle of ∼15° between the two planes. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the ground state of the dimer are delocalized over both porphyrins, indicating the existence of exciton coupling between them. Optical studies support this observation, suggesting H-type exciton coupling based on the spectral shift of the Soret band. Similar behavior is observed in electrochemical studies as well. Results from the femtosecond transient absorption studies support the existence of exciton coupling, while the triplet-state lifetimes measured by nanosecond transient absorption studies show only minor differences. Time-resolved electron paramagnetic resonance and DFT studies of the lowest triplet state show that it is localized on a single porphyrin. While the localization of the triplet state is consistent with the expected weak coupling between the triplet states due to their small transition dipoles, the optical data, DFT calculations, and structure of the dimer all indicate that the dipole-dipole model of the excitonic coupling is not capable of adequately describing the excited-state properties. Notably, the role of p-block Sb in cofacial porphyrin dimer promoting exciton coupling, irrespective of its higher oxidation state of +5, is borne out from this study.

The axial‐bonding ability of aluminum(III) porphyrin has been employed to construct two covalently linked homodimer systems. The center‐to‐center distance between the two porphyrins, which are connected via an oxalic bridge is ∼6.90 Å. Unlike most other meso‐ substituted porphyrins, which have aromatic substituents, the Al porphyrins were decorated with propyl or heptyl side groups to tune their electronic properties. Further, the Lewis acidity of the Al center was exploited to coordinate the Lewis base, imidazole, appended to the electron acceptor, C ₆₀ , to construct a self‐assembled supramolecular homodimer‐C ₆₀ donor‐acceptor conjugate. Steady state optical studies show that the exciton coupling between porphyrins is weak, and time‐resolved electron paramagnetic resonance (TREPR) measurements in the glass phase indicate that the triplet state of the dimer remains localized. When C ₆₀ is coordinated to the monomers and dimers, femtosecond and nanosecond transient absorption studies and TREPR measurements show that electron transfer between the porphyrin and fullerene occurs. Despite the weak coupling between the two porphyrins of the dimer, the presence of a second porphyrin enhances both the yield and lifetime of the charge‐separated state. Remarkably, the resulting charge‐separated lifetimes found to be between 2–5 µs in the investigated reaction center mimics.

Amphiphilic aluminum(III) porphyrin (AlC7P) has been designed, and its aggregation‐induced photophysics has been reported. The meso‐positions of the aluminum(III) porphyrin are functionalized with a heptyl chain (C7), while the central porphyrin features an axial OH group on the Al center. The hydrophilic (OH) and hydrophobic (C7) properties combined with the Lewis acid (Al center) and Lewis base (OH group) functionality to facilitate the formation of aggregation through self‐assembly. Spectroscopic studies indicate that aggregation can be modulated by solvent and temperature; it is more pronounced in long‐chain hydrocarbon solvents, such as hexanes and n‐heptane, and is favored at lower temperatures. At low temperature conditions, the aggregated molecules manifest low fluorescence yields, whereas at higher temperatures, the self‐assembly breaks down to exhibit high fluorescence yields. This temperature‐dependent aggregation‐induced photophysical behavior observed in hexanes makes this porphyrin ideal for a luminescent molecular thermometer.

High-potential hypervalent antimony(V) porphyrins have been covalently linked to the well-known electron acceptor C₆₀ molecule, resulting in the formation of antimony(V) porphyrin-fullerene conjugates: SbP-C₆₀ and SbPF₃-C₆₀. The two porphyrins, SbP and SbPF₃ contain four meso-phenyl and meso-3,4,5-trifluorophenyl units, respectively. These systems are designed to leverage the exceptionally high redox potentials of SbP and SbPF₃ to create an unusual pattern of excited state energies in a porphyrin-fullerene conjugate, in which the energy of the (porphyrin)^•--C₆₀^•+ state lies between the porphyrin and fullerene excited singlet states. Time-resolved spectral data show that ultrafast singlet-singlet energy transfer from the porphyrin to the C₆₀ unit occurs. The estimated energetics suggest that the ¹C₆₀* state could be populated from porphyrin excited singlet state by either the usual Förster mechanism or by electron transfer from the C₆₀ unit to the excited SbP/SbPF₃ moiety followed by charge recombination. However, spectral features associated with the charge-separated state are not observed, and the energy transfer rates calculated for the Förster mechanism are in reasonable agreement with the experimental values. Thus, direct energy transfer appears to be the dominant process in these novel dyads derived from high-potential antimony(V) porphyrins.

The time dependence of the light-induced spin polarization of a series of porphyrins is reported. The porphyrins contain central elements from three distinct groups in the Periodic Table with different oxidation states, types, and numbers of axial ligands, as well as different peripheral substituents on the porphyrin ring. Shortly after photoexcitation, in all cases, the primary multiplet polarization of the porphyrin triplet state evolves into long-lived net polarization whose lifetime is determined by the lifetime of the triplet state. The zero-field splitting parameters and sign of the multiplet polarization vary significantly among the porphyrins, but the transient EPR spectra taken at later times are remarkably similar. The development of long-lived net polarization is proposed to involve the dynamic Jahn–Teller effect, in which hopping between the two lowest triplet states occurs.