This work aims to design BODIPY-(Zn-porphyrin)2 conjugates that channel all harvested light energy into one conjugate moiety, providing a specific photoresponse with an extended excitation range. Three BODIPY-porphyrin conjugates have been prepared, and their excitation energy deactivation channels have been established. In two of them, the BODIPY pi-conjugation was extended at either the 2,6-positions or the 3,5-positions via Sonogashira coupling. Then the click reaction was applied to conjugate it to two Zn-porphyrins. The pi-conjugation extension shifts the BODIPY absorption spectra bathochromically, with the 3,5-substituted BODIPY spectra being more shifted compared to those of the 2,6-substituted ones. These BODIPY-(Zn-porphyrin)2 conjugates exhibit fluorescence spectra that are almost identical to those of the parent BODIPYs, independently of the excitation wavelength. The Zn-porphyrin transfers its own excitation energy to the BODIPY moiety and simultaneously acts as a BODIPY fluorescence quencher by enhancing the BODIPY radiationless transition rate in the conjugate. The third BODIPY-(Zn-porphyrin)2 conjugate was prepared by attaching two Zn-porphyrins via click reaction to azide groups in the meta-positions of the phenyl ring at the 8-position of BODIPY. Here, the BODIPY moiety harvests the excitation energy, which ultimately goes to the Zn-porphyrins with no traces of BODIPY emission. Notably, the intramolecular transition rates in the Zn-porphyrins remain unperturbed in the conjugate.

Corroles are the brightest representatives of the contracted tetrapyrrolic macrocycles whose electronic structure, spectral-luminescent and photophysical properties are quite different from those of porphyrins. To date the number of reviews has appeared dealing with the synthesis and applications studies, but much less attention has been paid to the results of the fundamental studies on the electronic structure, aromaticity and photophysics of these compounds. This review aims to summarize the peculiarities of the spectral-luminescent and photophysical properties of the free basecorroles unraveled in last twenty years

The possibility of four NH-tautomers formation in asymmetrically substituted 5,10-diaryl-corrole free bases has been studied. The molecular conformation optimization has been carried out for four NH-tautomers in both the ground singlet S 0 and the lowest excited triplet T 1 states with the density functional theory, and the sets of integral and local structure parameters have been evaluated. The results unambiguously demonstrate that each of the four NH-tautomers has its unique molecular conformation. However, the ground state energies of the NH-tautomers were found to differ strongly as a function of the rotation degree of freedom of aryl substituents. Almost the same ground state energies of all NH-tautomers have been found for the substitution with sterically constrained mesityl groups. In contrast, a large energy gap has been revealed between two pairs of NH-tautomers in case of substitution with freely rotating phenyl groups. Therefore, in the former case, the relative populations of all four NH-tautomers are of the same order of magnitude and all four NH-tautomers coexist, but only two NH-tautomers are expected in the latter at room temperatures. The aromaticity degree and the [Formula: see text]-conjugation pathways have been evaluated for each of four NH-tautomers in both ground S 0 and lowest excited triplet T 1 states, and similar behavior was found for the pairs of NH-tautomers those protons are localized either in the dipyrromethene fragment or in the dipyrrole fragment of the macrocycle. All NH-tautomers were found to be aromatic in the ground state, but the inversion of aromaticity takes place in the lowest triplet T 1 state for all of them. Finally, the energies of the four frontier molecular orbitals have been compared and analyzed in the framework of possible spectral differences between the NH-tautomers.

The influence of peripheral substitution and the nature of the macroheterocyclic heteroatoms on the basicity of hydrophilic 5,10,15,20-tetraarylporphyrins in the lowest excited singlet S1 state was studied using absorption and fluorescence spectroscopies. The basicity of the porphyrins themselves in the lowest excited singlet S1 state was found to decrease as compared to the ground S0 state. The difference in basicity constants pKa depended on the nature of electronic communication between the macrocycle and peripheral substituents. In the case of an inductive effect of the substituents, the difference in the basicity constants pKa was small, while mesomeric effects led to significant differences in the pKa values. The cooperative nature of protonation, leading to almost simultaneous addition of two protons, was shown to be preserved in both cases. Replacement of a pyrrole with a thiophene in the macroheterocycle led to a decrease in cooperativity in both the ground S0 and the lowest excited singlet S1 states. The basicity constants characterizing the addition of the second and third protons by thia-substituted porphyrin differed significantly in the S1 state (pKa2 – pKa1 = 2), while they were close in the ground state. The activation entropy change ΔΔS‡ upon protonation of the porphyrins in the lowest excited S1 state was compared to that in the ground S0 state.

The influence of the architecture of NH2-peripheral substitution of porphine derivatives on the intersystem T1 → S0-transition energy was studied theoretically. The molecular conformations of 15 porphine derivatives and 8 Zn-porphine derivatives in the ground singlet (S0) and lowest triplet (T1) states were optimized, the molecular orbital energies were determined, and the energies of the T1 → S0 transition were calculated using quantum chemical methods. The T1 → S0-transition energy was found to decrease from 11,700 to 6200 cm–1 upon increasing the number of NH2 groups in the macrocycle Cm-positions. The T1 → S0-transition energy was a linear function of the weighted sum of inductive and resonant Hammett constants 0.2σI + 0.8σR of the substituents. The ratio of inductive and resonant contributions of the NH2 groups depended on the method of attachment to the macrocycle, with the contribution of resonant interactions decreasing with increasing spacer length. The main reason for the bathochromic shift of the T1 → S0 transition was a significant increase in the energy of the b1-orbital, which had antinodes on the macrocycle Cm atoms. The dependence also held for Zn-porphyrins with the same peripheral substitution architecture. The energy of the T1 → S0 transition was noted to differ for both NH tautomers and conformers differing in the position of NH2 groups relative to the macrocycle mean plane. The calculations showed that experimental studies of aminoporphyrins were promising for obtaining new phosphors in the IR spectral region. A method for predicting the T1 → S0-transition energy for the synthesis of compounds with the required spectral and luminescent characteristics was proposed based on the results.

Peculiarities of absorption spectra formation of porphine derivatives upon attachment of an NO2 group to the C-m-position of the tetrapyrrole macrocycle have been studied. The molecular conformation of the substituted porphine molecule was optimized using quantum chemistry methods. The energies of molecular orbitals were determined. Absorption spectra were calculated. It was found that the electronic interaction between the macrocycle and the NO2 group is determined by the orientation of the nitro group relative to the macrocycle mean plane. It has been established that the energies of the LUMO and HOMO-1 orbitals depend significantly on the dihedral angle theta between the macrocycle mean plane and the nitro group plane, while the energies of the LUMO+1 and HOMO orbitals vary slightly. As a result, the S-0 -> S-1 and S-0 -> S-2 absorption bands have bathochromic shifts, the magnitudes of which are different and depend on the configurational composition of the transitions. The oscillator strength of the S-0 -> S-1 transition turns out to be greatest in the coplanar conformer, which has minimal configuration interaction. The oscillator strength of the S-0 -> S-1 transition increases sharply in the orthogonal conformer, in which the LUMO and LUMO+1 are quasi-degenerate. All spectral characteristics can be presented as functions of the weighted sum of cos(2) theta and cos(2)2 theta, taking into account the configuration composition of the electronic transitions.

The effect of sequential formation of protonated and deprotonated forms of 2,3,7,13,17,18-hexamethyl-8,12-di-n-butylcorrole in solution on their photophysical characteristics and spectral-luminescent properties was studied. The spectral characteristics of each of the two forms in the ground electronic state were found to be the same when they formed from the free base and from the antipodal form, while the fluorescence spectra of the two forms differed depending on the method of formation. Based on an analysis of spectral shifts in fluorescence spectra and fluorescence quantum yields, it was concluded that the protonated and deprotonated forms of corroles underwent specific interactions in the lower excited singlet S1 state due to the formation of a complex solvation shell. These specific interactions were shown to lead to hysteresis of the fluorescence characteristics of the protonated and deprotonated forms of corroles with their mutual transitions caused by (cyclic) changes in the acid–base equilibrium.

The role of the molecular structure in the formation of the bathochromic shift of the S 0 → S 1 transition for a family of N-substituted porphine derivatives was studied. Molecular conformations of porphine, four of its N-substituted derivatives, and two model porphines with selected fixed bond angles and lengths in the macrocycles were optimized, the energies of the molecular orbitals were determined, and electronic absorption spectra were calculated using quantum-chemistry methods. It was found that N-substitution led to significant pyramidalization of the nitrogen atom. The degree of hybridization λ 2 of the N atom depended on the volume of the N-substituent and reached a value of λ 2 = 2.729 in porphyrin H(N–CCl 3 )P. The degree of hybridization λ 2 of the N atom was established as the factor determining the energy of the long-wavelength S 0 → S 1 transition because the conjugation along the inner C a –N–C a fragment of the pyrrole ring decreased upon pyramidalization of the N atom while π-conjugation via the outer C a –C b –C b –C a fragment, which led to an increase in the size of the conjugated π-system, simultaneously strengthened. The tilt of the N-substituted pyrrole ring relative to the macrocycle mean plane and the electron-donating/electron-withdrawing properties of the N-substituents did not directly affect the bathochromic shift of the S 0 → S 1 transition.

The spectral and luminescent characteristics of a solution of 10-phenyl-5,15-di(4,6-dichloropyrimidinyl)-corrole at 77 K were studied. Effective NH-tautomerization was found to occur in the lower T 1 triplet state of the long-wavelength T1-tautomer, resulting in phosphorescence being emitted only from the short-wavelength T2-tautomer. A shift of the acid–base equilibrium and deprotonation of a certain fraction of the molecules were observed at 77 K. The fluorescence and phosphorescence spectra of the deprotonated form were identified. The energy gap Δ E ( S 1 – T 1 ) = 5570 cm –1 for the deprotonated form was found to be as large as that for the free bases. The molecular conformations of NH-tautomers for a series of corroles with various peripheral substitution architectures were optimized and their electronic absorption spectra and energy gap Δ E ( S 1 – T 1 ) were calculated by the density functional theory method. An increase in the energy gap Δ E ( S 1 – T 1 ) was established to be due to an increase in the energy mismatch Δ E (LUMO–LUMO+1). The observed trend was common for all types of studied molecular systems, i.e., both NH-tautomers of the free base and the deprotonated form. It was proposed that such a trend is an inherent property of the contracted corrole macrocycle, which has an excess of electron density as compared to the porphine system.

The origin of individual features in the ground state absorption spectra of two protonated corroles differing in the architecture of peripheral substitution (either Cm-aryl or Cb-alkyl) have been studied in detail with the ground state absorption spectroscopy and density functional theory calculations. The geometry optimization, molecular orbitals and absorption spectra calculation have been carried out. It was found that protonation leads to the saddle type macrocycle conformation in contrast with the wave type conformation known for the parent-free base corroles. The mean plane deviation parameter Δ23 for the macrocycle, pyrrole tilting angles and the degree of pyramidalization λ2 of all four pyrrole nitrogens was found to depend on the peripheral substitution architecture. Macrocycle conformation of the protonated forms has distinct asymmetrical features which are reflected by the sets of values of the tilting angles and values of pyramidalization degree. The pair of pyrroles B and C has smaller tilting angles and higher pyramidalization degree values, whereas the opposite trend was found for the pair of pyrroles A and D. Electronic effects and structural differences induced by substitution lead to the pronounced shifts of the molecular orbitals. In the Cb-alkylated corrole, almost-degenerated HOMO and HOMO-1 molecular orbitals lead to enhancement of the configuration interaction. As a result, the Qx transition oscillator strength goes down, becoming comparable to that of the Qy one. A large HOMO-HUMO-1gap in the Cm-aryl corrole minimizes the configuration interaction, giving rise to Qx band domination in the visible range spectrum.

The formation of Jaggregates of 21-thia-5,10,15,20-(tetra-4-sulfonatophenyl)-porphyrin in acidified aqueous solutions is reported for the first time. The spectral-luminescent properties of these species were measured and ascribed to excitonic interactions. The number of coherent interacting monomeric porphyrin molecules in the aggregate was determined. J-Aggregates of this heteroporphyrin were found to fluoresce. The fluorescence quantum yield Φ fl was found to be 1.8·10 –4 . The J-aggregates were found to be photolabile. Upon their photo-excitation to the absorption band at 503 nm, these species collapse to the monomeric doubly-protonated molecules. The photomonomerization process is reversible: the J-aggregates form again upon maintenance of the solution in the dark.

The π-conjugation pathways were identified and the degrees of aromaticity for NH-tautomers of corrole free bases were determined by quantum chemistry methods and absorption spectroscopy. The macrocycle skeletal atoms participated differently in the formation of the π-conjugation pathway. It was supposed that conjugation pathways consisting of 18 π-electrons were dominant. At the same time, each of two NH-tautomers possessed its own distinct π-conjugation pathway, which caused their degrees of aromaticity to differ. It was shown that the peripheral substitution architecture of the macrocycle influenced the degree of aromaticity. A method for controlling the equilibrium between the two NH-tautomers that consisted of designing the electron-density distribution in the macrocycle that was characteristic of one of the tautomers was proposed and proved experimentally.

The spectral and luminescent properties of hydrophilic 21-thia-5,10,15,20-tetra(4-sulfonatophenyl)porphyrin and 5,10,15,20-tetra(4-sulfonatophenyl)porphyrin in solutions at 293 K were compared. Peculiarities of halochromic effects due to the replacement of a pyrrole by a thiophene ring in the macrocycle were revealed for the first time. Multicenter interactions on the periphery and in the core of the macrocycle led to modulation of spin–orbit couplings, which became apparent in changes of the fluorescence quenching efficiency. Fluorescence of the doubly protonated heteroporphyrins was shown to be quenched as compared to the free bases whereas fluorescence enhancement was observed for the doubly protonated forms.

The formation of J-aggregates of 21-thia-5,10,15,20-tetra-(4-sulfonatophenyl)-porphyrin in acidified water solutions is herein revealed for the first time and their spectral-luminescent properties are measured. It is shown that the spectral-luminescent properties of J-aggregates are due to the excitonic interactions, and the number of coherent interacting monomeric porphyrin molecules in the aggregate is evaluated. It is found that J-aggregates of heteroporphyrin have fluorescence, and the fluorescence quantum yield Фfl is found to be as low as 1.8 . 10–4. The photolability of J-aggregates is found, namely, upon J-aggregate photoexcitation to the absorption band at 503 nm they collapse to the monomeric doubly protonated molecules. The process of photomonomerisation is reversible: upon keeping the solution in the dark, J-aggregates form again.

For two free base corroles with different architecture of peripheral substitution, the solvatochromic shifts of absorption bands in a series of solvents of different nature are herein determined, and the nature of the solvatochromic effects is analyzed by the Valentine method. It is found that the solvatochromism of the free bases of corroles originates due to universal nonspecific interactions, and the short-wavelength T2 tautomer experiences stronger solvation. It is shown that in polar aprotic solvents (acetone, acetonitrile, dimethylformamide, and dimethylsulfoxide), specific acid-base interactions occur simultaneously, leading to the formation of the deprotonated form. It is found that both deprotonated and protonated forms of corroles also show nonspecific solvation, leading to solvatochromic shifts, the magnitude of which exceeds that for free base corroles. It is proposed that this feature is due to an excess (negative or positive) electronic charge of the macrocycle.

The aromaticity in the lowest triplet T 1 -state of NH-tautomers of corrole free bases with different peripheral substitution architecture was investigated using quantum chemistry methods. It was established that the dominant π-conjugation pathways differed for the NH-tautomers although the dominant π-conjugation pathways in the ground singlet ( S 0 ) and excited triplet ( T 1 ) states for each of the two tautomers were the same. The degree of aromaticity of the macrocycle in the triplet T 1 -state was found to decrease distinctly as compared to the ground S 0 -state. It was shown that the macrocycle of the corrole free bases in the triplet T 1 -state should be considered antiaromatic. Relationships of the degree of aromaticity with the macrocycle conformation and electronic effects of peripheral substituents were discussed.

The temperature dependence of absorption and fluorescence spectra of 5,10,15,20-tetrakis-(4-sulfonatophenyl)-porphyrin in weakly acidic aqueous solutions was studied in the range 288–333 K. The fraction of molecules in the free-base form in the ground (S 0 ) and lower excited singlet states (S 1 ) was found to increase as the temperature increased because the fraction of molecules in the doubly protonated form decreased. The deprotonation was caused by a shift in the acid–base equilibrium in the macrocycle core due to decreases in pKa(S 0 ) and pKa(S 1 ) with increasing temperature. The difference pK a (S 1 ) – pK a (S 0 ) < 0 for solution temperatures >293 K with pKa(S 1 ) – pKa(S 0 ) > 0 for T < 293 K. The activation energies of deprotonation in the S 0 - and S 1 -states for T > 293 K were E a = 5.0 and 3.4 kJ/mol; for T < 293 K, they increased to 20.3 and 56.2 kJ/mol. These differences were explained by different specific solvation of the tetrapyrrole macrocycle in the ground S 0 - and lower excited S 1 -states because of a change in the ratio of two forms of water. So-called form A with disordered H-bonds dominated at higher temperatures. The fractions of forms A and B, which possessed a strongly ordered H-bond system, were comparable as the temperature decreased. Stabilization of the porphyrin free base prevailed at high temperatures; of the doubly protonated form, at low temperatures.

Boron dipyrromethene (BODIPY) dyes represent a particular class within the broad array of potential photosensitizers. Their highly fluorescent nature opens the door for theragnostic applications, combining imaging and therapy using a single, easily synthesized chromophore. However, near-infrared absorption is strongly desired for photodynamic therapy to enhance tissue penetration. Furthermore, singlet oxygen should preferentially be generated without the incorporation of heavy atoms, as these often require additional synthetic efforts and/or afford dark cytotoxicity. Solutions for both problems are known, but have never been successfully combined in one simple BODIPY material. Here, we present a series of compact BODIPY-acridine dyads, active in the phototherapeutic window and showing balanced brightness and phototoxic power. Although the donor-acceptor design was envisioned to introduce a charge transfer state to assist in intersystem crossing, quantum-chemical calculations refute this. Further photophysical investigations suggest the presence of exciplex states and their involvement in singlet oxygen formation.

Steady-state fluorescence measurements and quantum-chemical DFT geometry optimizations are applied to extend the structure–property relationships between the free-base corrole macrocycle conformation and its basicity to the lowest excited S[Formula: see text] and T[Formula: see text] states. Direct basicity estimation in the lowest excited S[Formula: see text] state is demonstrated by means of fluorescence quantum yield measurements. The long wavelength T1 tautomer is found to retain its basicity in the S[Formula: see text] state, whereas the short wavelength T2 tautomer shows a noticeable decrease in basicity in the S[Formula: see text] state, which is related to the in-plane tilting of the pyrrole ring to be protonated. The conformational changes upon going from the ground to the lowest excited T[Formula: see text] state and the influence of the meso-aryl substitution pattern on the overall degree of distortions and tilting of the pyrrole ring to be protonated are also discussed from the point of view of macrocycle basicity.

The molecular structures and spectral and luminescent properties of 2,3,7,13,17,18-hexamethyl-8,12-di-nbutylcorrole and 7,13-dimethyl-8,12-di-n-butylcorrole free bases in solutions at 288–328 K were studied using luminescence and absorption spectroscopy and quantum chemistry. Absorption and fluorescence spectra of the compounds were shown to be superimposed spectra of two NH-tautomers. Individual tautomer spectra were identified. Bands were assigned to specific transitions. NH-tautomer equilibria in the ground S0 and lower excited S1 singlet states were characterized.