Dual fluorescence in 9-amino-2,7,12,17-tetraphenylporphycene

Spectroscopic investigation of photophysics and tautomerism of amino- and nitroporphycenes

Parent, unsubstituted porphycene and its two derivatives: 2,7,12,17-tetra-n-propylporphycene and 2,7,12,17-tetra-t-butylporphycene were substituted at the meso position with amino and nitro groups. These two families of porphycenes were characterized in detail with respect to their spectral, photophysical, and tautomeric properties. Two trans tautomers of similar energies coexist in the ground electronic state, but only one form dominates in the lowest excited singlet state. Absorption, magnetic circular dichroism (MCD), and emission anisotropy combined with quantum-chemical calculations led to the assignment of S₁ and S₂ transitions in both tautomers. Compared with the parent porphycene, the S₁-S₂ energy gap significantly increases; for one tautomeric form, the effect is twice as large as for the other. Both amino- and nitroporphycenes emit single fluorescence; previously reported dual emission of aminoporphycenes is attributed to a degradation product. Introduction of bulky t-butyl groups leads to a huge decrease in fluorescence intensity; this effect, arising from the interaction of the meso substituent with the adjacent t-butyl moiety, is particularly strong in the nitro derivative.

2 + 2 Can Make Nearly a Thousand! Comparison of Di- and Tetra-Meso-Alkyl-Substituted Porphycenes

Two porphycenes, substituted at the meso positions with two and four methyl groups, respectively, reveal similar absorption spectra, but their photophysical properties are completely different. 9,20-dimethylporphycene emits fluorescence with about 20% quantum yield, independent of the solvent. In contrast, fluorescence of 9,10,19,20-tetramethylporphycene is extremely weak in nonviscous solvents, but it can be recovered by placing the chromophore in a rigid environment. We propose a model that explains these differences, based on calculations and structural analogies with other extremely weakly emitting derivatives, dibenzo[cde,mno]porphycenes. The efficient S₁ deactivation involves delocalization of two inner cavity protons coupled with proton translocation toward a high-energy cis tautomer. The latter process leads to distortion from planarity. The probability of deactivation increases with the strength of the intramolecular NH···N hydrogen bonds. The model also explains the observation of biexponential fluorescence decay in weakly emitting porphycenes. It can be extended to other derivatives, in particular, the asymmetrically substituted ones. We also point to the possibility of using specific porphycenes as viscosity sensors, in particular, when working in single molecule regime.

Rational Synthesis of 5,10-Diazaporphyrins via Nucleophilic Substitution Reactions of α,α'-Dibromotripyrrin and Dihydrogenation to Give 5,10-Diazachlorins

We report here the first rational synthesis of 5,10-diazaporphyrins via nucleophilic substitution reactions of α,α'-dibromotripyrrin. Uses of 1,3-diiminoisoindoline and 3,4-di(ethylsulfanyl)pyrrole-2,5-diimine as nucleophiles allowed for synthesis of 5,10-diazabenzoporphyrin (2) and 5,10-diaza-7,8-di(ethylsulfanyl)porphyrin (3). 3 was reduced to 5,10-diazaporphyrin (4), 5,10-diaza-2,3-dihydrogenated porphyrin (5), and 5,10-diaza-7,8-dihydrogenated porphyrin (6) with yields that were dependent upon reduction conditions. All the structures of these products were confirmed by X-ray crystallographic analysis. Their optical and electrochemical properties have been comparatively studied with those of 5,15-diazaporphyrin (7) and 5,15-diazachlorin (8). Furthermore, NH tautomers of 2 and 4 were observed as different species in solution, and the dynamic NH tautomeric behavior was studied in 4.

On the excitation dependence of fluorescence spectra of meso-tetrapyridyl zinc (II) porphyrin and its relation with hydrogen bonding and outlying decoration

Zinc porphyrins are potential candidates for boosting the advancement of various technological applications, including those exploring the molecule's radiative emissions. In this work, the excitation dependence of fluorescence spectra from 5,10,15,20-meso-tetrapyridyl zinc(II) porphyrin dissolved in a binary solvent mixture of CHCl₃: MeOH, is reported. Important modifications in the profiles of the fluorescence bands are observed after exciting the molecules in a broad wavelength range from 350 to 565 nm. We attribute such modifications to the existence of two distinct relaxation pathways, related to two quasi-degenerated potential energy surfaces (PES) in the ZnTPyP's first excited state whose population rates changes for different excitation wavelengths. We also observed that by changing the CHCl₃:MeOH proportion in the binary mixture, a quenching mechanism mediated by the MeOH hydrogen bondings and ZnTPyP takes place, which allows for tuning the excitation dependence of the aforementioned relaxations pathways. Moreover, our data confirm that the addition of outlying RuCl(dppb)(bipy) ruthenium complex linked to the pyridyl moieties of the ZnTPyP ring is also an excellent strategy to modify the excitation dependence of the fluorescence relaxation pathways.

Chromatographically separable ruffled non-planar isomeric octaalkylporphycenes: consequences of unsymmetrical substitution upon structure and photophysical properties

Two symmetry reduction induced unsymmetrical porphyrins (2.0.2.0) were synthesized and characterized with distinct structural, optoelectronic and nonlinear optical properties.

Supersonic jet spectroscopy of parent hemiporphycene: Structural assignment and vibrational analysis for S0 and S1 electronic states

Hemiporphycene (HPc), a constitutional isomer of porphyrin, is studied under supersonic expansion conditions by means of laser-induced fluorescence, visible-visible hole-burning experiments, single vibronic level fluorescence techniques, and quantum chemical calculations. Only one trans form of jet-cooled HPc is observed, in contrast to solution studies that evidence a mixture of two trans tautomeric forms separated in energy by ∼1 kcal/mol. Reliable structural assignment is provided by simulating absorption and emission patterns at the density functional theory and time-dependent density functional theory levels of theory. The vibronic spectra are nicely reproduced for both electronic ground and lowest excited singlet states for the most stable trans form. In contrast to another porphyrin isomer, porphycene (Pc), no tunneling or photo-induced hydrogen transfer is detected. The lower symmetry of HPc compared with Pc and the concomitant non-equivalent positions of the inner-cavity nitrogen atoms result in a non-symmetric double minimum potential for tautomerization, larger energy barrier, and a longer tunneling distance, with the average intramolecular hydrogen bond length larger in HPc than in Pc. HPc readily forms hydrates that show red-shifted absorption relative to the bare molecule.

Gram-scale synthesis of porphycenes through acid-catalyzed oxidative macrocyclizations of E/Z-mixed 5,6-diaryldipyrroethenes

A set of novel porphycenes was synthesized on a gram-scale in high overall yields by optimization of macrocyclization of E/Z-mixed 5,6-diaryldipyrroethenes.

β-Hexaalkylporphycenes: Positional Effect of Alkyl Groups toward Design and Control of Structural and Photophysical Properties in Isomeric Hexaethylporphycenes

Two novel β-hexaalkylated porphycenes, i.e., 2,3,7,12,13,17- (HOT) and 2,3,6,12,13,16-hexaethylporphycenes (HIT) were introduced for the first time in porphycene chemistry. These were synthesized through McMurry coupling reactions of new isomeric unsymmetrically substituted triethylbipyrrole dialdehydes. The positional effects of alkyl groups could be manifested through significant alteration in structure of porphycene cores and, as a consequence their photophysical properties, not noticed in β-octaethylporphycene. HOT displays significant fluorescence accompanied by reasonable singlet oxygen generation ability.

Acid- and hydrogen-bonding-induced switching between 22-π and 18-π electron conjugations in 2-aminothiazolo[4,5-c]porphycenes

2-Aminothiazolo[4,5-c]porphycenes undergo a 22-π to 18-π electron conjugation switch in the presence of acids and strong H-bonding solvents.

The first synthesis of meso-dicycloalkylporphycenes: ring strain effects on structural and optical properties of isomeric porphyrins

Two novel meso-dicycloalkylporphycenes were synthesized for the first time, which showed remarkable ring-strain-induced switching of their fluorescence behaviors in the solution phase.

Spectroscopic and microscopic investigations of tautomerization in porphycenes: condensed phases, supersonic jets, and single molecule studies

Tautomerization of porphycene, coherent in supersonic jets and a rate process in solutions, can be controlled for single molecules on surfaces.

A novel fluoro-chromogenic click reaction for the labelling of proteins and nanoparticles with near-IR theranostic agents

Binding of red-absorbing porphycene isothiocyanates to proteins and nanoparticles leads to near-IR fluorescent and photosensitising conjugates.

The long and winding road to new porphycenes

We describe attempts — not always successful — made over the years to improve the efficiency of porphycene synthesis and to produce novel compounds, custom-designed for specific purposes. New porphycenes are reported, some of them obtained rather unexpectedly as by-products of the planned reactions. Structure and energy computations of possible tautomeric forms in porphycenes substituted by one, two, three, and four tert-butyl groups lead to predictions regarding the kinetics and mechanisms of intramolecular double hydrogen transfer. The occurrence of tautomerization in single molecules of tert-butylsubstituted porphycenes is demonstrated by using fluorescence polarization techniques.

Tautomerism and dual fluorescence in 9-substituted n-propyl- and methoxyethyl-porphycenes

Dual fluorescence is described and characterized for several 9-substituted porphycenes. Using both spectroscopic and computational studies, the phenomenon is related to the differential stabilization of the two trans tautomers in the excited state. The electronic nature of the 9-substituents affects both the energy and the lifetime of the two tautomers as well as their interconversion. The results are consistent with a two-step model for excited-state tautomerization that involves the population of an upper excited state, tentatively assigned to a cis species, which in turn provides an efficient non-radiative pathway to the ground state.