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.

Triarylisocyanurate-Based Fluorescent Two-Photon Absorbers.

The synthesis and characterization of six triarylisocyanurates, featuring 2,7-fluorenyl or 9,10-anthracenyl groups incorporated in their peripheral arms are reported. Photophysical studies reveal that these new octupolar derivatives are more fluorescent (ΦF ≥0.60 for all new compounds except for 1,3,5-tri(9H-fluoren-2-yl)-1,3,5-triazinane-2,4,6-trione 3) and present a red-shifted lowest absorption and emission compared to their known phenyl analogues of comparable size. Depending on the nature of the terminal substituent, fast intramolecular energy transfer among the three arms or localization of the excitation on a single branch occurs after population of their first singlet excited state. The latter effect was only observed in the presence of strongly electron-releasing substituents in polar media. These new chromophores are also better two-photon absorbers than the 1,4-phenylene-based isocyanurates reported so far, with cross sections σ2 ≥500 GM at 770 nm for 4-NPh2 the fluorenyl group containing (13) and the anthracenyl group containing (14) chromophores. All these spectroscopic features, analyzed with the help of quantum chemical calculations, are crucial for the design of new biphotonic fluorescent dyes.

Parent, Unsubstituted Hemiporphycene: Synthesis and Properties.

Among seven possible nitrogen-in constitutional isomers of porphyrin only one, porphycene, has been obtained so far in the free, unsubstituted form. Herein, the synthesis of another isomer, parent hemiporphycene (HPc), and its thorough structural, spectral, photophysical, electrochemical, and theoretical characterization are reported. Most of the properties of HPc are intermediate between those of porphyrin and porphycene, as evidenced by the values of inner-cavity dimensions, orbital-energy splittings, absorption coefficients, magnetic circular dichroism parameters, NH-stretching frequencies, fluorescence quantum yields, tautomerization rates, and redox potentials. The largest differences arise with respect to tautomerism, due to the low symmetry of HPc and inequivalence of the four nitrogen atoms that define the inner cavity. Two trans tautomers are observed, separated in energy by about 1 kcal mol-1 . Tautomerization from the higher- to the lower-energy form is detected in the lowest-excited singlet state and occurs at a rate that is about four orders of magnitude lower than that observed for porphycene. Hemiporphycene is a very good model for the investigation of inequivalent intramolecular H-bonds present in one molecule; two such bonds in HPc reveal unusual characteristics, and the bond strength results from the interplay between the N⋅⋅⋅N distance and the N-H-N angle.

Substituent and solvent effects on the excited state deactivation channels in anils and boranils.

Differently substituted anils (Schiff bases) and their boranil counterparts lacking the proton-transfer functionality have been studied using stationary and femtosecond time-resolved absorption, fluorescence, and IR techniques, combined with quantum mechanical modelling. Dual fluorescence observed in anils was attributed to excited state intramolecular proton transfer. The rate of this process varies upon changing solvent polarity. In the nitro-substituted anil, proton translocation is accompanied by intramolecular electron transfer coupled with twisting of the nitrophenyl group. The same type of structure is responsible for the emission of the corresponding boranil. A general model was proposed to explain different photophysical responses to different substitution patterns in anils and boranils. It is based on the analysis of changes in the lengths of CN and CC bonds linking the phenyl moieties. The model allows predicting the contributions of different channels that involve torsional dynamics to excited state depopulation.

Enhancing fluorescence by using pluronic block copolymers as carriers of monomeric porphycenes.

Porphycenes, structural isomers of porphyrins, usually exhibit strong fluorescence in organic solvents. However, in water they are practically insoluble or form only weakly emitting aggregates. We show that embedding porphycenes inside pluronic micelles in water solutions leads to the recovery of strong monomeric fluorescence, of which the decay times and quantum yields are similar to those observed for homogeneous solvents. One of the investigated porphycenes serves as a fluorescence sensor of the microenvironment viscosity, revealing that the viscosity inside pluronic micelles is quite high. Using confocal fluorescence microscopy, we obtained images of single pluronic micelles containing monomeric porphycene chromophores.

Tautomerization in 2,7,12,17-tetraphenylporphycene and 9-amino-2,7,12,17-tetraphenylporphycene: influence of asymmetry on the direction of the transition moment.

Femtosecond transient absorption anisotropy studies have been performed for two porphycenes of different symmetry. In 2,7,12,17-tetraphenylporphycene, the chemical identity of two trans forms implies a change in the S(0)-S(1) transition-moment direction upon tautomerization. Exploiting this phenomenon, the rates of double hydrogen transfer in both the S(0) and S(1) states (1.4×10(12)  s(-1) and 2.7×10(11)  s(-1) , respectively) have been determined by performing time-resolved anisotropy studies. In the asymmetric 9-amino-2,7,12,17-tetraphenylporphycene, tautomerization occurs with a similar rate in the ground state. In the S(1) state, the reaction is hindered in its vibrationally relaxed form, but the excitation spectra suggest that it may occur for an unrelaxed molecule. Unlike all porphycenes that have been studied so far, 9-amino-2,7,12,17-tetraphenylporphycene does not reveal significant changes in anisotropy owing to intramolecular double hydrogen transfer; rather, the transition-moment directions are similar in the two tautomeric forms. Analysis of the molecular orbitals allows for an explanation of the "locking" of the transition moments: it is due to a large splitting of the two HOMO orbitals, which retain the order of their energies in the two tautomers.

Bridging the gap between porphyrins and porphycenes: substituent-position-sensitive tautomerism and photophysics in meso-diphenyloctaethylporphyrins.

2,3,7,8,12,13,17,18-Octaethyl-5,15-diphenylporphyrin (1) is characterized by an inner cavity with a rectangular shape and small NH⋅⋅⋅N distances. It resembles porphycene, which is a constitutional isomer of porphyrin known for its strong intramolecular hydrogen bonds and rapid tautomerization. Such distortion of the porphyrin cavity leads to tautomeric properties of 1 that are intermediate between those of porphyrin and porphycene. In particular, a tautomerization in the lowest excited singlet state of 1 has been discovered, occurring with a rate three orders of magnitude lower than that in porphycene, but three to four orders of magnitude higher than that in porphyrin. An isomer of 1, 2,3,7,8,12,13,17,18-octaethyl-5,10-diphenylporphyrin (2), exhibits a different kind of geometry distortion. This molecule is nonplanar, but the inner cavity shape and dimensions are similar to those of the parent porphyrin. The same hydrogen-bonding strength as that in porphyrin is observed for 2. In contrast, the nonplanarity of 2 significantly influences the photophysics, leading to a decrease in fluorescence quantum yield and lifetime. Absorption, magnetic circular dichroism, and fluorescence spectra are similar for 1 and 2 and resemble those of parent porphyrin. This is a consequence of comparable energy splittings of the frontier orbitals, ΔHOMO≈ΔLUMO. The results demonstrate that judicious selection of substituents and their position enables a controlled modification of geometry, hydrogen-bonding strength, tautomerization rate, and photophysical and spectral parameters of porphyrinoids.

From bifunctional nucleophilic behavior of DBU to a new heterocyclic fluorescent platform.

[structure: see text] An unexpected discovery of a novel cyclocondensation reaction of 1,8-diazabicyclo[5.4.0]undec-8-ene (DBU) with activated 1,2-dichloro compounds is described. The 2-aminopyrrole skeleton is generated through the concomitant formation of new nitrogen-carbon and carbon-carbon bonds. A new pentacyclic derivative formed upon the reaction of 2,3-dichloroquinoxaline with DBU exhibits strong fluorescence both in solutions (Phi in hexane = 0.4) and in the solid state.

Electronic structure, spectra, and magnetic circular dichroism of cyclohexa-, cyclohepta-, and cyclooctapyrrole.

Three recently obtained expanded porphyrins represent nice examples of compounds for which the electronic and spectral properties can be predicted from symmetry considerations alone. Perimeter-model-based theoretical analysis of the electronic structure of doubly protonated cyclo[6], cyclo[7], and cyclo[8]pyrrole leads to the anticipation of qualitatively the same electronic absorption and magnetic circular dichroism patterns for all three compounds. These predictions are fully confirmed by experiments, as well as DFT and INDO/S calculations. Due to a characteristic pattern of frontier molecular orbitals, a degenerate HOMO and a strongly split LUMO pair, the three cyclopyrroles show comparable absorption intensity in the Q and Soret regions. Magnetic circular dichroism spectra reveal both A and B Faraday terms, of which the signs and magnitudes are in remarkably good agreement with theoretical expectations. The values of the magnetic moments of the two lowest degenerate excited states have also been obtained.