On the Nature of the Fluorescent State in β-Nitrotetraarylporphyrins

Abstract

2-NO 2 -5,10,15,20-tetraphenylporphyrin (H 2 TPP-NO 2 ) is shown to exist in solution as an equilibrium mixture of two NH tautomers with different spectral and photophysical properties. At 77 K in a rigid glass solution the fluorescence spectra of the tautomers contain two well-resolved narrow bands that are slightly (∼300 cm -1 ) Stokes-shifted with respect to the corresponding Q X00 absorption bands, with the spectrum of the more stable tautomer being ∼500 cm -1 red-shifted as compared to the spectrum of the less stable tautomer. At room temperature, the tautomers show almost identical broad and structureless fluorescence spectra, markedly red-shifted relative to the longest wavelength absorption band. The Stokes shift increases with an increase of solvent polarity, being as large as ∼2000 cm -1 in N,N-dimethylformamide. Fluorescence lifetimes of the tautomers are found to be markedly (about a factor of 2) different, with both decreasing with an increase of solvent polarity. The reasons for these peculiar fluorescence properties can be understood on the basis of semiempirical quantum chemical calculations. Charge-transfer (CT) states are found to be located between the porphyrinic Q and B states for both tautomers, and a different energy of the CT states gives rise to different fluorescence lifetimes of the tautomers. The calculations also predict a lowering of the Q X state energy and an increase of its dipole moment µ with a decrease of the angle θ between the plane of the NO 2 group and the porphyrin plane (relative to the ground-state equilibrium geometry with θ ) 80°). We suggest that, in solution, such an increase in the µ value should result in an increase of the stabilization interaction between the polar porphyrin and solvent molecules and, consequently, in flattening of the Q X state energy curve as a function of θ and a shift of the minimum of this curve to θ values less than 80°. Both quantum chemical calculations and picosecond transient absorption measurements of the S 1 f S n absorption show that the fluorescent Q X state of H 2 TPP-NO 2 has mainly 1 (π,π*) character with a relatively small charge-transfer admixture.

A Structural Model of Nitro‐Porphyrin Dyes Based on Spectroscopy and Density Functional Theory

“…The data in Table show that adding nitro substituents at any position on H 2 TPP or ZnTPP quenches the fluorescence, consistent with a broad swath of literature . This quenching has been attributed to multiple, potentially concurrent mechanisms, including mixing between S 1 and CT states, spin‐orbit coupling, and enhanced vibrational or rotational relaxation.…”

“…The most salient fact about the time‐resolved emission data is the appearance of two fluorescence lifetimes in each of the nitrated Pors (see Table ). Similar dynamics have previously been reported for β‐nitro Pors and attributed either to the interconversion of tautomers with different photophysics, or to a distribution of conformers with respect to the dihedral angle of the nitro group . In any case, there is no reason to expect emission from higher S n states, so the two lifetimes must correspond to distinct molecular structures, whether they are tautomers, atropisomers, or some other kind of conformers.…”

“…The strongly electron withdrawing nitro groups polarize the molecule in the ground state, with this effect being even stronger in the excited state. While there is some controversy over the nature and CT character of the emissive S 1 state of a nitro‐Por, there is little doubt that it has a substantial dipole moment . A more polar solvent like DMSO should stabilize this kind of polar excited state, red‐shifting the emission relative to toluene.…”

“…Significant attention has been paid to the excited state dynamics of Por compounds, but accurate measurements still present a substantial challenge . There are numerous reports of Pors showing appreciable aggregation in solution, which makes it difficult to distinguish the photophysical properties of the monomeric species from the aggregates .…”

“…In analyzing these compounds, we are faced with a significant body of literature offering a range of descriptive frameworks that must be reconciled. These reports all describe several characteristics common to these kinds of compounds, including broad and redshifted absorbance and emission spectra, quenched fluorescence quantum yield, shortened singlet lifetimes, and strong solvent dependence.…”