Photophysics of a Butadiyne-Linked Porphyrin Dimer:  Influence of Conformational Flexibility in the Ground and First Singlet Excited State

Abstract

The photophysics of a butadiyne-linked porphyrin dimer has been investigated by spectroscopy and quantum mechanical calculations. Primarily, the influence of conformation on the ground and first singlet excited states was studied, and two spectroscopically distinct limiting cases were identified. Experiments show that the twisted and planar conformers are separate spectroscopic species that can be selectively excited and have unique absorption and emission spectra. Calculated ground-state spectra compare well with experimental spectra of the two species. A spectrum of the planar conformer was obtained by the addition of a dipyridyl pyrrole ligand, which forms a 1:1 complex with the dimer and thus forces it to stay planar. The absorption spectrum of the twisted conformer could be deduced from the excitation spectrum of its emission. The interpretation of the ground-state spectrum of the free noncomplexed dimer is that it represents an average of a broad distribution of conformations. Calculations support this conclusion by indicating that the barrier for rotation is relatively small in the ground state (0.7 kcal/mol). Studies of the temperature dependence of the fluorescence spectrum of the dimer indicate a mother-daughter relationship between the twisted and planar conformations in the excited state, where the former has approximately 3.9 kcal/mol higher energy. Furthermore, time-correlated single-photon counting experiments also suggest that the twisted population adopts a planar configuration in the first singlet excited state with a rate constant of k rot ) 8.8 × 10 9 s -1 in 2-MTHF at room temperature. The temperature dependence of the fluorescence lifetimes indicated that an activation energy barrier of approximately 2 kcal/mol, in part related to solvent viscosity, is associated with this rate constant.

Imaging intracellular viscosity of a single cell during photoinduced cell death

“…2a. The spectral changes that occur with an increase in viscosity are consistent with the formation of increasing amounts of the twisted conformer of 1, 22 as torsional rotation about the butadiyne link becomes slower in viscous media. Thus, we assign the emission maximum of 1 at 710 nm to the twisted conformation and the emission maximum at 780 nm to the planar conformation.…”

“…22 Each conformer is characterized by distinctive absorption and emission spectra. The emission of the dimer in fluid solution was dominated by the lower energy planar conformation, whereas the emission of the less stable twisted conformer could be observed from glassy matrices where rotation of the two porphyrin units relative to each other is restricted.…”

Dual mode quantitative imaging of microscopic viscosity using a conjugated porphyrin dimer

“…17 The working principle of this type of molecular rotor is based on the fact that two fluorescent conformers of a dimer exist, with twisted and planar position of the porphyrin rings, which can interconvert with viscosity dependent rates. 27 Here we tested the molecular rotor properties of 1, a dimer analogous to 2, with excellent biocompatibility properties. 28 The absorption and fluorescence spectra of 1 along with the proposed energy levels diagram are shown in Figure 1b,c.…”

Distortions and Deformations of Metaled Meso-Substituted and Unsubstituted Porphyrins and Derivatives in Crystal Structures

“…Porphyrins show much promise in many areas of molecular engineering and have been suggested as building blocks for many diverse applications such as artificial photosynthesis, novel optical materials, and molecular-scale electronics [1] [2]. Their structures contain an aromatic macrocyclic tetrapyrrole (Figure 1) [3], characterized by three kinds of carbon atoms α, β and meso.…”

“…Thus, few researches had considered the substitution approaches [16] [17]. In addition, most of studies we can find have performed by considering the substituent effects on specific experimental phenomena and generally electronic structure [2] [8] [18] [19]. Crystallographic and molecular studies have been the most used means in the search for their unique properties [6] [20].…”