Time-resolved EPR and ENDOR study of the photoexcited triplet state of free-base tetraphenylchlorin in a crystalline toluene matrix †

Abstract

A time-resolved EPR and ENDOR study of the photoexcited triplet state of the free-base tetraphenylchlorin has been made in a polycrystalline toluene matrix at 120 K. Crystallization of the toluene results in a partially aligned sample. The nature of the orientation of the solute molecules is investigated by timeresolved EPR spectroscopy using the anisotropy of the zero-field splitting tensor of the triplet state as the observable parameter. It is determined that 55% of the triplet molecules are oriented in a single crystal-like domain with the triplet z-axes oriented within 15Њ of the magnetic field. In the ENDOR study selective excitation, of only those molecules which have their triplet z-axes parallel to the magnetic field, has permitted the measurement of the A zz component of the hyperfine coupling tensor of protons, in the reference frame of the zero-field splitting tensor. The sign and magnitude of the matrix couplings are also determined. The use of the partially oriented sample drastically enhanced the signal intensity over that achieved in a randomly oriented sample by increasing the number of molecules with their triplet z-axes parallel to the magnetic field. Additionally, time-resolved ENDOR spectroscopy allowed the hyperfine interactions to be determined at far higher temperatures than usual for the study of triplet states.

The Role of the Electronic Structure of the Porphyrin as Viewed by EPR/ENDOR Methods in the Efficiency of Biomimetic Model Compounds for Photosynthesis

“…The triplet state contains the same number of electrons as the excited singlet state and it is paramagnetic, with an unpaired electron in the HOMO and one in the LUMO, as shown in Figure 3. The hfcs of the triplet state can be determined by EN-DOR, as shown by Kay et al [36] Unfortunately, at present, the assignment of hfcs to specific molecular positions has turned out to be an even more difficult task than for the cation and anion radicals described above, which complicates the interpretation of the data. The hfcs of the triplet state can be determined by EN-DOR, as shown by Kay et al [36] Unfortunately, at present, the assignment of hfcs to specific molecular positions has turned out to be an even more difficult task than for the cation and anion radicals described above, which complicates the interpretation of the data.…”

Pentacene in 1,3,5-Tri(1-naphtyl)benzene: A Novel Standard for Transient EPR Spectroscopy at Room Temperature

“…Thus, sample preparation only requires that the two components are mixed together in the desired ratio, melted using a heat gun or oil bath under anaerobic conditions, and then cooled back down to room temperature. Because the shortrange order of the liquid state is maintained in glassy samples, not only is the sample optically transparent but the solute molecules do not become aligned; as typically occurs in crystalline matrices [15,16].…”

Twenty years of biophysics of photosynthesis in Padova, Italy (1984–2005): A tale of two brothers

“…She had also worked in Klaus Mö bius laboratory under the above-mentioned European project. Here, the first time resolved direct-detection ENDOR spectrum of the P680 triplet was obtained among other interesting results on in vitro systems (Di Valentin et al 1996, Kay et al 1997. Marilena was fond of looking at synthetic systems and GG had a great interest in what he used to call the Boltzmann dream 7 , the hope that chemists would sometime, in the future, compete with Nature to synthesize sugars with the help of light.…”