Electron spin echo modulation of the photoexcited triplets of anthracene in p-terphenyl crystals

Abstract

Electron spin echo envelope modulations (ESEEM) of photoexcited anthracene in p-terphenyl crystals at room temperature are reported. Sources of contribution to the observed ESEEM have been established. The hyperfine tensor elements of the anthracene triplet have been determined and compared with the results of the previous ENDOR study.excitation source was the third harmonic of a Nd:YAG laser (X = 355 nm) with 30-mJ maximum energy. The FFT program is a Nicolet NMR software package. Electron Spin Echo Envelope Modulations and ENDOR Frequencies Below we shall present only the results of two-pulse (tt/2 -t -) experiments. For a one nucleus (I = 1/2) system,

Photoexcited triplet states of twisted acenes investigated by Electron Paramagnetic Resonance

“…5). The experimental hyperfine coupling values of about À9.6 MHz along the ZFS X axis and of À7.7 MHz along the out-of-plane Z axis observed for the twisted compounds are relatively close to the literature values of À10.512 MHz/À10.22 MHz and À8.643 MHz/À8.69 MHz determined for the anthracene triplet based on the ENDOR 26 and ESEEM 35 studies, respectively (see Fig. S8 in the ESI † for a direct comparison).…”

“…In the anthracene triplet state, the two types of b protons are almost equivalent, with out-of-plane hyperfine couplings of À4.689 MHz 26 /À4.65 MHz 35 and in-plane couplings of 1.34 MHz along X and À5.14 MHz along Y that could only be determined in the ESEEM study. 35 The peak at À4.4 MHz in the ENDOR spectrum of the open reference compound recorded at the Z À field position can therefore be assigned to the b protons, which also appear to be essentially equivalent here. However, as the twist angle increases in the Ant-Cn series, the single peak splits into two, one with a slightly larger hyperfine coupling and one with a smaller coupling.…”

“…The hyperfine couplings of the photoexcited triplet state were investigated independently by two groups: Clarke and Hutchison performed ENDOR measurements on anthracene in a phenazine single crystal 26 and Yu et al used ESEEM (Electron Spin Echo Envelope Modulation) spectroscopy to determine the proton hyperfine couplings for anthracene in p-terphenyl crystals. 35 The largest hyperfine couplings in Ant-Cn are expected to be due to the a protons with the largest contribution along the long in-plane axis, X, a similarly large contribution along the out-of-plane orientation, Z, and a much smaller contribution along the short in-plane axis Y (see Fig. 5).…”

Electron spin echo spectroscopy of organic triplets

“…31 An ENDOR study of anthracene triplets in a phenazine crystal at liquid helium temperatures has been reported in ref 29. We performed our ESE studies of anthracene triplets in p-terphenyl crystals at room temperature.20, 21 The studies enable us to measure ENDOR frequencies from the FFT of ESEEM which can be used to evaluate the HF tensor elements and the spin density of anthracene triplets. Our studies were intended to make a comparative test of the capability of the ESE technique to map ENDOR frequencies of triplets at room temperature.…”

Time-resolved ESR of intersystem crossing and energy transfer processes

“…Furthermore, they applied this technique to other orthodiaza-aromatics which were known to be non-phosphorescent and discussed the 3 * 3 * strong vibronic coupling between the ~,~ and n,~ states [31]. Electron spin echo envelope modulation is also a powerful method to determine hyperfine tensors of photoexcited triplet states [32][33][34][35]. Bos et al [36] applied this technique to the pyridine triplet, which is also non-phosphorescent, and 3 * determined that the triplet state has strong n,~ character.…”