The hyperfine tensors of the triplet states of several guest donor molecules in naphthaleneitetracyanobenzene host crystals have been measured by EPR s~ctroscopy. From these data the degree of charge transfer (6') in the excited state can be derived. in a straightforward way. Whereas anthracene yieIds values almost identical with those characteristic for the isolated molecule (b' c O.OS), phenanthrene shows a significant degree af charge transfer in the triplet state (b* = 0.47).
1. Zxtroduction
Although considerable progress has recently been made in the understanding of the optical, photoelectrical, and magnetic properties of weak charge-transfer (CT) complexes, there is stih some uncertainty about the amount of CT in the excited states. This Iack of detailed understanding is due partly to the lack of specific experimental data, particularly on excited CT-triplet states. For instance, Stark effect studies ofopticaI spectra which have provided direct measurements of the CT-character in excited CT-singlet states [ l] , have not yet been reported for the corresponding triplet states. For the CT-triplet states unequivocal evidence for the electron-transfer has been deduced mainly from EPR measurements of the fine structure parameters D and c [2][3][4]. This method is based on the fact that the absolute value of D reflects the average dipolar interacticn of the two unpaired electrons forming the triplet state. Hence, the D value is reduced significantly when these two electrons are partially delocalized over both the donor and the acceptor molecules. However, it is known, [3,5,6] that there are difficulties in relating the measured value of D to the-amount of CT-char-* IBM Postdoctord FeUow'(1974-75). *? Reprint request to this guthor.