Abstract. We have calculated solvent effects on the zero- field splitting (ZFS) constants induced by electron spin– spin coupling (SSC) in the low-lying triplet states of azaaromatic molecules in solutions using multiconfigu- ration self-consistent-field wave functions and the polarizable continuum model. The second-order spin– orbit coupling (SOC) contribution to the splitting of the 3 pp  states is found to be almost negligible, and the calculations therefore provide a good estimate of the ZFS parameters and their solvent dependence based only on the electron spin–spin coupling expectation values. The correlation between the shift in the ZFS and the phosphorescence frequency that has been observed in optically detected magnetic resonance experiments in low-temperature glasses is supported by our direct SSC calculations without taking SOC into account. This makes it possible to distinguish between the two theories that earlier were proposed to explain the inhomogeneous broadening of triplet state spectra, and discard the one that is exclusively based on the SOC-induced mixing of the singlet and triplet states. Keywords: Zero-field splitting – Solvent effects – Triplet spin label – Optically detected magnetic resonance – Electron spin–spin coupling 1 Introduction Electron paramagnetic resonance (EPR) spectroscopy has found numerous applications to biomedical and biochemical problems as a sensitive tool for the detec- tion of free radicals and other paramagnetic species [1, 2]. This applies to the natural occurrence of free-radical intermediates in metabolic processes, to the observation of stable transition-metal ions and to the analysis of paramagnetic probes introduced into biosystems. The success of the spin-probe technique is determined by the ability of the environment to influence the EPR spec- trum of the probe. It is therefore important to under- stand how the probe–substrate interaction can modify the parameters of the spin Hamiltonian. A number of ab initio calculations have been carried out in order to simulate the effects on the EPR parameters (g-factor and hyperfine constants) arising from the interaction between the unpaired spin of a free radical and a diamagnetic environment, see for example Refs. [3, 4, 5]. In this paper we are going to investigate the solvent dependence of the parameters of the spin Hamiltonian for triplet excited-state spin labels. Triplet spin labels are often used in a form of nitr- oxide biradicals [1] and also as natural constituents of biopolymers that contain chromophores which can be excited to the triplet state [6, 7, 8]. von Schutz et al. [6] observed optical detection of magnetic resonance (ODMR) for tryptophan phosphorescence in horse liver alcohol dehydrogenase and in hen egg-white lysozyme. The ODMR technique was used by Alfredson and Maki [7, 8] to investigate the effects of complex formation between DNAs and several species of Streptomyces antibiotics. These species contain two quinoxaline [7] and two quinoline moieties [8] that are attached by peptide linkages to the depsipeptide ring through a pair of serine residues. Among other natural products of Streptomyces the quinomycins (echinomycin and its bisquinolone analogues) were studied. Complexation with DNAs was found to influence the triplet state zero- field splitting (ZFS) of the phosphorescent quinoxaline and quinoline residues [7, 8]. It has been shown that the ODMR signals of organic molecules hosted as a dilute impurity in glassy or polycrystalline guests at low tem- peratures are strongly inhomogeneously broadened [6, 9, 10] as are the optical spectral lines [11]. The relative magnitude of the line broadening, Dm/m, is of the same order of magnitude for both types of spectra [9, 12]. In the excited triplet states of polyatomic molecules the ZFS operator H S (the effective spin Hamiltonian) is given by [24] Contribution to the Jacopo Tomasi Honorary Issue Correspondence to: Hans A ˚ gren e-mail: agren@theochem.kth.se Regular article Solvent effects on optically detected magnetic resonance in triplet spin labels Boris Minaev 1 , Oleksandr Loboda 1 , Olav Vahtras 1 , Kenneth Ruud 2 , Hans A ˚ gren 1 1 Laboratory of Theoretical Chemistry, Department of Biotechnology, SCFAB, The Royal Institute of Technology, 10691, Sweden 2 Department of Chemistry, University of Tromsø, 9037 Tromsø, Norway Received: 20 December 2002 / Accepted: 30 April 2003 / Published online: 30 January 2004 Ó Springer-Verlag 2004 Theor Chem Acc (2004) 111:168–175 DOI 10.1007/s00214-003-0532-5