Density Functional Theory Prediction of Enhanced Photomagnetic Properties of Nitronyl Nitroxide and Imino Nitroxide Diradicals with Substituded Dihydropyrene Couplers Md. Ehesan Ali and Sambhu N. Datta* Department of Chemistry, Indian Institute of Technology-Bombay, Powai, Mumbai, Maharashtra, India 400076 ReceiVed: August 1, 2006; In Final Form: August 9, 2006 We predict the photoswitching magnetic properties of four substituted dihydropyrenes from density functional broken-symmetry calculations. The magnetic exchange coupling constants differ up to 9.44 cm -1 . The intramolecular exchange interactions are ferromagnetic in nature. The calculated coupling constants are much larger than those reported earlier for photomagnetic organic molecules. Photochromism is the reversible photon-induced transition of a chemical species between two different forms having different absorption spectra. Photochromic materials change their geometries and physical properties with irradiation. They are useful in potential photoswitching. If a photoswitchable molecule is used as a spin coupler between two magnetic units, the magnetism of the resulting species can change upon irradiation. 1 Perfluorocyclopentene is one of the widely studied photo- chromic spin couplers. Matsuda et al. have synthesized a large number of nitronyl nitroxide diradicals with perfluorocyclo- pentene. 1 In these diradicals, the intramolecular exchange interaction is very weak, and the coupling constant J is of the order of the hyperfine coupling constant (hfcc). The J value differs nearly 150-fold between the open- and closed-ring isomers. Its absolute magnitude is generally found to be <10 -3 cm -1 for open ring isomers and ∼10 -2 cm -1 for closed ring isomers except for NN diradicals with 1,2-bis(2-methyl-1- benzothiophene-3-yl)perfluorocyclopentene where J equals -0.76 and -4.03 cm -1 , respectively. 1b,d,g As |J| is very small in the ground state, the photomagnetic properties of these molecules are not expected to find a great usage. This has led to the investigation of photoexcited states of diradicals. Teki et al. have investigated the magnetic properties of excited states of nitronyl nitroxide diradicals with diphenylanthracene coupler. 2 Huai et al. have also investigated similar excited states by theoretical means. 3 The substituted pyrene molecule exists in two different forms, namely, cyclophanediene (CPD) and dihydropyrene (DDP), as shown in Figure 1. 4 The restricted lifetime of CPD limits the utility of these molecules. The thermal return of CPD to DDP belongs to the category of Woodward-Hoffmann orbital symmetry forbidden processes. Nevertheless, the barrier created from the correlation of the occupied reactant orbitals with the virtual product orbitals and vice versa is not too high for CPD f DDP conversion. Recently, Williams et al. have found that proper substitutions can increase the activation barrier to hinder the thermal conversion. 5 In this work, we have investigated the ground-state photo- magnetic properties of nitronyl nitroxide diradicals and imino- nitroxide diradicals with substituted pyrene couplers. The four sets of diradicals are illustrated in Figure 2. One novelty of this work is in the choice of the coupler. To our knowledge, these molecules have not been synthesized so far. The J values of the isomers differ by 4.7-9.6 times for each pair. The magnitude of J for the closed ring isomers is significantly large, which constitutes the second novelty. Besides, the points of attachment of the NN and IN groups are decided from the rule of spin alternation 6 such that the resulting diradicals are ferromagnetic in nature. The theoretical evaluation of the magnetic exchange coupling constant has been performed using broken-symmetry (BS) density functional (DFT) methodology proposed by Noodle- man. 7 The exchange interaction J between two magnetic sites 1 and 2 is normally expressed by the Heisenberg effective spin Hamiltonian H ˆ)-2JS ˆ 1 ‚S ˆ 2 where S ˆ 1 and S ˆ 2 are the respective spin angular momentum operators. A spin-polarized, unrestricted formalism and a broken-symmetry solution is needed for the lowest spin state in the BS method. 8 The BS state is not an * Corresponding author. E-mail: sndatta@chem.iitb.ac.in. Figure 1. Conversion of CPD to DDP. Figure 2. Diradicals under investigation. 10525 2006, 110, 10525-10527 Published on Web 08/22/2006 10.1021/jp064913c CCC: $33.50 © 2006 American Chemical Society