Article pubs.acs.org/JPCA
Quantum Chemical Investigation of Light-Activated Spin State Change in Pyrene Coupled to Oxoverdazyl Radical Center Tumpa Sadhukhan, Anindya Datta, and Sambhu N. Datta* Department of Chemistry, Indian Institute of Technology−Bombay, Powai, Mumbai 400 076, India
Downloaded by KTH ROYAL INST OF TECHNOLOGY on August 30, 2015 | http://pubs.acs.org Publication Date (Web): August 25, 2015 | doi: 10.1021/acs.jpca.5b06052
S Supporting Information *
ABSTRACT: Low-spin ground states and low-lying excited states of higher spin were investigated for four pyrene oxoverdazyl monoradicals 1−4 and eight pyrene dioxoverdazyl diradicals 5−12. The ground states for quartet and quintet spin symmetries that are in reality excited states were found in the region of 565−775 nm above the respective electronic ground states. We calculated the “adiabatic” magnetic exchange coupling constant in the electronic ground state of each isolated biradical (5−12) by unrestricted density functional theory. A number of hybrid functionals such as B3LYP, PBE0, M06, and M06-2X were used. We also used range-separated functionals such as LC-ωPBE and ωB97XD to compare their effects on the coupling constant and the relative energy of the high-spin state. Molecular geometries were optimized for the doublet and quartet spin states of every monoradical (1−4), and the broken symmetry and triplet solutions were optimized for every biradical (5−12), by systematically using 6-311G, 6-311G(d,p), and 6-311++G(d,p) basis sets with each functional. The geometry of each quintet diradical (5−12) was optimized using 6-311G basis set. B3LYP produced the best spin values. The excited state (quartet or quintet)−ground state energy difference (ΔE) increases in the presence of paraphenylene connectors. These energy differences were predicted here. The nature of spin coupling and consequently the ground state spin agree with spin alternation rule and the calculated atomic spin population. The adiabatic coupling constants were predicted for the biradicals (5−12) in their electronic ground states. Electron paramagnetic resonance parameters were determined at 6-311++G** level for the ground state and the quartet state of 1 and compared with the available experimental data. Low-lying excited states were found for the radical center (oxoverdazyl), pyrene, molecule 1, and diradical 5 by timedependent density functional theory (TDDFT) method using B3LYP hybrid, 6-311++G(d,p) basis set, and the molecular geometry in the electronic ground state. Data from these calculations were used to discuss possible mechanisms for the achievement of the high-spin (excited) states in 1 and 5 and to predict a similar outcome for radicals 2−4 and 6−12 upon excitation. A comprehensive mechanism for the first excitation is proposed here. In particular, we show that the initial excitation of 1 involves large contributions from mixed transitions between pyrene and oxoverdazyl moieties, whereas the initial excitation of 5 is basically that of only the pyrene fragment. Subsequent internal conversion and intersystem crossing are likely to lead to the high-spin states of lower energy. Sample spin-flip TDDFT calculations were also done to confirm the energetic location and composition of the quartet state of 1 and the quintet state of 5.
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INTRODUCTION In recent years, spin alignment in the ground state and that in an excited state of a conjugated organic molecular radical has attracted great attention and become an important topic in the field of molecular magnetism. Although spin polarization dictates the alignment of spin in neutral ground states of organic molecular magnets, external stimulations such as thermal perturbation,1−6 electrochemical redox reactions,7−10 charge doping,11−13 and photoexcitations14−16 have been used for spin manipulation in the ground as well as excited states. Among these techniques, light-induced perturbation has gained relative popularity. On irradiation, the following phenomena can take place: photophysical processes like formation of a high-spin state; redox processes like oxidation or reduction following an electron transfer; and magnetic processes like spincrossover.17,18 One may indeed use a specific excitation to achieve a particular spin alignment. This strategy can be employed in the construction of a magnetic device such as a © XXXX American Chemical Society
quantum computer at the molecular level. Materials obtained from conjugated organic molecules would be useful for this purpose. Compounds with low-lying high-spin excited states can be divided into topologically bound species such as fullerene,19,20 coordination metal complexes such as metalloporphyrins,21−24 and molecules like anthracene, pentacene, and pyrene with strong and extensive π conjugation.25−31 In this work we focus on substituted pyrenes. The conjugated π-systems have several distinct advantages.32 The π-conjugation is accompanied by strong exchange coupling that can generate bulk magnetism at a finite temperature.33 Desired spin systems can be designed by the well-developed synthetic routes of organic chemistry.34 Attachment of radical centers can enhance intersystem crossing Received: June 24, 2015 Revised: August 13, 2015
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DOI: 10.1021/acs.jpca.5b06052 J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
Downloaded by KTH ROYAL INST OF TECHNOLOGY on August 30, 2015 | http://pubs.acs.org Publication Date (Web): August 25, 2015 | doi: 10.1021/acs.jpca.5b06052
The Journal of Physical Chemistry A (ISC), which allows a photoexcited state that is silent in the parent species to be directly detected by electron spin resonance.26,35 The networking of conjugation plays an important role in determining the magnetic characteristics. The nature of coupling between the radical centers can be tuned by changing the magnetic nature of the conjugation network on excitation. Teki et al.25−31 have designed, synthesized, and characterized several conjugated molecules that on excitation by light get into low-lying high-spin states. In such molecules, the conjugated part acts as a photosensitizer and a transient spin carrier (TSC) in the excited state, and the organic radical center is a spin carrier (SC) that may not be fully conjugated and therefore appears as “dangling.” Furthermore, a connector (B) that links the SC with TSC may or may not be present. Teki et al.27 reported the excited high-spin quartet state of a molecule containing pyrene in an apparently triplet (excited) state and a “dangling” oxoverdazyl radical. A ferromagnetic coupling between the pyrene moiety and the monoradical exists in the quartet state. In this work we used pyrene moiety as a TSC, the wellknown oxoverdazyl radical group as the SC, and the paraphenylene group as a possible connector. The chosen connector is of course known as an efficient antiferromagnetic coupler, and provides a smooth spin oscillation between successive atoms starting from one end to the other. The TSC and the oxoverdazyl radical are shown in Figure 1. We selected
Figure 2. Optimized structures of the monoradicals under investigation in their respective ground states at UB3LYP/6-311+ +G(d,p) level. The spin state for each species is indicated in parentheses (D = doublet). The oxo-verdazyl group is not in the plane of pyrene frame, and because of the relatively “free” rotation around the connecting bond it appears to be “dangling”.
mechanism for attaining the quartet state of triradical 1 and the quintet state of tetraradical. Sample spin-flip TDDFT calculations were done to confirm the energetic location and composition of the quartet state of 1 and the quintet state of 5 from laser-induced excitation.
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THEORY AND METHODOLOGY The Heisenberg effective spin Hamiltonian for N unpaired electrons can be written as H = −2J
∑
Si ·Sj (1)
1≤i
