CODECS 2013 Workshop. San Lorenzo de El Escorial, Madrid, 18th –22nd April, 2013

Computational study on the mechanism of population of the triplet species responsible of the photomagnetism in Biindenylidenediones (BIDs)

Pedro J. Castro and Mar Reguero

Departament de Química Física i Inorgànica, Universitat Rovira i Virgili, C/Marcel·lí Domingo s/n, Tarragona 43007.

Photochromism is a phenomenon that takes place when a compound irradiated with UV produces a photoproduct with different absorption spectra. Organic photochromic compounds are molecules with considerable interest because they are expected to offer routes to new functional materials that take advantage of changes induced by irradiation. The interest is increasing particularly in those materials in which irradiation can also induce magnetic properties, because they are of particular interest in the generation of new technologies such as photoregulated devices for use in electronic technology, molecular switches, optical memories, metal detectors, etc. Photochromism is usually observed in solution, and in most of the photochromic reactions the mechanism involves a non-adiabatic singlet to singlet population transfer. Nevertheless, biindenylidenediones (BID) are photochromic organic compounds that show magnetic properties switchable with irradiation, but only in crystalline state. Different experimental groups have synthesized and studied different subgroups of the BID family.[1-4] J. Meng observed that the starting yellow crystals of the BID changed to reddish-brown crystals without considerable structural changes.[1] These brown crystals show an EPR signal after irradiation, that disappears when the crystals are cooled with liquid nitrogen and reappears when heated again,[1,2] as schematized in the Figure 1.

Fig. 1. Experimental fact observed by Meng’s group.

This observation indicates that there must exist an excited state with high multiplicity, possibly a triplet, which is populated after irradiation and that is stable enough to give place to the EPR signal. There are several hypotheses about the species that originates the EPR signal[2,4], and the controversy continues while the mechanism of population and depopulation of this state has not yet been clarified.

The aim of this work is to elucidate computationally the geometrical and electronic structure on the intermediates and the mechanism of the processes that explain the observed behaviour. In order to do it we have used a strategy that combines several multiconfigurational methods such as CASSCF CASPT2 and IDDCI[5,6] . We have applied the Davidson correction[7] (DC) to decrease the size-extensivity error. Given the high computational requirements of these methods, we have used a model (shown in Figure 2) to perform this study, and check its validity by comparison with IDDCI+DC calculations on the Franck-Condon geometry for full system (Figure 2).

Fig. 2. Model and Full system studied.

We have explored the potential energy surfaces of the lowest singlet and triplet excited states which are of (no→π*) and (π→π*) nature. Our results show that the lowest 1Au state, of (π→π*) character, is the one populated after the initial absorption. However, other singlet and triplet states are also low in energy and cross the initially populated state along its relaxation path. Consequently interconversion between singlet and triplet states is possible and EPR signal is predicted when a stable triplet minimum is populated. The computational results can explain satisfactorily in this way the experimental observations.

References

1.  L. Xu, T. Sugiyama, H. Huang, Z. Song, J. Meng and T. Matsuura. ChemComm. 2328-2329, (2002).

2.  J. Han and J. Meng. Journal of Photochem. And Photobiol. C: Photochemistry 10, 141-147 (2009).

3.  K. Tanaka, and F. Toda. J. Chem. Soc. Perkin Trans. 1, 873-874 (2000).

4.  K. Fujii, K. Aruga, A. Sekine, H. Uekusa, K. Sohno and K. Tanaka. CrystEngComm. 13, 731 (2011).

5.  J. Miralles, O. Castell, R. Caballol and J. P. Malrieu. Chem. Phys., 172, 33 (1993)

6.  V. M. Garcia, O. Castell, R. Caballol and J. P. Malrieu, Chem. Phys. Lett. 238, 222 (1995)

7.  (a) S. R: Langhoff and E. R. Davidson, Intl. J. Quantum Chem. 8, 61 (1974). (b) J. Cabrero, R. Caballol and J. P. Malrieu, Molecular Physics 100, 919-926 (2002)