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.
pedrojavier.castro@urv.cat
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.
No EPR
signal
EPR signal
No EPR
signal
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
CODECS 2013 Workshop. San Lorenzo de El Escorial, Madrid, 18th –22nd April, 2013
(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)