Excitations in oligosilanes with photochromic side groups

Excitations in oligosilanes with photochromic side groups

ARTICLE IN PRESS Journal of Luminescence 112 (2005) 386–390 www.elsevier.com/locate/jlumin Excitations in oligosilanes with photochromic side groups...

194KB Sizes 0 Downloads 44 Views

ARTICLE IN PRESS

Journal of Luminescence 112 (2005) 386–390 www.elsevier.com/locate/jlumin

Excitations in oligosilanes with photochromic side groups P. Tomana,, S. Nesˇ pu˚reka, W. Bartkowiakb, J. Sworakowskib a

Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovsky´ Sq. 2, 162 06 Prague 6, Czech Republic Institute of Physical and Theoretical Chemistry, Wroclaw University of Technology,Wyb. Wyspianskiego 27, 50-370 Wroclaw, Poland

b

Available online 14 October 2004

Abstract A quantum mechanical study of conformations, electronic densities, and absorption spectra of neutral and cationradical hepta[methyl(phenyl)silanes] (7MPSi) substituted with a photochromic spiropyran derivative (SP/MR) was performed. 7MPSi-SP exhibit similar differences between the neutral and cation-radical conformation as unsubstituted 7MPSi, while 7MPSi-MR do not show any significant main chain conformation differences between the neutral and positively charged macromolecule. Population analysis shows that the positive charge in the cation-radical of 7MPSi-SP is located on the main chain, while in the cation-radical of 7MPSi-MR it is located on the photochromic unit. r 2004 Elsevier B.V. All rights reserved. PACS: 36.20.Ey; 36.20.Kd Keywords: Photochromism; Cation-radical; Polaron; Ab initio calculation

1. Introduction A major challenge in the current molecular electronic research lies in finding multistable molecular systems that can act as logical units in molecular-scale devices. Carter [1] proposed the original idea of molecular switch utilizing the tunneling principle. In our previous papers [2,3], we presented an alternative approach. The proCorresponding author. Institute of Macromolecular Chem-

istry, Academy of Sciences of the Czech Republic, Ma´chova 7, 120 00 Prague 2, Czech Republic. Tel.: +420 222511696; fax: +420 222516969. E-mail address: [email protected] (P. Toman).

posed molecular switch consists of a molecular wire (polysilane) with photochromic side units (spiropyran derivatives) attached via a spacer. The charge carrier (hole) transport proceeds predominantly along the s-conjugated silicon backbone with participation of interchain hopping and polaron formation. The on-chain charge carrier mobility is modulated by charge–dipole interactions. In this paper, we investigate the effect of a positive charge introduced on oligo[methyl(phenyl)silane] (OMPSi) macromolecule substituted with a photochromic spiropyran derivative (30 ,30 dimethyl-6-nitro-2,3-dihydrospiro[benzothiazole2,20 -[2 H]chromene]) via a –C(O)–C(H)(OH)– spacer linking the phenyl of the oligomer and the

0022-2313/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2004.09.029

ARTICLE IN PRESS P. Toman et al. / Journal of Luminescence 112 (2005) 386–390

polymers in general, where the effect of polaron formation has been well documented. Furthermore, because of charge relaxation, it is not sure whether the HOMO of the neutral macromolecule is the place where the positive charge of the cationradical will be localized.

indole moiety of spiropyran (cf. Scheme 1a). The resulting changes in the energy and geometry of the macromolecule mimic the polaron formation on an infinite chain of polysilane, and may be considered as an asymptotic (static) approximation of a dynamic problem. Upon irradiation, the photochromic unit undergoes a ring-opening reaction from a closed form (SP) to an open merocyanine form (MR) (see Scheme 1b), whereas in the ground-state potential energy surface, SP has a lower energy than MR, the latter form is energetically favored on the excited-state potential energy surface. The MR–SP reaction is accompanied by changes of the localization of molecular orbitals and charge redistribution resulting in a significant increase of the dipole moment. For neutral macromolecules, HOMO of OMPSi substituted with SP is located on the oligomer main chain and is of the same type as HOMO of the unsubstituted OMPSi. On the other hand, HOMO of OMPSi substituted with MR is located on the substituent. In the latter case, the highest occupied molecular orbital located on the oligomer main chain is HOMO-1. Its oneelectron energy level is shifted down in comparison with the unsubstituted OMPSi by a value comparable with the energy of electrostatic interaction between a unit charge on the main chain and a dipole formed by the substituent MR. This rearrangement of the molecular orbital structure leads to the formation of hole traps on the MR units [3]. The previous calculations were, however, performed for neutral systems only. In other words, any deformation associated with the presence of charged species has been neglected. This is obviously not the case in polysilanes (see, e.g., the discussion in Refs. [4,5]) nor in conjugated

2. Computational details Computations were performed on oligomers containing 7 repeating units with the photochromic substituent SP/MR attached to the middle unit (7MPSi-SP, 7MPSi-MR). The oligomer chains were capped by methyls. The macromolecules were assumed to be isolated. All computations were performed with the Gaussian 98 program [6]. The conformations of macromolecules were optimized by the Hartree–Fock (HF) method at the 3-21G(*) level. For the polaron (cation-radical) calculations, spin-unrestricted wavefunction (UHF) was used. The absorption spectra were calculated by means of semiempirical ZINDO/S method. Transition energies were calculated from configuration interaction where all singly substituted determinants, in a restricted active space of 100 highest occupied and 100 lowest virtual orbitals, were included.

3. Results and discussion 3.1. Conformations The conformations of the neutral and cationradical 7MPSi-SP and 7MPSi-MR were described by means of the Si–Si bond lengths (Fig. 1) and

CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3

Si

Si

Si

Si

Si

Si

Si

H3 C

C O

(a)

δ−

CH3 S

CH3

NO HO C H R

387

R = SP, MR

H S NO2

+

δ

N H3 C

SP

O

H3C

NO2

MR

(b)

Scheme 1. The macromolecule under study (a) and the photochromic reaction of the SP–MR moiety (b).

ARTICLE IN PRESS 388

P. Toman et al. / Journal of Luminescence 112 (2005) 386–390

Si–Si–Si bond angles (Fig. 2) in the main chain. The results were compared with the structure of the oligomer without the photochromic substituent (7MPSi). Our calculations show that in the cation-radical 7MPSi, the Si-Si bonds in the middle of the chain are longer than the corresponding bonds in the neutral oligomer. Simultaneously, Si-Si-Si bond angles are smaller in the cation-radical 7MPSi. Changes of the bond lengths and angles in 7MPSiSP are very similar to those found the in unsubstituted oligomer. On the other hand, the conformations of the main chain of the cationradical and neutral 7MPSi-MR are almost the same. These conformational changes can be explained by means of Mulliken population analysis. The differences of the atomic charges between the cation-radical and neutral oligomers, summed on main chain (Si atoms), phenyls and methyl, and photochromic unit respectively, are shown in Table 1. In case of cation-radical 7MPSi and 7MPSi-SP, the major part of the differential charge is localized on phenyls and methyls. On the other hand, the atomic spin densities were found to be localized predominantly on the Si atoms with the maximum in the middle part of the oligomer. This means that the unpaired electron is

Fig. 2. Si–Si–Si bond angles in the main chain of 7MPSi, 7MPSi-SP, and 7MPSi-MR: (a) cation-radical 7MPSi-MR, (b) neutral 7MPSi-MR, (c) neutral 7MPSi-SP, (d) neutral 7MPSi, (e) cation-radical 7MPSi-SP, and (f) cation-radical 7MPSi.

localized mainly on the silicon chain. However, the introduced positive charge of the cation-radical silicon chain redistributes the electron density among the silicons, and phenyls and methyls by the attraction of the electron pairs forming the Si–C bonds, having no impact on the spin density distribution. This is why the atomic charges on Si atoms are so small. For 7MPSi and 7MPSi-SP, this fact provides an explanation of the Si–Si–Si bond angle differences between the neutral and cation-radical macromolecule. Lower electron density of Si–Si bonds and higher electron density near the Si atoms in the directions of the Si–C bonds lead to a decrease of the Si–Si–Si bond angles. In case of cation-radical 7MPSi-MR, the differential charge as well as spin density are localized almost only on the photochromic unit MR. For this reason, the presence of the hole has no impact on the main chain geometry. 3.2. Absorption spectra

Fig. 1. Si–Si bond lengths in the main chain of 7MPSi, 7MPSiSP, and 7MPSi-MR. Because the values for some macromolecules are almost the same, only 2 curves are given: (a) for cation-radical 7MPSi and cation-radical 7MPSi-SP, (b) for cation-radical 7MPSi-MR and all neutral macromolecules.

The conformational differences between neutral and cation-radical macromolecules are also reflected in the absorption spectra that are the important characteristics of the materials perspective for the molecular switch construction. The computed absorption spectra are shown in Fig. 3.

ARTICLE IN PRESS P. Toman et al. / Journal of Luminescence 112 (2005) 386–390

389

Table 1 Differences of the Mulliken atomic charges between the cation-radical and neutral oligomers and atomic spin densities of the cationradicals Part of the macromolecule

Si atoms Phenyls+methyls SP/MR

Differences of the atomic charges

Atomic spin densities

7MPSi

7MPSi-SP

7MPSi-MR

7MPSi

7MPSi-SP

7MPSi-MR

0.17 0.83 —

0.18 0.76 0.06

0.01 0.03 0.96

0.94 0.06 —

0.94 0.05 0.01

0.00 0.00 1.00

All values are summed on Si atoms, phenyls and methyls, and photochromic unit SP/MR, respectively. Values are given in fractions of the unit charge +e.

Fig. 3. Computed absorption spectra of the neutral and cation-radical 7MPSi-SP and 7MPSi-MR.

The main absorption transitions in the spectrum of the cation-radical 7MPSi-SP correspond to the transitions in the unsubstituted cation-radical 7MPSi spectrum (374 and 430 nm) and in the stand alone neutral SP (321 and 393 nm). The spectrum of the cation-radical 7MPSi-SP also contains several peaks in the near infrared region

(1006, 1254, and 1715 nm), corresponding to the transitions from the main chain valence band to the polaron level in the gap. The main transitions in the cation-radical 7MPSi-MR spectrum lie below 350 nm, in analogy to the transitions in the neutral 7MPSi and in the standalone MR cation-radical. Contrary to the

ARTICLE IN PRESS 390

P. Toman et al. / Journal of Luminescence 112 (2005) 386–390

neutral macromolecule, the cation-radical 7MPSiMR has no absorption around 450 nm. This information is important for the molecular switch philosophy because it probably means that only neutral 7MPSi-MR can undergo the reverse photochromic reaction initiated by a light pulse. It seems that MR can be bleached by the cationradical formation, e.g. by hole injection.

4. Conclusions The results of the ab initio calculations show that the charge in cation-radical of 7MPSi-SP is located on the main chain, while it is located on the photochromic unit in the cation-radical of 7MPSi-MR. The differences in the conformations of the neutral and cation-radical 7MPSi-SP are very similar to those in the unsubstituted 7MPSi. The neutral as well as cation-radical 7MPSi-MR conformations possess the same main chain geometry as the neutral 7MPSi. Absorption spectrum of the cation-radical 7MPSi-MR does not involve the transition around 450 nm, i.e., around the wavelength which is used to drive the reverse photochromic reaction of the neutral MR macromolecule.

Acknowledgements This work was supported by Project no. 203/02/ D074 of the Grant Agency of the Czech Republic, by Projects nos. 49 and 50 (Czech–Polish cooperation) of the Ministry of Education, Youth, and Sports, and by the Polish Committee for Scientific Research (Grant No. T09A 132 22, and the Cooperation Grants 18/2004/CZ and 19/2004/ CZ). The computer time in MetaCenter (Prague and Brno) and in Joint Supercomputing Center (Czech Technical University, Prague) is gratefully acknowledged. References [1] F.L. Carter, Conformational switching at the molecular level, in: F.L. Carter (Ed.), Molecular Electronic Devices, M. Dekker, New York, 1982, p. 51. [2] S. Nesˇ pu˚rek, P. Toman, J. Sworakowski, Thin Solid Films 438–439C (2003) 268. [3] P. Toman, W. Bartkowiak, J. Sworakowski, S. Nesˇ pu˚rek, R. Zales´ ny, in preparation. [4] H. Ba¨ssler, P.M. Borsenberger, R.J. Perry, J. Polym. Sci. Polym. Phys. 32 (1994) 1677. [5] S. Nesˇ pu˚rek, J. Sworakowski, A. Kadashchuk, IEEE Trans. Diel. Electr. Insul. 8 (2001) 422. [6] M.J. Frisch, et al., Computer program Gaussian 98, Revision A.7, Gaussian, Inc., Pittsburgh PA, 1998.