- Email: [email protected]
Site effects on the electronic relaxation of aromatic molecules in van der Waals solids
Journal of Luminescence 94–95 (2001) 457–460
Site effects on the electronic relaxation of aromatic molecules in van der Waals solids C. Cre! pina,*, C. Ge! ea, A. Cuisseta, L. Divaya, A. Tramera, P. de Pujob a
! # Laboratoire de PhotoPhysique Moleculaire -CNRS, Bat.210, Universite! Paris-Sud, 91405 Orsay Cedex, France ! Laboratoire de Chimie Theorique, DSM/DRECAM/SPAM, CEA-Saclay, 91191 Gif sur Yvette Cedex, France
b
Abstract Fluorescence and phosphorescence of naphthalene and aniline molecules trapped in cryogenic matrices present sitedependent features. The isolation in solid argon has been especially investigated. Experimental results for naphthalene are summarized and related to the site geometries obtained from molecular dynamics calculations. In the case of aniline, site effects are mainly revealed through the phosphorescence spectra. In particular sites, the measurement of the inversion frequency can be directly extracted. r 2001 Elsevier Science B.V. All rights reserved. Keywords: UV luminescence; van der Waals solids; Naphthalene; Aniline
1. Introduction Rare gas and inert molecules such as nitrogen are usual host materials in the matrix isolation spectroscopy. They do not react with the trapped molecules, the main molecular properties are preserved. Nevertheless, there is always an environment effect on the guest species, as the usual frequency shift in the electronic transition between the gas phase and the condensed phase. In the host lattice, large aromatic molecules are trapped in different cavities wherefrom several host atoms or molecules are removed. The structure of such cavities cannot be deduced from experiment but may be determined by simulation of deposition of *Corresponding author. Tel.: +33-1691-57539; fax: +331691-56777. E-mail address: [email protected] (C. Cr!epin).
guest–host mixtures. The S1 -S0 fluorescence and the T1 -S0 phosphorescence may be recorded upon a selective S1 ’S0 excitation of different sites by a tunable dye laser. We have performed such experiments on naphthalene (C10 H8 ) and aniline (C6 H5 NH2 ) isolated in van der Waals solids [1,2]. Naphthalene was chosen because of the peculiar properties of its first excited states: the S1 2S0 transition is very weakly allowed ð f B104 [3]) and the efficiency of the S1 *T1 intersystem crossing is very low in gas phase [4]. An environment effect on the S1 electronic relaxation was then expected. Aniline exhibits a large amplitude motion in its ground state: the wagging of the NH2 group up and down the aromatic plane [5]. This vibrational mode, the inversion mode I, is involved in the tunneling effect between the two equivalent bent configurations. The influence of the environment on such a vibrational mode can be deduced from the fluorescence and phosphorescence spectra.
0022-2313/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 0 1 ) 0 0 3 3 5 - 0
458
! C. Crepin et al. / Journal of Luminescence 94–95 (2001) 457–460
Experimental results on naphthalene and their interpretation were described elsewhere [1,6]. The detailed study of aniline in argon matrices will be also reported in Ref. [2]. This paper points out the site effects observed in argon matrices for both molecules.
1.0 0.8
A-sites
0.6 0.4 0.2
Several parameters in fluorescence and phosphorescence spectra of the naphthalene molecule in Ar matrices depend on the excitation frequency. This may be explained by the presence of two main families of sites with excitation spectra corresponding to the red and blue wings of the S1 ’S0 vibronic bands, noted A-sites and B-sites, respectively [1]. The singlet and triplet emission frequencies are red-shifted in the A-sites in comparison with the B-sites emission. More surprising, the intensity distribution between ‘‘allowed’’ (in Condon approximation) and ‘‘forbidden’’ (induced by the Herzberg–Teller mechanism) [7–9] vibronic bands in the S1 -S0 transition is also sitedependent. The environment effect on both (Condon and Herzberg–Teller) transition probabilities depends on the site structure. The intersystem crossing S1 *T1 rate kISC is modified in the solid, but the enhancement of the kISC rate is more pronounced in the A-sites ðkISC ¼ 38 106 s1 ) than in the B-sites ðkISC ¼ 8 106 s1 ), the gas phase value being kISC ¼ 2:45 106 s1 [4]. At last, the widths of vibronic bands in the phosphorescence spectra depend on the family of sites as shown in Fig. 1. These features have been associated with different geometries of sites obtained by a molecular dynamic simulation of the matrix isolation of naphthalene [6]: the molecule is found to be contained either in the f0 0 1g or the f1 1 1g crystallographic plane of the argon lattice. The first configuration, corresponding to more cramped sites, was assigned to the A-sites where the molecule–lattice interaction, inducing an increase of kISC ; is more important. On the other hand, the sharpness of the phosphorescence bands in the A-sites indicates that the site selection in S1 2S0 transition frequencies is equivalent in this case with a site selection in T1 2S0 transition
intensity (a.u.)
2. Naphthalene in Ar matrices 0.0 21000
20000
19000
1.0
B-sites
0.8 0.6 0.4 0.2 0.0 21000
20000
19000
-1
cm
Fig. 1. Naphthalene phosphorescence spectra in argon matrix at T ¼ 10 K: Upper spectrum (strong intensity): A-sites excitation (nlaser ¼ 32 295 cm1 ). Lower spectrum (weak intensity): B-sites excitation (nlaser ¼ 32 335 cm1 ), narrow bands are due to the simultaneous excitation of the wing of the A-sites absorption.
frequencies. This is also correlated with betterdefined and more rigid structures obtained when the molecule is trapped in a f0 0 1g plane.
3. Aniline in Ar matrices As previously, we distinguish two main families of sites in the aniline/argon system. In this case, these families have a clear spectral signature in the S1 ’S0 absorption: instead of inhomogeneously broadened vibronic bands in the absorption spectrum of naphthalene, the aniline spectrum exhibits two narrow lines, corresponding to the zero-phonon lines of each family of sites, with a broad phonon side band (Fig. 2) [2]. The electron
! C. Crepin et al. / Journal of Luminescence 94–95 (2001) 457–460
Aniline /Ar 1.0
1
10
0
1
1
00
6a 0
120
B-sites
O. D.
phonons 0.5
A-sites
0.0
35100
34600
34100
33600
0.2
O. D.
8b1g 0.1
1
7b1g0
1
Naphthalene/Ar
0
B-sites A-sites 1
8a1g0
0
00
0.0
33000
32500
-1
32000
31500
cm
Fig. 2. Origin region of aniline and naphthalene S1 ’S0 absorption spectra in argon matrix at T ¼ 10 K
density distribution is deeply modified in the S0 –S1 transition so that the geometry of the molecule is changed [10] (aniline is bent in the S0 state and quasi-planar in the S1 state). A strong electron– phonon coupling is then involved in the S1 2S0 transition. As in absorption, the site selected fluorescence spectra present intense phonon bands on each vibronic transition. As in the case of naphthalene, we observed a site-dependent enhancement of the intersystem crossing efficiency. The singlet lifetimes are very short and our experimental setup allows only a rough evaluation of the non-radiative rate: kISC B9 108 s1 in the A-sites and kISC B4 108 s1 in the B-sites as compared to kISC ¼ 0:8 108 s1 in gas phase [11]. The most important site effects occur in the phosphorescence spectra (Fig. 3), especially in the vibronic intensity distribution. This emission involves vibrational modes of two different symmetries (totally symmetric in-plane a1 modes and outof-plane b1 modes in the C2v symmetry group of
459
the molecule). The presence of out-of-plane modes suggests two different mechanisms of intensity borrowing by spin–orbit and spin–orbit–vibronic singlet–triplet coupling. If most of these modes appear in both site-selected emission, they exhibit different intensities depending on the excited site. The vibrational structure is more clearly observed in B-sites than in A-sites. In particular, the origin part of the spectrum is composed of two well resolved ‘‘doublets’’. Experiments on deutered species (C6 H5 ND2 and C6 H5 NHD) confirmed that the first doublet can be assigned to the 0þ -0þ ð000 Þ and 0þ -0 ðI01 ) transition, where 0þ ð0 ) represents the symmetric (antisymmetric) vibrational level in the inversion mode [12]. A direct evaluation of the inversion frequency is then obtained in the B-sites: Dð0þ 0 Þ ¼ 46 cm1 for the hydrogenated species as compared to 41 cm1 in the free molecule. The second doublet involves the 10b out-of-plane mode and is assigned to the 10b01 and 10b01 I01 vibronic transitions. As the inversion mode is a b1 mode, the mode symmetries are a1 and b1 in the decreasing energy order in the first doublet whereas they are b1 and a1 in the second one. The intensity ratio of the two components are inverted from one doublet to the other, in correlation with symmetry considerations. Fig. 3 shows that these ratios are also different in the A-sites. As in the case of the naphthalene/argon system, if we assume that the aromatic plane of aniline is embedded either in a f0 0 1g or in a f1 1 1g crystallographic plane of the argon lattice, the A-sites (resp. B-sites) are assigned to the first (resp. second) case following the experimental results on the intersystem crossing efficiency. Then, the C2v molecular symmetry group is probably reduced to the Cs group in the B-sites because of the close non-symmetric argon planes above and below the guest molecule, which break down the aromatic plane symmetry. If it is so, the different site symmetries could influence the vibronic intensity distribution in the phosphorescence spectra. 4. Conclusion These two examples show the importance of the close molecular neighborhood in the electronic
! C. Crepin et al. / Journal of Luminescence 94–95 (2001) 457–460
460
phosphorescence B-sites νlaser = 33950 cm-1
phosphorescence (arbitrary unit)
10000
5000
0 28000
27500
27000
26500
26000
25500
25000
6000 4000 phosphorescence A-sites νlaser = 33920 cm-1
2000 0 28000
27500
27000
26500
26000
25500
25000
-1
wavenumber (cm)
Fig. 3. Aniline phosphorescence spectra in argon matrix at T ¼ 10 K: The upper spectrum is the phosphorescence induced by laser excitation on B-sites (nlaser ¼ 33 950 cm1 ) of the 000 transition-bold labels: b1 vibronic modes, other labels: a1 vibronic modes-. The lower one is a phosphorescence induced by laser excitation on A-sites (nlaser ¼ 33 920 cm1 ).
relaxation dynamic. Even in an inert material, beyond a host effect, a site effect is clearly observed. Simulations on the naphthalene/argon system have underlined the importance of the repulsive forces between the guest and the host lattice which are only weakly locally perturbed [6]. The study of aniline is still in progress. First experiments in other rare gas matrices exhibit different phosphorescence spectra, showing the great influence of the environment on this forbidden transition.
References [1] C. Cr!epin, A. Tramer, Chem. Phys. 272 (2001) 227. [2] C. G!ee, C. Cr!epin, A. Cuisset, L. Divay, in preparation.
[3] M. Rubio, M. Merchan, E. Orti, B.O. Roos, J. Chem. Phys. 179 (1994) 395. [4] M. Stockburger, H. Gatterman, W. Klusman, J. Chem. Phys. 63 (1975) 4529. [5] N.W. Larsen, E. Hansen, F. Nicolaisen, Chem. Phys. Lett. 43 (1976) 584. [6] C. Cr!epin, P. de Pujo, B. Bouvier, V. Brenner, Ph. Milli!e, Chem. Phys. 272 (2001) 243. [7] M. Stockburger, H. Gatterman, W. Klusman, J. Chem. Phys. 63 (1975) 4519. [8] G. Holneicher, J. Wolf, Ber. Bunsenges., Phys. Chem. 99 (1995) 366. [9] F. Negri, M.Z. Zgierski, J. Chem. Phys. 104 (1996) 3486. [10] J.M. Hollas, M.R. Howson, T. Ridley, L. Halonen, Chem. Phys. Lett. 98 (1983) 611. [11] R. Scheps, D. Florida, S. Rice, J. Chem. Phys. 61 (1974) 1730. [12] C. G!ee, C. Cr!epin, private communication.
Site effects on the electronic relaxation of aromatic molecules in van der Waals solids C. Cre! pina,*, C. Ge! ea, A. Cuisseta, L. Divaya, A. Tramera, P. de Pujob a
! # Laboratoire de PhotoPhysique Moleculaire -CNRS, Bat.210, Universite! Paris-Sud, 91405 Orsay Cedex, France ! Laboratoire de Chimie Theorique, DSM/DRECAM/SPAM, CEA-Saclay, 91191 Gif sur Yvette Cedex, France
b
Abstract Fluorescence and phosphorescence of naphthalene and aniline molecules trapped in cryogenic matrices present sitedependent features. The isolation in solid argon has been especially investigated. Experimental results for naphthalene are summarized and related to the site geometries obtained from molecular dynamics calculations. In the case of aniline, site effects are mainly revealed through the phosphorescence spectra. In particular sites, the measurement of the inversion frequency can be directly extracted. r 2001 Elsevier Science B.V. All rights reserved. Keywords: UV luminescence; van der Waals solids; Naphthalene; Aniline
1. Introduction Rare gas and inert molecules such as nitrogen are usual host materials in the matrix isolation spectroscopy. They do not react with the trapped molecules, the main molecular properties are preserved. Nevertheless, there is always an environment effect on the guest species, as the usual frequency shift in the electronic transition between the gas phase and the condensed phase. In the host lattice, large aromatic molecules are trapped in different cavities wherefrom several host atoms or molecules are removed. The structure of such cavities cannot be deduced from experiment but may be determined by simulation of deposition of *Corresponding author. Tel.: +33-1691-57539; fax: +331691-56777. E-mail address: [email protected] (C. Cr!epin).
guest–host mixtures. The S1 -S0 fluorescence and the T1 -S0 phosphorescence may be recorded upon a selective S1 ’S0 excitation of different sites by a tunable dye laser. We have performed such experiments on naphthalene (C10 H8 ) and aniline (C6 H5 NH2 ) isolated in van der Waals solids [1,2]. Naphthalene was chosen because of the peculiar properties of its first excited states: the S1 2S0 transition is very weakly allowed ð f B104 [3]) and the efficiency of the S1 *T1 intersystem crossing is very low in gas phase [4]. An environment effect on the S1 electronic relaxation was then expected. Aniline exhibits a large amplitude motion in its ground state: the wagging of the NH2 group up and down the aromatic plane [5]. This vibrational mode, the inversion mode I, is involved in the tunneling effect between the two equivalent bent configurations. The influence of the environment on such a vibrational mode can be deduced from the fluorescence and phosphorescence spectra.
0022-2313/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 0 1 ) 0 0 3 3 5 - 0
458
! C. Crepin et al. / Journal of Luminescence 94–95 (2001) 457–460
Experimental results on naphthalene and their interpretation were described elsewhere [1,6]. The detailed study of aniline in argon matrices will be also reported in Ref. [2]. This paper points out the site effects observed in argon matrices for both molecules.
1.0 0.8
A-sites
0.6 0.4 0.2
Several parameters in fluorescence and phosphorescence spectra of the naphthalene molecule in Ar matrices depend on the excitation frequency. This may be explained by the presence of two main families of sites with excitation spectra corresponding to the red and blue wings of the S1 ’S0 vibronic bands, noted A-sites and B-sites, respectively [1]. The singlet and triplet emission frequencies are red-shifted in the A-sites in comparison with the B-sites emission. More surprising, the intensity distribution between ‘‘allowed’’ (in Condon approximation) and ‘‘forbidden’’ (induced by the Herzberg–Teller mechanism) [7–9] vibronic bands in the S1 -S0 transition is also sitedependent. The environment effect on both (Condon and Herzberg–Teller) transition probabilities depends on the site structure. The intersystem crossing S1 *T1 rate kISC is modified in the solid, but the enhancement of the kISC rate is more pronounced in the A-sites ðkISC ¼ 38 106 s1 ) than in the B-sites ðkISC ¼ 8 106 s1 ), the gas phase value being kISC ¼ 2:45 106 s1 [4]. At last, the widths of vibronic bands in the phosphorescence spectra depend on the family of sites as shown in Fig. 1. These features have been associated with different geometries of sites obtained by a molecular dynamic simulation of the matrix isolation of naphthalene [6]: the molecule is found to be contained either in the f0 0 1g or the f1 1 1g crystallographic plane of the argon lattice. The first configuration, corresponding to more cramped sites, was assigned to the A-sites where the molecule–lattice interaction, inducing an increase of kISC ; is more important. On the other hand, the sharpness of the phosphorescence bands in the A-sites indicates that the site selection in S1 2S0 transition frequencies is equivalent in this case with a site selection in T1 2S0 transition
intensity (a.u.)
2. Naphthalene in Ar matrices 0.0 21000
20000
19000
1.0
B-sites
0.8 0.6 0.4 0.2 0.0 21000
20000
19000
-1
cm
Fig. 1. Naphthalene phosphorescence spectra in argon matrix at T ¼ 10 K: Upper spectrum (strong intensity): A-sites excitation (nlaser ¼ 32 295 cm1 ). Lower spectrum (weak intensity): B-sites excitation (nlaser ¼ 32 335 cm1 ), narrow bands are due to the simultaneous excitation of the wing of the A-sites absorption.
frequencies. This is also correlated with betterdefined and more rigid structures obtained when the molecule is trapped in a f0 0 1g plane.
3. Aniline in Ar matrices As previously, we distinguish two main families of sites in the aniline/argon system. In this case, these families have a clear spectral signature in the S1 ’S0 absorption: instead of inhomogeneously broadened vibronic bands in the absorption spectrum of naphthalene, the aniline spectrum exhibits two narrow lines, corresponding to the zero-phonon lines of each family of sites, with a broad phonon side band (Fig. 2) [2]. The electron
! C. Crepin et al. / Journal of Luminescence 94–95 (2001) 457–460
Aniline /Ar 1.0
1
10
0
1
1
00
6a 0
120
B-sites
O. D.
phonons 0.5
A-sites
0.0
35100
34600
34100
33600
0.2
O. D.
8b1g 0.1
1
7b1g0
1
Naphthalene/Ar
0
B-sites A-sites 1
8a1g0
0
00
0.0
33000
32500
-1
32000
31500
cm
Fig. 2. Origin region of aniline and naphthalene S1 ’S0 absorption spectra in argon matrix at T ¼ 10 K
density distribution is deeply modified in the S0 –S1 transition so that the geometry of the molecule is changed [10] (aniline is bent in the S0 state and quasi-planar in the S1 state). A strong electron– phonon coupling is then involved in the S1 2S0 transition. As in absorption, the site selected fluorescence spectra present intense phonon bands on each vibronic transition. As in the case of naphthalene, we observed a site-dependent enhancement of the intersystem crossing efficiency. The singlet lifetimes are very short and our experimental setup allows only a rough evaluation of the non-radiative rate: kISC B9 108 s1 in the A-sites and kISC B4 108 s1 in the B-sites as compared to kISC ¼ 0:8 108 s1 in gas phase [11]. The most important site effects occur in the phosphorescence spectra (Fig. 3), especially in the vibronic intensity distribution. This emission involves vibrational modes of two different symmetries (totally symmetric in-plane a1 modes and outof-plane b1 modes in the C2v symmetry group of
459
the molecule). The presence of out-of-plane modes suggests two different mechanisms of intensity borrowing by spin–orbit and spin–orbit–vibronic singlet–triplet coupling. If most of these modes appear in both site-selected emission, they exhibit different intensities depending on the excited site. The vibrational structure is more clearly observed in B-sites than in A-sites. In particular, the origin part of the spectrum is composed of two well resolved ‘‘doublets’’. Experiments on deutered species (C6 H5 ND2 and C6 H5 NHD) confirmed that the first doublet can be assigned to the 0þ -0þ ð000 Þ and 0þ -0 ðI01 ) transition, where 0þ ð0 ) represents the symmetric (antisymmetric) vibrational level in the inversion mode [12]. A direct evaluation of the inversion frequency is then obtained in the B-sites: Dð0þ 0 Þ ¼ 46 cm1 for the hydrogenated species as compared to 41 cm1 in the free molecule. The second doublet involves the 10b out-of-plane mode and is assigned to the 10b01 and 10b01 I01 vibronic transitions. As the inversion mode is a b1 mode, the mode symmetries are a1 and b1 in the decreasing energy order in the first doublet whereas they are b1 and a1 in the second one. The intensity ratio of the two components are inverted from one doublet to the other, in correlation with symmetry considerations. Fig. 3 shows that these ratios are also different in the A-sites. As in the case of the naphthalene/argon system, if we assume that the aromatic plane of aniline is embedded either in a f0 0 1g or in a f1 1 1g crystallographic plane of the argon lattice, the A-sites (resp. B-sites) are assigned to the first (resp. second) case following the experimental results on the intersystem crossing efficiency. Then, the C2v molecular symmetry group is probably reduced to the Cs group in the B-sites because of the close non-symmetric argon planes above and below the guest molecule, which break down the aromatic plane symmetry. If it is so, the different site symmetries could influence the vibronic intensity distribution in the phosphorescence spectra. 4. Conclusion These two examples show the importance of the close molecular neighborhood in the electronic
! C. Crepin et al. / Journal of Luminescence 94–95 (2001) 457–460
460
phosphorescence B-sites νlaser = 33950 cm-1
phosphorescence (arbitrary unit)
10000
5000
0 28000
27500
27000
26500
26000
25500
25000
6000 4000 phosphorescence A-sites νlaser = 33920 cm-1
2000 0 28000
27500
27000
26500
26000
25500
25000
-1
wavenumber (cm)
Fig. 3. Aniline phosphorescence spectra in argon matrix at T ¼ 10 K: The upper spectrum is the phosphorescence induced by laser excitation on B-sites (nlaser ¼ 33 950 cm1 ) of the 000 transition-bold labels: b1 vibronic modes, other labels: a1 vibronic modes-. The lower one is a phosphorescence induced by laser excitation on A-sites (nlaser ¼ 33 920 cm1 ).
relaxation dynamic. Even in an inert material, beyond a host effect, a site effect is clearly observed. Simulations on the naphthalene/argon system have underlined the importance of the repulsive forces between the guest and the host lattice which are only weakly locally perturbed [6]. The study of aniline is still in progress. First experiments in other rare gas matrices exhibit different phosphorescence spectra, showing the great influence of the environment on this forbidden transition.
References [1] C. Cr!epin, A. Tramer, Chem. Phys. 272 (2001) 227. [2] C. G!ee, C. Cr!epin, A. Cuisset, L. Divay, in preparation.
[3] M. Rubio, M. Merchan, E. Orti, B.O. Roos, J. Chem. Phys. 179 (1994) 395. [4] M. Stockburger, H. Gatterman, W. Klusman, J. Chem. Phys. 63 (1975) 4529. [5] N.W. Larsen, E. Hansen, F. Nicolaisen, Chem. Phys. Lett. 43 (1976) 584. [6] C. Cr!epin, P. de Pujo, B. Bouvier, V. Brenner, Ph. Milli!e, Chem. Phys. 272 (2001) 243. [7] M. Stockburger, H. Gatterman, W. Klusman, J. Chem. Phys. 63 (1975) 4519. [8] G. Holneicher, J. Wolf, Ber. Bunsenges., Phys. Chem. 99 (1995) 366. [9] F. Negri, M.Z. Zgierski, J. Chem. Phys. 104 (1996) 3486. [10] J.M. Hollas, M.R. Howson, T. Ridley, L. Halonen, Chem. Phys. Lett. 98 (1983) 611. [11] R. Scheps, D. Florida, S. Rice, J. Chem. Phys. 61 (1974) 1730. [12] C. G!ee, C. Cr!epin, private communication.