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Laser-induced fluorescence excitation spectra of 1,4-Di(1-naphthyl)propane and 1-buthylnaphthalene in a supersonic jet
Journal of Luminescence 102–103 (2003) 273–277
Laser-induced fluorescence excitation spectra of 1,4-Di(1-naphthyl)propane and 1-buthylnaphthalene in a supersonic jet David Groswasser, Gershon Rosenblum, Amnon Stanger, Shammai Speiser* Department of Chemistry, Technion-Israel Institute of Technology, Haifa 32000, Israel
Abstract Laser-induced fluorescence excitation spectra of the bichromophoric molecule, 1,4-Di(1-naphthyl)propane (N4N(1,10 )), and of 1-buthylnaphthalene (1-BuN ) in a supersonic jet are reported. Comparison between the two spectra suggests that the two naphthalene moieties in the bichromophoric molecule do not interact strongly. Exciton splitting or evidence for excimer formation was not observed. However, the aliphatic substitution alone, cannot account for the significant red shift of the origin in the spectrum of the bichromophoric molecule. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Supersonic jet spectroscopy; Laser-induced fluorescence; Naphthalene bichromophoric molecules
1. Introduction Interchromophoric interactions between naphthalene chromophores were the subject of many studies in a variety of systems: Excitonic interactions were observed in naphthalene crystals [1,2] and in molecular clusters of naphthalene in a supersonic jet [3–7]. Jonkman and Wiersma observed intramolecular excitonic interaction between naphthalene chromophores in binaphthyl (BN) [8]. Exciton splittings are known to provide valuable information about the interchromophor geometry [3,5]. Another type of process that was studied in dinaphthyl bichromophoric systems was excimer formation. Intramolecular excimer forma*Corresponding author. Tel.: +972-4-8293735; fax: +972-48233735. E-mail address: [email protected] (S. Speiser).
tion strongly depends on the separation and on the relative orientation of the chromophores. In clusters, a face-to-face ‘‘sandwich’’ geometry is optimal for the excimer formation [9]. In the case of bichromophoric molecules where the spacer is an aliphatic chain, three methylene units is the optimal length to reach this configuration. Excimer formation in dinapthyl bichromophoric systems has already been reported in solution [10–14]. However, processes that involve charge transfer (CT) strongly depend on the medium polarity, thus in order to learn about pure interchromophoric interactions, supersonic jet and isolated molecules conditions are required. We have already obtained the laser-induced fluorescence (LIF) spectra of a series of dinaphthyl bicromophoric molecules of the general type NnN(i,j 0 ) [15]. Here, N denotes a naphthalene moiety, n is the number of methylene units in the
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D. Groswasser et al. / Journal of Luminescence 102–103 (2003) 273–277
bridge, and i; j 0 are the position of the bridge at each of the naphthalene chromophores. In general, the spectra of these molecules exhibit an intense origin in the range of B31500–31800 cm1. In the case of N1N(2,20 ), N1N(2,10 ), N2N(2,20 ), N2N(2,10 ), we have observed activity in this spectral range that we have assigned to low-frequency modes of the methylene bridge (B10 cm1), or to the appearance of more than one conformer. The spectrum of N6N(2,20 ) is also very complex near the origin, again due to the excitation of a large number of conformers, while that of N6N(1,10 ) exhibits two series of progression in the vicinity of the origin. As model compounds in this work, we have used 1-methylnaphthalene, and 2-methylnaphthalene which have been investigated by Warren et al. [16], and by Jacobson et al. [17]. In this paper, we focus on the spectrum of N4N(1,10 ) which is exceptional in three aspects compared to those of other compound of this series: (a) the structure of the spectrum is very simple and resembles that of 1-methylnaphthalene [16], (b) the origin is significantly shifted from that of 1-methylnaphthalene (31773 cm1) to 31404 cm1, (c) intrachromophoric vibrations such as the 8(b1g) are intense. We have measured the spectrum of 1-buthylnaphthalene in order to study the effects of the long methylene chain on the spectrum, and in particular, verify by how much it can red shift the origin.
2. Experimental N4N(1,10 ) 1-buthylnaphthalene was synthesized in the Technion department of chemistry by a method described elsewhere [17]. 1-buthylnaphthalene was synthesized by Diels–Alder reaction of 7,8-dibromocyclobutabenzene with 1-pentene according to the procedure used for other substituted naphthalene [18]. The compounds were found to be over 95% pure by GCMS and NMR measurements. In all experiments we used 3 atm as the backing pressure, and He as the carrier gas. N4N(1,10 ) was heated to 1901C and 1-butylnaphthalene to 1241C in order to obtain sufficient vapor pressure.
A frequency-doubled dye laser pumped by a Nd–Yag laser was used for the excitation. The fluorescence signal was collected by Hamamatsu IP-28 photomultiplier and measured by a Tektronix TDS-220 digital oscilloscope. In order to reduce noise from laser reflections, the spectrum of N4N(1,10 ) was collected in a time gate of 75–450 ns after the excitation. In the spectrum of 1-butylnaphthalene, we used a 340 nm cutoff filter and collected the signal B25–450 ns from the excitation.
3. Results and discussion The spectra of 1-buthylenaphthalene and N4N(1,10 ) are shown in Fig. 1. The spectrum of N4N(1,10 ) is very typical to naphthalene and naphthalene derivatives such as 1-methylnaphthalene (1MN) [19] as can be seen from Table 1. This kind of a spectrum implies that the two chromophores are identical and are not strongly interacting, as in an open symmetric structure (Fig. 2a). The absence of low-frequency activity near the origin implies that only one conformation is excited, and that bridge vibrations are not coupled to the electronic transition. This is very surprising when considering the flexible nature of the aliphatic bridge. In addition, the origin of the spectrum is significantly shifted to the red compared to the origin of 1-methylnaphthalene, we identify the 0–0 transition at 31404 cm1. We note that a similar red shift and spectrum was observed by Chakraborty et al. [20] in acenaphthene. Such a red shift is excepted in ‘‘sandwich’’ geometry (Fig. 2b) due to a stabilizing interaction between the naphthalene moieties. However, we do not detect any exciton splitting or red shifted excimer fluorescence which may result from such a structure. The origin of the spectrum of 1-buthylnaphthalene is at 31740 cm1, similar to that of 1MN. This means that the effect of the methyl substitution on the spectral shift is not significantly changed by varying the length of the methylene chain. The 0–0 transition is broad and there is also a broad structure around 420 cm1 which is the region of
Relative fluorescence intensity / a.u.
D. Groswasser et al. / Journal of Luminescence 102–103 (2003) 273–277
275
4 3 2
31740 1 0
(a)
0
6
200
400
600
800
600
800
31404
5 4 3 2 1 0
(b)
0
200
400 -1
shift / cm
Fig. 1. (a) LIF spectrum of 1-buthylnaphthalene, the 0–0 transition is at 31740 cm1, (b) the spectrum of N4N(1,10 ) is shifted further to the blue, the 0–0 transition is at 31404 cm1.
Table 1 Comparison between 1-methylnaphthalene and N4N(1,10 ) Tentative assignment
Line position/cm1 1MN/N4N
Shift from 0–0/cm1 1MN/N4N
Normalized relative intensitya
000 8ðb1g Þ10 2ðb2g Þ10 9ðAg Þ10 8ðAg Þ10
31773.6/31404.4 32192.8/31816.8 32257.6/31887.2 32316.6/31937.0 32435.2/32061.4
0/0 419.2/412.4 484.0/482.8 543.1/532.6 661.6/657
1/1 5.35/0.59 0.99/0.069 0.245/0.11 0.92/0.046
a
Relative to the laser intensity, and normalized to the 0–0 intensity. 1-MN data is taken from Ref. [9].
the 8(b1g) transition, due to the excitation of different conformers. The lifetime of the 0–0 transition of N4N(1,10 ) is 14075 ns. The lifetimes of other vibronic transition is even shorter and drop to B100 ns. This is much shorter than the typical 300 ns measured for naphthalene or 1MN, but we note that in binaphthyl, for example, the lifetimes are also short and drop to B60 ns due to excitonic interaction [8,21]. Typical lifetimes for the bichromophoric molecules reported in Ref. [15] is over 200 ns. We have calculated the stable geometries of N4N. These are shown in Fig. 2. The calculated
geometries of N4N, corresponding to Fig. 2 and energies are at B3LYP/6-31G* theoretical level and are given in Table 2. All isomers have been verified by analytical frequency calculation to be real minima. The shortening of the lifetimes, the significant red shift of the origin, the simple spectrum which arise from the excitation of mainly one stable conformation and the absence of low-frequency bridge vibrations, point that the structure of the molecule is rigid and close to a ‘‘sandwich’’ type as in Fig. 2(3). The main arguments against this assignment are that in the spectrum we seem to observe excitation of only one chromophor, which
D. Groswasser et al. / Journal of Luminescence 102–103 (2003) 273–277
276
Fig. 2. Calculated conformations of N4N(1,10 ). Table 2 Calculated geometries of N4N, corresponding to Fig. 2. Geometries and energies are at B3LYP/6-31G* theoretical level
1 2 3
Total E
Relative E
ZPE correct.
Relative E ZPE correct.
927.838278 927.8409512 927.8376553
1.7 0.0 2.1
927.447431 927.450105 927.446633
1.7 0.0 2.2
Relative energies in kcal mol1. All isomers have been verified by analytical frequency calculation to be real minima.
means that the two are identical and hence the structure has an ‘‘open’’ symmetrical structure. Acknowledgements We thank Prof. M. B. Rubin for synthesizing the N4N compound used in this work. This work was supported by grant from the Technion’s VPR Fund and from the Technion Fund for Promotion of Research. References [1] S.D. Colson, D.M. Hanson, R. Kopelman, G.W. Robinson, J. Chem. Phys. 48 (1967) 2215. [2] D.M. Hanson, J. Chem. Phys. 52 (1970) 3409.
[3] J.A. Syage, J.E. Wessel, J. Chem. Phys. 89 (1988) 5962. [4] J.E. Wessel, J.A. Syage, J. Chem. Phys. 94 (1988) 737. [5] H. Saigusa, E.C. Lim, J. Chem. Phys. 98 (1994) 13470. [6] H. Saigusa, E.C. Lim, J. Chem. Phys. 99 (1995) 15738. [7] H. Saigusa, E.C. Lim, J. Chem. Phys. 103 (1995) 8793. [8] H.T. Jonkman, D.A. Wiersma, J. Chem. Phys. 81 (1984) 1573. [9] T. Matsuoka, K. Kosugi, K. Hino, M. Nishiguchi, K. Ohashi, N. Nishi, H. Sekiya, J. Phys. Chem. A 102 (1988) 7598. [10] E.A. Chandross, C.J. Dempster, J. Am. Chem. Soc. 92 (1969) 3586. [11] H. Itagaki, N. Obukata, A. Okamoto, K. Horie, I. Mita, J. Am. Chem. Soc. 104 (1982) 4469. [12] T.A. Smith, D.A. Shipp, G.D. Scholes, K.P. Ghiggino, J. Photochem. Photobiol. A. 80 (1994) 177. [13] Y. Tsujji, A. Tsuchida, M. Yamamoto, T. Momose, T. Shida, J. Phys. Chem. 95 (1991) 8635.
D. Groswasser et al. / Journal of Luminescence 102–103 (2003) 273–277 [14] H. Itagaki, N. Obukata, A. Okamoto, K. Horie, I. Mita, J. Am. Chem. Soc. 104 (1982) 4469. [15] D. Groswasser, S. Speiser, J. Fluoresc. 10 (2000) 113. [16] J.A. Warren, J.M. Hayes, G.J. Small, J. Chem. Phys. 80 (1984) 1786. [17] R. Rona , U. Feldman, J. Chem. Soc. (1958) 1737. [18] G. Rosenblum, I. Zaltsman, A. Stanger, S. Speiser, J. Photochem. Photobiol. A: Chem. 143 (2001) 245.
277
[19] B.A. Jacobson, J.A. Guest, F.A. Novak, S.A. Rice, J. Chem. Phys. 87 (1987) 269. [20] T. Chakraborty, M. Chowdhury, Chem. Phys. 159 (1992) 439. [21] D. Groswasser, Photophysics of naphthalene bichromophoric molecules and clusters, Ph.D. Thesis, TechnionIsrael Institute of Technology, 1999.
Laser-induced fluorescence excitation spectra of 1,4-Di(1-naphthyl)propane and 1-buthylnaphthalene in a supersonic jet David Groswasser, Gershon Rosenblum, Amnon Stanger, Shammai Speiser* Department of Chemistry, Technion-Israel Institute of Technology, Haifa 32000, Israel
Abstract Laser-induced fluorescence excitation spectra of the bichromophoric molecule, 1,4-Di(1-naphthyl)propane (N4N(1,10 )), and of 1-buthylnaphthalene (1-BuN ) in a supersonic jet are reported. Comparison between the two spectra suggests that the two naphthalene moieties in the bichromophoric molecule do not interact strongly. Exciton splitting or evidence for excimer formation was not observed. However, the aliphatic substitution alone, cannot account for the significant red shift of the origin in the spectrum of the bichromophoric molecule. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Supersonic jet spectroscopy; Laser-induced fluorescence; Naphthalene bichromophoric molecules
1. Introduction Interchromophoric interactions between naphthalene chromophores were the subject of many studies in a variety of systems: Excitonic interactions were observed in naphthalene crystals [1,2] and in molecular clusters of naphthalene in a supersonic jet [3–7]. Jonkman and Wiersma observed intramolecular excitonic interaction between naphthalene chromophores in binaphthyl (BN) [8]. Exciton splittings are known to provide valuable information about the interchromophor geometry [3,5]. Another type of process that was studied in dinaphthyl bichromophoric systems was excimer formation. Intramolecular excimer forma*Corresponding author. Tel.: +972-4-8293735; fax: +972-48233735. E-mail address: [email protected] (S. Speiser).
tion strongly depends on the separation and on the relative orientation of the chromophores. In clusters, a face-to-face ‘‘sandwich’’ geometry is optimal for the excimer formation [9]. In the case of bichromophoric molecules where the spacer is an aliphatic chain, three methylene units is the optimal length to reach this configuration. Excimer formation in dinapthyl bichromophoric systems has already been reported in solution [10–14]. However, processes that involve charge transfer (CT) strongly depend on the medium polarity, thus in order to learn about pure interchromophoric interactions, supersonic jet and isolated molecules conditions are required. We have already obtained the laser-induced fluorescence (LIF) spectra of a series of dinaphthyl bicromophoric molecules of the general type NnN(i,j 0 ) [15]. Here, N denotes a naphthalene moiety, n is the number of methylene units in the
0022-2313/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 0 2 ) 0 0 5 0 1 - X
274
D. Groswasser et al. / Journal of Luminescence 102–103 (2003) 273–277
bridge, and i; j 0 are the position of the bridge at each of the naphthalene chromophores. In general, the spectra of these molecules exhibit an intense origin in the range of B31500–31800 cm1. In the case of N1N(2,20 ), N1N(2,10 ), N2N(2,20 ), N2N(2,10 ), we have observed activity in this spectral range that we have assigned to low-frequency modes of the methylene bridge (B10 cm1), or to the appearance of more than one conformer. The spectrum of N6N(2,20 ) is also very complex near the origin, again due to the excitation of a large number of conformers, while that of N6N(1,10 ) exhibits two series of progression in the vicinity of the origin. As model compounds in this work, we have used 1-methylnaphthalene, and 2-methylnaphthalene which have been investigated by Warren et al. [16], and by Jacobson et al. [17]. In this paper, we focus on the spectrum of N4N(1,10 ) which is exceptional in three aspects compared to those of other compound of this series: (a) the structure of the spectrum is very simple and resembles that of 1-methylnaphthalene [16], (b) the origin is significantly shifted from that of 1-methylnaphthalene (31773 cm1) to 31404 cm1, (c) intrachromophoric vibrations such as the 8(b1g) are intense. We have measured the spectrum of 1-buthylnaphthalene in order to study the effects of the long methylene chain on the spectrum, and in particular, verify by how much it can red shift the origin.
2. Experimental N4N(1,10 ) 1-buthylnaphthalene was synthesized in the Technion department of chemistry by a method described elsewhere [17]. 1-buthylnaphthalene was synthesized by Diels–Alder reaction of 7,8-dibromocyclobutabenzene with 1-pentene according to the procedure used for other substituted naphthalene [18]. The compounds were found to be over 95% pure by GCMS and NMR measurements. In all experiments we used 3 atm as the backing pressure, and He as the carrier gas. N4N(1,10 ) was heated to 1901C and 1-butylnaphthalene to 1241C in order to obtain sufficient vapor pressure.
A frequency-doubled dye laser pumped by a Nd–Yag laser was used for the excitation. The fluorescence signal was collected by Hamamatsu IP-28 photomultiplier and measured by a Tektronix TDS-220 digital oscilloscope. In order to reduce noise from laser reflections, the spectrum of N4N(1,10 ) was collected in a time gate of 75–450 ns after the excitation. In the spectrum of 1-butylnaphthalene, we used a 340 nm cutoff filter and collected the signal B25–450 ns from the excitation.
3. Results and discussion The spectra of 1-buthylenaphthalene and N4N(1,10 ) are shown in Fig. 1. The spectrum of N4N(1,10 ) is very typical to naphthalene and naphthalene derivatives such as 1-methylnaphthalene (1MN) [19] as can be seen from Table 1. This kind of a spectrum implies that the two chromophores are identical and are not strongly interacting, as in an open symmetric structure (Fig. 2a). The absence of low-frequency activity near the origin implies that only one conformation is excited, and that bridge vibrations are not coupled to the electronic transition. This is very surprising when considering the flexible nature of the aliphatic bridge. In addition, the origin of the spectrum is significantly shifted to the red compared to the origin of 1-methylnaphthalene, we identify the 0–0 transition at 31404 cm1. We note that a similar red shift and spectrum was observed by Chakraborty et al. [20] in acenaphthene. Such a red shift is excepted in ‘‘sandwich’’ geometry (Fig. 2b) due to a stabilizing interaction between the naphthalene moieties. However, we do not detect any exciton splitting or red shifted excimer fluorescence which may result from such a structure. The origin of the spectrum of 1-buthylnaphthalene is at 31740 cm1, similar to that of 1MN. This means that the effect of the methyl substitution on the spectral shift is not significantly changed by varying the length of the methylene chain. The 0–0 transition is broad and there is also a broad structure around 420 cm1 which is the region of
Relative fluorescence intensity / a.u.
D. Groswasser et al. / Journal of Luminescence 102–103 (2003) 273–277
275
4 3 2
31740 1 0
(a)
0
6
200
400
600
800
600
800
31404
5 4 3 2 1 0
(b)
0
200
400 -1
shift / cm
Fig. 1. (a) LIF spectrum of 1-buthylnaphthalene, the 0–0 transition is at 31740 cm1, (b) the spectrum of N4N(1,10 ) is shifted further to the blue, the 0–0 transition is at 31404 cm1.
Table 1 Comparison between 1-methylnaphthalene and N4N(1,10 ) Tentative assignment
Line position/cm1 1MN/N4N
Shift from 0–0/cm1 1MN/N4N
Normalized relative intensitya
000 8ðb1g Þ10 2ðb2g Þ10 9ðAg Þ10 8ðAg Þ10
31773.6/31404.4 32192.8/31816.8 32257.6/31887.2 32316.6/31937.0 32435.2/32061.4
0/0 419.2/412.4 484.0/482.8 543.1/532.6 661.6/657
1/1 5.35/0.59 0.99/0.069 0.245/0.11 0.92/0.046
a
Relative to the laser intensity, and normalized to the 0–0 intensity. 1-MN data is taken from Ref. [9].
the 8(b1g) transition, due to the excitation of different conformers. The lifetime of the 0–0 transition of N4N(1,10 ) is 14075 ns. The lifetimes of other vibronic transition is even shorter and drop to B100 ns. This is much shorter than the typical 300 ns measured for naphthalene or 1MN, but we note that in binaphthyl, for example, the lifetimes are also short and drop to B60 ns due to excitonic interaction [8,21]. Typical lifetimes for the bichromophoric molecules reported in Ref. [15] is over 200 ns. We have calculated the stable geometries of N4N. These are shown in Fig. 2. The calculated
geometries of N4N, corresponding to Fig. 2 and energies are at B3LYP/6-31G* theoretical level and are given in Table 2. All isomers have been verified by analytical frequency calculation to be real minima. The shortening of the lifetimes, the significant red shift of the origin, the simple spectrum which arise from the excitation of mainly one stable conformation and the absence of low-frequency bridge vibrations, point that the structure of the molecule is rigid and close to a ‘‘sandwich’’ type as in Fig. 2(3). The main arguments against this assignment are that in the spectrum we seem to observe excitation of only one chromophor, which
D. Groswasser et al. / Journal of Luminescence 102–103 (2003) 273–277
276
Fig. 2. Calculated conformations of N4N(1,10 ). Table 2 Calculated geometries of N4N, corresponding to Fig. 2. Geometries and energies are at B3LYP/6-31G* theoretical level
1 2 3
Total E
Relative E
ZPE correct.
Relative E ZPE correct.
927.838278 927.8409512 927.8376553
1.7 0.0 2.1
927.447431 927.450105 927.446633
1.7 0.0 2.2
Relative energies in kcal mol1. All isomers have been verified by analytical frequency calculation to be real minima.
means that the two are identical and hence the structure has an ‘‘open’’ symmetrical structure. Acknowledgements We thank Prof. M. B. Rubin for synthesizing the N4N compound used in this work. This work was supported by grant from the Technion’s VPR Fund and from the Technion Fund for Promotion of Research. References [1] S.D. Colson, D.M. Hanson, R. Kopelman, G.W. Robinson, J. Chem. Phys. 48 (1967) 2215. [2] D.M. Hanson, J. Chem. Phys. 52 (1970) 3409.
[3] J.A. Syage, J.E. Wessel, J. Chem. Phys. 89 (1988) 5962. [4] J.E. Wessel, J.A. Syage, J. Chem. Phys. 94 (1988) 737. [5] H. Saigusa, E.C. Lim, J. Chem. Phys. 98 (1994) 13470. [6] H. Saigusa, E.C. Lim, J. Chem. Phys. 99 (1995) 15738. [7] H. Saigusa, E.C. Lim, J. Chem. Phys. 103 (1995) 8793. [8] H.T. Jonkman, D.A. Wiersma, J. Chem. Phys. 81 (1984) 1573. [9] T. Matsuoka, K. Kosugi, K. Hino, M. Nishiguchi, K. Ohashi, N. Nishi, H. Sekiya, J. Phys. Chem. A 102 (1988) 7598. [10] E.A. Chandross, C.J. Dempster, J. Am. Chem. Soc. 92 (1969) 3586. [11] H. Itagaki, N. Obukata, A. Okamoto, K. Horie, I. Mita, J. Am. Chem. Soc. 104 (1982) 4469. [12] T.A. Smith, D.A. Shipp, G.D. Scholes, K.P. Ghiggino, J. Photochem. Photobiol. A. 80 (1994) 177. [13] Y. Tsujji, A. Tsuchida, M. Yamamoto, T. Momose, T. Shida, J. Phys. Chem. 95 (1991) 8635.
D. Groswasser et al. / Journal of Luminescence 102–103 (2003) 273–277 [14] H. Itagaki, N. Obukata, A. Okamoto, K. Horie, I. Mita, J. Am. Chem. Soc. 104 (1982) 4469. [15] D. Groswasser, S. Speiser, J. Fluoresc. 10 (2000) 113. [16] J.A. Warren, J.M. Hayes, G.J. Small, J. Chem. Phys. 80 (1984) 1786. [17] R. Rona , U. Feldman, J. Chem. Soc. (1958) 1737. [18] G. Rosenblum, I. Zaltsman, A. Stanger, S. Speiser, J. Photochem. Photobiol. A: Chem. 143 (2001) 245.
277
[19] B.A. Jacobson, J.A. Guest, F.A. Novak, S.A. Rice, J. Chem. Phys. 87 (1987) 269. [20] T. Chakraborty, M. Chowdhury, Chem. Phys. 159 (1992) 439. [21] D. Groswasser, Photophysics of naphthalene bichromophoric molecules and clusters, Ph.D. Thesis, TechnionIsrael Institute of Technology, 1999.