- Email: [email protected]
Green fluorescence from perylene liquid in the molten state
Chemical Physics Letters 734 (2019) 136751
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Green fluorescence from perylene liquid in the molten state Kenta Sato, Ryuzi Katoh
⁎
T
College of Engineering, Nihon University, Koriyama, Fukushima 963-8642, Japan
H I GH L IG H T S
liquid in the molten state shows green fluorescence. • Perylene green fluorescence is emitted from an excimer with non-parallel geometry. • The lifetime and quantum yield of the green fluorescence are evaluated. • Fluorescence orange fluorescent excimer co-exists with the green one in the liquid. • The • The green and orange fluorescent excimers are in thermal equilibrium.
A B S T R A C T
Green fluorescence from perylene liquid in the molten state was observed, which differed substantially from the orange fluorescence from the sandwich excimer generally observed in concentrated solutions and in stable α-phase crystals. The peak position of the green fluorescence was similar to those in unstable β-phase crystals and in densely doped polymer films. Results from analysis of spectral components indicate that the green fluorescence is emitted from an excimer with nonparallel geometry, populated by thermal activation of the excimer with sandwich geometry.
1. Introduction An excimer is a molecular complex formed between excited and ground state molecules. Excimers are well known to emit a characteristic broad, structureless fluorescence that is red-shifted relative to the monomer’s fluorescence. Excimers are suitable prototypes with which to study intermolecular interaction in the excited state; therefore, much research has been carried out on excimers, both experimentally and theoretically [1–3]. Based on the dramatic spectral change of fluorescence upon excimer formation, excimer fluorescence has been applied as a fluorescence probe for microenvironments in various chemical systems, such as the viscosity of supramolecular assemblies [2,4], chemical sensing in solutions [2,5] and the hybridization analysis of nucleic acids as molecular beacons [6]. Perylene (C20H12) is known to be a model of excimer study, and excimer fluorescence of perylene in various environments has been observed. In dilute solutions, no excimer formation occurs and blue fluorescence around 445 nm from the excited state of the monomer can be seen, with a high fluorescence quantum yield, Φf (e.g., Φf = 0.94 in cyclohexane [2]). In concentrated solutions, weak excimer fluorescence with a peak of around 640 nm has been detected [7,8]. In the crystalline phase of perylene, orange fluorescence around 590 nm has been observed for the stable α-phase crystals, in which sandwich dimers are ⁎
packed in a herringbone structure [9–11]. Perylene is polymorphic, and unstable β-phase crystals, in which individual perylene molecules are packed in a herringbone structure, fluoresce green [9,11–13]. Green excimer fluorescence has also been observed from perylene molecules densely dispersed in polymer films [14,15] and in Langmuir-Blodgett (LB) films [16], in which the molecules are oriented randomly. These observations imply that there are two kinds of excimers of perylene, i.e., orange and green fluorescent excimers. The orange fluorescent excimer has a parallel stacked structure, and the green one, a non-parallel structure. This is a remarkable characteristic of the perylene excimer, as only the parallel stacked structure is known to be stable for the excimers of various other aromatic molecules, such as naphthalene, anthracene and pyrene [1,2]. The molten (high temperature liquid) state of aromatic compounds is another suitable environment for excimer study, for which only a few results have been reported [17,18]. Several characteristic properties for excimers in the molten state are expected. The concentration of molecules in the molten state is very high because there are no solvent molecules, and the degree of freedom for molecular motion is higher than that in the solid state. Melting points of conventional aromatic crystals are above 400 K; thus, thermal activation processes would play an important role in the formation and relaxation processes of excimers, for example, the thermal dissociation of the excimer to the monomer
Corresponding author. E-mail address: [email protected] (R. Katoh).
https://doi.org/10.1016/j.cplett.2019.136751 Received 7 July 2019; Received in revised form 18 August 2019; Accepted 8 September 2019 Available online 09 September 2019 0009-2614/ © 2019 Elsevier B.V. All rights reserved.
Chemical Physics Letters 734 (2019) 136751
K. Sato and R. Katoh
excited state. For anthracene in the molten state, very weak excimer fluorescence has been observed [18]. The weakness of this fluorescence is attributed to fluorescence quenching by a photo-dimerization reaction in the excimer state [19]. For pyrene in the molten state, excimer fluorescence similar to that in solution has been observed simultaneously with monomer fluorescence from the thermal dissociation of the excimer [18]. In order to clarify the characteristic properties of the perylene excimer in the molten state, we studied the fluorescence of a high-temperature perylene liquid. 2. Materials and methods The perylene was purified by extensive zone refining [20]. To eliminate thermal degradation at high temperature, a glass tube containing sample powder was sealed under a nitrogen atmosphere. The sample tube was heated with a home-made heater up to just above the melting point (553 K) and the fluorescence of perylene in the molten state was measured. Densely doped polymer (DP) films used as a reference were prepared by a drop-cast method from a highly concentrated toluene solution of perylene (3 mM) contained with polystyrene (Nacalai Tesque, 10 g L–1). The sample was excited with 365-nm light from a UV-LED (Thorlabs, M365L2). The fluorescence spectra were measured with a cooled CCD detector (Princeton Instruments, PIXIS 250) equipped with a monochromator (Acton, Spectra Pro150). The fluorescence decay profiles were measured with a time-resolved luminescence spectrometer based on a streak camera (Hamamatsu, C4334). Time resolution was about 500 ps. The excitation wavelength was 400 nm (second harmonic of the output from a Ti:sapphire laser; Spectra-Physics, Tsunami). The repetition rate of the oscillator was reduced from 80 to 8 MHz with a pulse selector (Spectra-Physics, Model 3980). 3. Results and discussion Fig. 1 shows a fluorescence spectrum of perylene liquid in the molten state (a) along with spectrum of a DP film measured in this study (b), the monomer and excimer in solution (from reference [8]) (c) and α- and β-crystals (from reference [13]) (d). For the liquid, green fluorescence could be seen clearly (photograph in the inset of Fig. 1). The color of the liquid sample was orange, which differs notably from the greenish color of the solutions, DP films and β-crystals but is similar to the orange color of the α -crystals. This difference clearly indicates that intermolecular interaction plays an important role for the electronic structure of perylene in the molten state and that the molten perylene molecules could have a sandwich dimer configuration in their ground state, similar to that of the α-crystals. It is noted that the orange/red fluorescence is expected to be observed from the orange color liquid, whereas the green fluorescence was observed. This suggests that a thermal activation process is included in the emission process of the green fluorescence. The fluorescence spectrum of the perylene liquid was broad and the shoulder at around 600 nm could be seen (Fig. 1a, solid red line). The peak was observed at 530 nm (Fig. 1a), which is similar to those in the DP films (530 nm, Fig. 1b) and β-crystals (525 nm, Fig. 1d). However, the spectrum differed significantly from that of the blue fluorescence of the monomer (445 nm, Fig. 1c), indicating that the excited perylene molecules in the molten state form excimers. In addition, the peak position was different from the peaks of both the orange fluorescence of the stable excimer in solution (640 nm, Fig. 1c) and the α-crystal (590 nm, Fig. 1d), indicating that the green fluorescence is emitted from the excimer having non-parallel geometry. Although broad and structureless fluorescence spectrum is expected to be observed in the excmer, vibrational structure is observed in the fluorescence spectrum of β-crystal. This would be due to structural restriction of the excimer in crystal lattice. The perylene excimer in LB films has been systematically studied as
Fig. 1. Fluorescence spectra of perylene liquid in the molten state (solid red line) along with simulated spectral components of green (Ex1, solid green line) and the orange (Ex2, solid orange line) fluorescent excimer and the sum of the two spectral components (dashed black line) (a), DP films (b), monomer and excimer in solution (c) and α- and β-crystals (d). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
a function of the concentration of the perylene moiety in the film [16]: green (Ex1) and the orange (Ex2) fluorescent excimers were identified from the analysis of the fluorescence spectra and Ex2 dominated at high concentrations. After careful analysis including time-resolved fluorescence spectroscopy, Ex1 and Ex2 were assigned to the fluorescence from the non-parallel excimer and sandwich excimer, respectively [16]. We analyzed the fluorescence spectrum of the perylene liquid in a similar manner (Fig. 1a). A wavenumber scale was used for the fitting by using gaussian functions and then the result was plotted against wavelength scale. The fluorescence spectrum was reproduced by using two gaussian functions (Fig. 1a, dashed black line), in which the peak positions and the band widths were similar but not the same as the values for the analysis of the LB films. This clearly indicates that there is a fluorescence component of the sandwich excimer (Ex2) in addition to that of the non-parallel excimer (Ex1). It is noted that the shoulder at around 600 nm could not well reproduced by the fitting, suggesting that the fitting using two gaussian functions is too simple. Although the origin of the shoulder is not clear, this is probably due to vibrational structure of excimer fluorescence similar to that observed in the β-crystals. The fluorescence decay profiles of the perylene liquid observed at around the peaks of Ex1 (510–530 nm) and Ex2 (560–600 nm) are clearly similar to each other (Fig. 2). This indicates that the 2
Chemical Physics Letters 734 (2019) 136751
K. Sato and R. Katoh
Table 1 Radiative lifetimes: λpeak, τf, Φf and τR of the perylene excimer in various phases.
Liquid Solution α-crystal β-crystal
λpeak (nm)
τf (ns)
Φf
τR (ns)
530a 640c 580d 525e
8.3a,b 17.6c 60d 12.3e
0.04a 0.02c 0.31d 0.6e
200 900 200 20
a
This work. Weighted average of (0.15 × 2.7 ns + 0.85 × 9.3 ns). c Ref. [8]. d Ref. [21]. e Ref. [13]. b
Fig. 2. Decay profiles of the fluorescence of perylene liquid observed at around the peak of Ex1 (510–530 nm, green closed circles) and Ex2 (560–600 nm, red open circles). The blue solid line is fit to the data with a double exponential function. The parameters used for the fitting are also shown. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
the
two
decay
components
[13]). This suggests that the mechanism of the green excimer fluorescence emission of the liquid is different from that of the β-crystal. To explore the mechanism of the green fluorescence emission from perylene liquid, we summarized the radiative lifetimes (τR = τf/Φf) of the excimers of perylene in various phases (Table 1). It is noted that τR for allowed transitions in conventional aromatic molecules are known to be 10–20 ns and thus the value of τR is useful for the consideration of allowedness of the optical transition. In the α-crystal and solution, the orange fluorescent excimer clearly has a long τR, indicating a forbidden transition due to the sandwich geometry of the excimer. The green fluorescent excimer in the β-crystal has a short τR, indicating an allowed transition due to non-parallel geometry. Although the liquid fluoresces green, note that τR is long. The long τR of the liquid suggests that the majority of excimers in the molten state have a sandwich geometry giving the Ex2 spectrum. The green fluorescence is emitted from a small amount of the excimer with non-parallel geometry giving the Ex1 fluorescence, which has been populated by thermal activation of the stable excimer with sandwich geometry. Due to the high Φf, the green fluorescence from the thermally activated state dominates the fluorescence color of perylene in the molten state despite its low population. The monomer fluorescence would be expected to be observed at around 450 nm after thermal dissociation of the stable sandwich excimer (Ex2). Note that no monomer fluorescence appears, in contrast with the case of pyrene in the molten state, in which monomer fluorescence has been clearly observed after thermal dissociation of the excimer in the molten state [18]. Thus, perylene molecules form excimers even when distorted from the sandwich geometry by thermal agitation; in other words, the non-parallel excimer is a meta-stable state for perylene. In conclusion, we observed that perylene liquid in the molten state fluoresces green. From spectral analysis, we found that in the molten state, there are two excimers of perylene in thermal equilibrium: an orange fluorescent excimer with sandwich geometry and a green fluorescent excimer with non-parallel geometry. The Φf of the fluorescence of the liquid is low (ca. 0.04), limited by the forbidden transition of the sandwich excimer; in other words, the green fluorescence is emitted from the non-parallel excimer populated by thermal activation.
fluorescence of the perylene liquid decays without spectral shape change. It is noted that no fast components were observed, suggesting that formation of the excimer occurs within the time resolution of the spectrometer (500 ps). From the fact that the Ex1 and Ex2 in LB films show different fluorescence lifetime [16], the similar lifetime observed in the liquid clearly indicates that the excimer states Ex1 and Ex2 are in thermal equilibrium. The decay profiles were well fitted by a double exponential function: the fluorescence lifetimes and relative ratio were evaluated to be τ1 = 2.7 ns (15%) and τ2 = 9.3 ns (85%). Although the origin of the fast decay component (τ1) is not clear at the present stage, it is probably due to the relaxation from the Franck-Condon state to the equilibrium excimer state. The thermal equilibrium exhibited in the molten liquid is in contrast to the fluorescence of the Ex1 and Ex2 components in LB films, and is reasonable because conformational change assisted by thermal activation is restricted in the films, in contrast to the free molecular motion available in the liquid. Fig. 3 shows the fluorescence spectrum of α-crystals at room temperature before melting and of the liquid observed under the same optical geometry for the measurements. Melting clearly decreased the fluorescence intensity of perylene. The quantum yield of fluorescence (Φf) of the liquid can be evaluated by comparing its integrated intensity with that of α-crystals as a standard sample. Although such a simple evaluation method is difficult to apply to the present study because of the different light scattering conditions between the liquid and crystal samples, we roughly estimated the Φf for the liquid to be 0.04 using the value of Φf = 0.31 for the α-crystal [21]. It is noted that this value is significantly lower than Φf of the green fluorescent β-crystals (Φf = 0.6
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments The authors gratefully acknowledge the supply of ultrapure perylene crystals by Professor Emeritus Masahiro Kotani (Gakushuin University). This work was supported by the JST-SENTAN program.
Fig. 3. Fluorescence spectra of α-crystals at room temperature before melting, and the liquid in the molten state observed under the same optical geometry. 3
Chemical Physics Letters 734 (2019) 136751
K. Sato and R. Katoh
References [12]
[1] J.B. Birks, Photophysics of Aromatic Molecules, Wiley, New York, 1970. [2] B. Valeur, M.N. Berberan-Santos, Molecular Fluorescence, Wiley-VCH, Weinheim, 2012. [3] E.S.S. Iyer, A. Sadybekov, O. Lioubashevski, A.I. Krylov, S. Ruhman, Rewriting the story of excimer formation in liquid benzene, J. Phys. Chem. A 121 (2017) 1962–1975. [4] J. Duhamel, New insights in the study of pyrene excimer fluorescence to characterize macromolecules and their supramolecular assemblies in solution, Langmuir 28 (2012) 6527–6538. [5] S. Karuppannan, J.-C. Chambron, Supramolecular chemical sensors based on pyrene monomer–excimer dual luminescence, Chem. Asian J. 6 (2011) 964–984. [6] I.O. Aparin, G.V. Proskurin, A.V. Golovin, A.V. Ustinov, A.A. Formanovsky, T.S. Zatseepin, V.A. Korshun, Fine tuning of pyrene excimer fluorescence in molecular beacons by alteration of the monomer structure, J. Org. Chem. 82 (2017) 10015–10024 and references therein. [7] J.B. Birks, L.G. Christophorou, 'Excimer’ fluorescence. IV. Solution spectra of polycyclic hydrocarbons, Proc. R. Soc. A 277 (1964) 571–582. [8] R. Katoh, S. Sinha, S. Murata, M. Tachiya, Origin of the stabilization energy of perylene excimer as studied by fluorescence and near-IR transient absorption spectroscopy, J. Photochem. Photobiol. A 145 (2001) 23–34. [9] J. Tanaka, The electronic spectra of aromatic molecular crystals. II. The crystal structure and spectra of perylene, Bull. Chem. Soc. Jpn. 36 (1963) 1237–1249. [10] B. Walker, H. Port, H.C. Wolf, The two-step excimer formation in perylene crystals, Chem. Phys. 92 (1985) 177–185. [11] H. Nishimura, T. Yamaoka, K. Mizuno, M. Iemura, A. Matsui, Luminescence of free
[13]
[14] [15]
[16] [17] [18]
[19] [20]
[21]
4
and self-trapped excitons in α- and β-perylene crystals, J. Phys. Chem. Jpn. 53 (1984) 3999–4008. T. Yago, Y. Tamaki, A. Furube, R. Katoh, Growth of β-perylene crystal, Chem. Lett. 36 (2007) 370–371. K. Sato, R. Katoh, Fluorescence properties of β-perylene crystals prepared by a physical vapor transport method under atmospheric pressure, Chem. Phys. Lett. 730 (2019) 312–315. H. Tachikawa, L.R. Faulkner, Anomalous luminescence from perylene in thin polymer films, Chem. Phys. Lett. 39 (1976) 436–441. F. Ito, K. Yamamoto, Y. Kogasaka, R. Katoh, Intermolecular Dynamics of perylene in polymer matrices during the drop-casting process probed by fluorescence and droplet mass changes, Langmuir 34 (2018) 8281–8287. S. Akimoto, A. Ohmori, I. Yamazaki, Dimer formation and excitation relaxation of perylene in Langmuir−Blodgett films, J. Phys. Chem. B 101 (1997) 3753–3758. J.B. Birks, J.B. Aladekomo, Excimer fluorescence spectra of aromatic liquids, Spectrochim. Acta 20 (1964) 15–21. R. Horiguchi, N. Iwasaki, Y. Maruyama, Time-resolved and temperature-dependent fluorescence spectra of anthracene and pyrene in crystalline and liquid states, J. Phys. Chem. 91 (1987) 5135–5139. R. Katoh, E. Katoh, N. Nakashima, M. Yuuki, M. Kotani, Near-IR absorption spectrum of aromatic excimers, J. Phys. Chem. 101 (1997) 7725–7728. M. Kotani, K. Kakinuma, M. Yoshimura, K. Ishii, S. Yamazaki, T. Kobori, H. Okuyama, H. Kobayashi, H. Tada, Charge carrier transport in high purity perylene single crystal studied by time-of-flight measurements and through field effect transistor characteristics, Chem. Phys. 325 (2006) 160–169. R. Katoh, K. Suzuki, A. Furube, M. Kotani, K. Tokumaru, Fluorescence quantum yield of aromatic hydrocarbon crystals, J. Phys. Chem. C 113 (2009) 2961–2965.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Green fluorescence from perylene liquid in the molten state Kenta Sato, Ryuzi Katoh
⁎
T
College of Engineering, Nihon University, Koriyama, Fukushima 963-8642, Japan
H I GH L IG H T S
liquid in the molten state shows green fluorescence. • Perylene green fluorescence is emitted from an excimer with non-parallel geometry. • The lifetime and quantum yield of the green fluorescence are evaluated. • Fluorescence orange fluorescent excimer co-exists with the green one in the liquid. • The • The green and orange fluorescent excimers are in thermal equilibrium.
A B S T R A C T
Green fluorescence from perylene liquid in the molten state was observed, which differed substantially from the orange fluorescence from the sandwich excimer generally observed in concentrated solutions and in stable α-phase crystals. The peak position of the green fluorescence was similar to those in unstable β-phase crystals and in densely doped polymer films. Results from analysis of spectral components indicate that the green fluorescence is emitted from an excimer with nonparallel geometry, populated by thermal activation of the excimer with sandwich geometry.
1. Introduction An excimer is a molecular complex formed between excited and ground state molecules. Excimers are well known to emit a characteristic broad, structureless fluorescence that is red-shifted relative to the monomer’s fluorescence. Excimers are suitable prototypes with which to study intermolecular interaction in the excited state; therefore, much research has been carried out on excimers, both experimentally and theoretically [1–3]. Based on the dramatic spectral change of fluorescence upon excimer formation, excimer fluorescence has been applied as a fluorescence probe for microenvironments in various chemical systems, such as the viscosity of supramolecular assemblies [2,4], chemical sensing in solutions [2,5] and the hybridization analysis of nucleic acids as molecular beacons [6]. Perylene (C20H12) is known to be a model of excimer study, and excimer fluorescence of perylene in various environments has been observed. In dilute solutions, no excimer formation occurs and blue fluorescence around 445 nm from the excited state of the monomer can be seen, with a high fluorescence quantum yield, Φf (e.g., Φf = 0.94 in cyclohexane [2]). In concentrated solutions, weak excimer fluorescence with a peak of around 640 nm has been detected [7,8]. In the crystalline phase of perylene, orange fluorescence around 590 nm has been observed for the stable α-phase crystals, in which sandwich dimers are ⁎
packed in a herringbone structure [9–11]. Perylene is polymorphic, and unstable β-phase crystals, in which individual perylene molecules are packed in a herringbone structure, fluoresce green [9,11–13]. Green excimer fluorescence has also been observed from perylene molecules densely dispersed in polymer films [14,15] and in Langmuir-Blodgett (LB) films [16], in which the molecules are oriented randomly. These observations imply that there are two kinds of excimers of perylene, i.e., orange and green fluorescent excimers. The orange fluorescent excimer has a parallel stacked structure, and the green one, a non-parallel structure. This is a remarkable characteristic of the perylene excimer, as only the parallel stacked structure is known to be stable for the excimers of various other aromatic molecules, such as naphthalene, anthracene and pyrene [1,2]. The molten (high temperature liquid) state of aromatic compounds is another suitable environment for excimer study, for which only a few results have been reported [17,18]. Several characteristic properties for excimers in the molten state are expected. The concentration of molecules in the molten state is very high because there are no solvent molecules, and the degree of freedom for molecular motion is higher than that in the solid state. Melting points of conventional aromatic crystals are above 400 K; thus, thermal activation processes would play an important role in the formation and relaxation processes of excimers, for example, the thermal dissociation of the excimer to the monomer
Corresponding author. E-mail address: [email protected] (R. Katoh).
https://doi.org/10.1016/j.cplett.2019.136751 Received 7 July 2019; Received in revised form 18 August 2019; Accepted 8 September 2019 Available online 09 September 2019 0009-2614/ © 2019 Elsevier B.V. All rights reserved.
Chemical Physics Letters 734 (2019) 136751
K. Sato and R. Katoh
excited state. For anthracene in the molten state, very weak excimer fluorescence has been observed [18]. The weakness of this fluorescence is attributed to fluorescence quenching by a photo-dimerization reaction in the excimer state [19]. For pyrene in the molten state, excimer fluorescence similar to that in solution has been observed simultaneously with monomer fluorescence from the thermal dissociation of the excimer [18]. In order to clarify the characteristic properties of the perylene excimer in the molten state, we studied the fluorescence of a high-temperature perylene liquid. 2. Materials and methods The perylene was purified by extensive zone refining [20]. To eliminate thermal degradation at high temperature, a glass tube containing sample powder was sealed under a nitrogen atmosphere. The sample tube was heated with a home-made heater up to just above the melting point (553 K) and the fluorescence of perylene in the molten state was measured. Densely doped polymer (DP) films used as a reference were prepared by a drop-cast method from a highly concentrated toluene solution of perylene (3 mM) contained with polystyrene (Nacalai Tesque, 10 g L–1). The sample was excited with 365-nm light from a UV-LED (Thorlabs, M365L2). The fluorescence spectra were measured with a cooled CCD detector (Princeton Instruments, PIXIS 250) equipped with a monochromator (Acton, Spectra Pro150). The fluorescence decay profiles were measured with a time-resolved luminescence spectrometer based on a streak camera (Hamamatsu, C4334). Time resolution was about 500 ps. The excitation wavelength was 400 nm (second harmonic of the output from a Ti:sapphire laser; Spectra-Physics, Tsunami). The repetition rate of the oscillator was reduced from 80 to 8 MHz with a pulse selector (Spectra-Physics, Model 3980). 3. Results and discussion Fig. 1 shows a fluorescence spectrum of perylene liquid in the molten state (a) along with spectrum of a DP film measured in this study (b), the monomer and excimer in solution (from reference [8]) (c) and α- and β-crystals (from reference [13]) (d). For the liquid, green fluorescence could be seen clearly (photograph in the inset of Fig. 1). The color of the liquid sample was orange, which differs notably from the greenish color of the solutions, DP films and β-crystals but is similar to the orange color of the α -crystals. This difference clearly indicates that intermolecular interaction plays an important role for the electronic structure of perylene in the molten state and that the molten perylene molecules could have a sandwich dimer configuration in their ground state, similar to that of the α-crystals. It is noted that the orange/red fluorescence is expected to be observed from the orange color liquid, whereas the green fluorescence was observed. This suggests that a thermal activation process is included in the emission process of the green fluorescence. The fluorescence spectrum of the perylene liquid was broad and the shoulder at around 600 nm could be seen (Fig. 1a, solid red line). The peak was observed at 530 nm (Fig. 1a), which is similar to those in the DP films (530 nm, Fig. 1b) and β-crystals (525 nm, Fig. 1d). However, the spectrum differed significantly from that of the blue fluorescence of the monomer (445 nm, Fig. 1c), indicating that the excited perylene molecules in the molten state form excimers. In addition, the peak position was different from the peaks of both the orange fluorescence of the stable excimer in solution (640 nm, Fig. 1c) and the α-crystal (590 nm, Fig. 1d), indicating that the green fluorescence is emitted from the excimer having non-parallel geometry. Although broad and structureless fluorescence spectrum is expected to be observed in the excmer, vibrational structure is observed in the fluorescence spectrum of β-crystal. This would be due to structural restriction of the excimer in crystal lattice. The perylene excimer in LB films has been systematically studied as
Fig. 1. Fluorescence spectra of perylene liquid in the molten state (solid red line) along with simulated spectral components of green (Ex1, solid green line) and the orange (Ex2, solid orange line) fluorescent excimer and the sum of the two spectral components (dashed black line) (a), DP films (b), monomer and excimer in solution (c) and α- and β-crystals (d). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
a function of the concentration of the perylene moiety in the film [16]: green (Ex1) and the orange (Ex2) fluorescent excimers were identified from the analysis of the fluorescence spectra and Ex2 dominated at high concentrations. After careful analysis including time-resolved fluorescence spectroscopy, Ex1 and Ex2 were assigned to the fluorescence from the non-parallel excimer and sandwich excimer, respectively [16]. We analyzed the fluorescence spectrum of the perylene liquid in a similar manner (Fig. 1a). A wavenumber scale was used for the fitting by using gaussian functions and then the result was plotted against wavelength scale. The fluorescence spectrum was reproduced by using two gaussian functions (Fig. 1a, dashed black line), in which the peak positions and the band widths were similar but not the same as the values for the analysis of the LB films. This clearly indicates that there is a fluorescence component of the sandwich excimer (Ex2) in addition to that of the non-parallel excimer (Ex1). It is noted that the shoulder at around 600 nm could not well reproduced by the fitting, suggesting that the fitting using two gaussian functions is too simple. Although the origin of the shoulder is not clear, this is probably due to vibrational structure of excimer fluorescence similar to that observed in the β-crystals. The fluorescence decay profiles of the perylene liquid observed at around the peaks of Ex1 (510–530 nm) and Ex2 (560–600 nm) are clearly similar to each other (Fig. 2). This indicates that the 2
Chemical Physics Letters 734 (2019) 136751
K. Sato and R. Katoh
Table 1 Radiative lifetimes: λpeak, τf, Φf and τR of the perylene excimer in various phases.
Liquid Solution α-crystal β-crystal
λpeak (nm)
τf (ns)
Φf
τR (ns)
530a 640c 580d 525e
8.3a,b 17.6c 60d 12.3e
0.04a 0.02c 0.31d 0.6e
200 900 200 20
a
This work. Weighted average of (0.15 × 2.7 ns + 0.85 × 9.3 ns). c Ref. [8]. d Ref. [21]. e Ref. [13]. b
Fig. 2. Decay profiles of the fluorescence of perylene liquid observed at around the peak of Ex1 (510–530 nm, green closed circles) and Ex2 (560–600 nm, red open circles). The blue solid line is fit to the data with a double exponential function. The parameters used for the fitting are also shown. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
the
two
decay
components
[13]). This suggests that the mechanism of the green excimer fluorescence emission of the liquid is different from that of the β-crystal. To explore the mechanism of the green fluorescence emission from perylene liquid, we summarized the radiative lifetimes (τR = τf/Φf) of the excimers of perylene in various phases (Table 1). It is noted that τR for allowed transitions in conventional aromatic molecules are known to be 10–20 ns and thus the value of τR is useful for the consideration of allowedness of the optical transition. In the α-crystal and solution, the orange fluorescent excimer clearly has a long τR, indicating a forbidden transition due to the sandwich geometry of the excimer. The green fluorescent excimer in the β-crystal has a short τR, indicating an allowed transition due to non-parallel geometry. Although the liquid fluoresces green, note that τR is long. The long τR of the liquid suggests that the majority of excimers in the molten state have a sandwich geometry giving the Ex2 spectrum. The green fluorescence is emitted from a small amount of the excimer with non-parallel geometry giving the Ex1 fluorescence, which has been populated by thermal activation of the stable excimer with sandwich geometry. Due to the high Φf, the green fluorescence from the thermally activated state dominates the fluorescence color of perylene in the molten state despite its low population. The monomer fluorescence would be expected to be observed at around 450 nm after thermal dissociation of the stable sandwich excimer (Ex2). Note that no monomer fluorescence appears, in contrast with the case of pyrene in the molten state, in which monomer fluorescence has been clearly observed after thermal dissociation of the excimer in the molten state [18]. Thus, perylene molecules form excimers even when distorted from the sandwich geometry by thermal agitation; in other words, the non-parallel excimer is a meta-stable state for perylene. In conclusion, we observed that perylene liquid in the molten state fluoresces green. From spectral analysis, we found that in the molten state, there are two excimers of perylene in thermal equilibrium: an orange fluorescent excimer with sandwich geometry and a green fluorescent excimer with non-parallel geometry. The Φf of the fluorescence of the liquid is low (ca. 0.04), limited by the forbidden transition of the sandwich excimer; in other words, the green fluorescence is emitted from the non-parallel excimer populated by thermal activation.
fluorescence of the perylene liquid decays without spectral shape change. It is noted that no fast components were observed, suggesting that formation of the excimer occurs within the time resolution of the spectrometer (500 ps). From the fact that the Ex1 and Ex2 in LB films show different fluorescence lifetime [16], the similar lifetime observed in the liquid clearly indicates that the excimer states Ex1 and Ex2 are in thermal equilibrium. The decay profiles were well fitted by a double exponential function: the fluorescence lifetimes and relative ratio were evaluated to be τ1 = 2.7 ns (15%) and τ2 = 9.3 ns (85%). Although the origin of the fast decay component (τ1) is not clear at the present stage, it is probably due to the relaxation from the Franck-Condon state to the equilibrium excimer state. The thermal equilibrium exhibited in the molten liquid is in contrast to the fluorescence of the Ex1 and Ex2 components in LB films, and is reasonable because conformational change assisted by thermal activation is restricted in the films, in contrast to the free molecular motion available in the liquid. Fig. 3 shows the fluorescence spectrum of α-crystals at room temperature before melting and of the liquid observed under the same optical geometry for the measurements. Melting clearly decreased the fluorescence intensity of perylene. The quantum yield of fluorescence (Φf) of the liquid can be evaluated by comparing its integrated intensity with that of α-crystals as a standard sample. Although such a simple evaluation method is difficult to apply to the present study because of the different light scattering conditions between the liquid and crystal samples, we roughly estimated the Φf for the liquid to be 0.04 using the value of Φf = 0.31 for the α-crystal [21]. It is noted that this value is significantly lower than Φf of the green fluorescent β-crystals (Φf = 0.6
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments The authors gratefully acknowledge the supply of ultrapure perylene crystals by Professor Emeritus Masahiro Kotani (Gakushuin University). This work was supported by the JST-SENTAN program.
Fig. 3. Fluorescence spectra of α-crystals at room temperature before melting, and the liquid in the molten state observed under the same optical geometry. 3
Chemical Physics Letters 734 (2019) 136751
K. Sato and R. Katoh
References [12]
[1] J.B. Birks, Photophysics of Aromatic Molecules, Wiley, New York, 1970. [2] B. Valeur, M.N. Berberan-Santos, Molecular Fluorescence, Wiley-VCH, Weinheim, 2012. [3] E.S.S. Iyer, A. Sadybekov, O. Lioubashevski, A.I. Krylov, S. Ruhman, Rewriting the story of excimer formation in liquid benzene, J. Phys. Chem. A 121 (2017) 1962–1975. [4] J. Duhamel, New insights in the study of pyrene excimer fluorescence to characterize macromolecules and their supramolecular assemblies in solution, Langmuir 28 (2012) 6527–6538. [5] S. Karuppannan, J.-C. Chambron, Supramolecular chemical sensors based on pyrene monomer–excimer dual luminescence, Chem. Asian J. 6 (2011) 964–984. [6] I.O. Aparin, G.V. Proskurin, A.V. Golovin, A.V. Ustinov, A.A. Formanovsky, T.S. Zatseepin, V.A. Korshun, Fine tuning of pyrene excimer fluorescence in molecular beacons by alteration of the monomer structure, J. Org. Chem. 82 (2017) 10015–10024 and references therein. [7] J.B. Birks, L.G. Christophorou, 'Excimer’ fluorescence. IV. Solution spectra of polycyclic hydrocarbons, Proc. R. Soc. A 277 (1964) 571–582. [8] R. Katoh, S. Sinha, S. Murata, M. Tachiya, Origin of the stabilization energy of perylene excimer as studied by fluorescence and near-IR transient absorption spectroscopy, J. Photochem. Photobiol. A 145 (2001) 23–34. [9] J. Tanaka, The electronic spectra of aromatic molecular crystals. II. The crystal structure and spectra of perylene, Bull. Chem. Soc. Jpn. 36 (1963) 1237–1249. [10] B. Walker, H. Port, H.C. Wolf, The two-step excimer formation in perylene crystals, Chem. Phys. 92 (1985) 177–185. [11] H. Nishimura, T. Yamaoka, K. Mizuno, M. Iemura, A. Matsui, Luminescence of free
[13]
[14] [15]
[16] [17] [18]
[19] [20]
[21]
4
and self-trapped excitons in α- and β-perylene crystals, J. Phys. Chem. Jpn. 53 (1984) 3999–4008. T. Yago, Y. Tamaki, A. Furube, R. Katoh, Growth of β-perylene crystal, Chem. Lett. 36 (2007) 370–371. K. Sato, R. Katoh, Fluorescence properties of β-perylene crystals prepared by a physical vapor transport method under atmospheric pressure, Chem. Phys. Lett. 730 (2019) 312–315. H. Tachikawa, L.R. Faulkner, Anomalous luminescence from perylene in thin polymer films, Chem. Phys. Lett. 39 (1976) 436–441. F. Ito, K. Yamamoto, Y. Kogasaka, R. Katoh, Intermolecular Dynamics of perylene in polymer matrices during the drop-casting process probed by fluorescence and droplet mass changes, Langmuir 34 (2018) 8281–8287. S. Akimoto, A. Ohmori, I. Yamazaki, Dimer formation and excitation relaxation of perylene in Langmuir−Blodgett films, J. Phys. Chem. B 101 (1997) 3753–3758. J.B. Birks, J.B. Aladekomo, Excimer fluorescence spectra of aromatic liquids, Spectrochim. Acta 20 (1964) 15–21. R. Horiguchi, N. Iwasaki, Y. Maruyama, Time-resolved and temperature-dependent fluorescence spectra of anthracene and pyrene in crystalline and liquid states, J. Phys. Chem. 91 (1987) 5135–5139. R. Katoh, E. Katoh, N. Nakashima, M. Yuuki, M. Kotani, Near-IR absorption spectrum of aromatic excimers, J. Phys. Chem. 101 (1997) 7725–7728. M. Kotani, K. Kakinuma, M. Yoshimura, K. Ishii, S. Yamazaki, T. Kobori, H. Okuyama, H. Kobayashi, H. Tada, Charge carrier transport in high purity perylene single crystal studied by time-of-flight measurements and through field effect transistor characteristics, Chem. Phys. 325 (2006) 160–169. R. Katoh, K. Suzuki, A. Furube, M. Kotani, K. Tokumaru, Fluorescence quantum yield of aromatic hydrocarbon crystals, J. Phys. Chem. C 113 (2009) 2961–2965.