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A new family of light-emissive symmetric squarylium dyes in the solid state
Dyes and Pigments 122 (2015) 134e138
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Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig
A new family of light-emissive symmetric squarylium dyes in the solid state Yutaka Ohsedo a, *, Kowichiro Saruhashi b, Hisayuki Watanabe a, b a
Advanced Materials Research Laboratory, Collaborative Research Division, Art, Science and Technology Center for Cooperative Research, Kyushu University, 4-1 Kyudaishinmachi, Nishi-ku, Fukuoka 819-0388, Japan b Nissan Chemical Industries, Ltd., 2-10-1 Tsubonishi, Funabashi, Chiba 274-8507, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 April 2015 Received in revised form 15 June 2015 Accepted 16 June 2015 Available online 25 June 2015
High-intensity light emission was observed from symmetric squarylium dyes bearing diaryl amine moieties in the solid state unlike typical squarylium analogues. The enhanced light emission in the solid state may be ascribed to the propeller-like dye architectures, which sterically hinder the intermolecular p ep interactions rather than the intramolecular charge transfer. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Squarylium dyes Fluorescence Intramolecular charge transfer Radical cation Fluorescence quantum yield pep interaction
1. Introduction Recently, the structureeproperty relationship investigations of luminescent p-conjugated functional dyes have resulted in high performance dyes that find application as optoelectronic devices and medical imaging tools for affected and targeted regions. Moreover, certain dyes have exhibited higher luminescence upon aggregation or in the solid state rather than the typical fluorescence quenching [1]. This unusual strong light emission of some organic dyes in the solid state has prompted extensive efforts in designing and synthesizing new photoluminescent compounds [2e16]. Certain dyes have also demonstrated enhanced fluorescence upon selective sensing of biomolecules. This aggregation-induced emission can be used to diagnose specific health conditions [1a,1b,17]. Squarine or squarylium dyes (SQDs) are an extensively studied family of functional dyes [18]. These dyes are expected to be used as organic electroactive and photoactive devices such as electroluminescent devices [19], bulk heterojunction organic photovoltaic cells [20], and dye-sensitized solar cells [21]. In the field of medical science, SQDs have been studied as a sensitizer for photodynamic therapy [22] and fluorophores for sensors [23]. Furthermore, * Corresponding author. Tel.: þ81 92 400 4381; fax: þ81 92 400 4382. E-mail address: [email protected] (Y. Ohsedo). http://dx.doi.org/10.1016/j.dyepig.2015.06.025 0143-7208/© 2015 Elsevier Ltd. All rights reserved.
investigations of their use as supramolecular modifiers have revealed their long-lived durability, which is sufficient for medical imaging [24]. Despite extensive synthetic work and photophysical studies, there have been few reports on the solid-state light emission of these dyes. However, studies have reported SQDs bearing an indolenium moiety showed red fluorescence with a weak quantum yield of only 0.02 [25a], whereas indolenine-derived semi-squaric acid showed a quantum yield of 0.2 [25b]. Recently, a new family of asymmetric-type squarylium dyes showing larger fluorescence quantum yield ~0.36 in the solid state was reported [25c]. In this study, we report the enhanced fluorescence of symmetric SQDs. Fluorescence quantum yields increased from <0.01 in DMF to 0.05e0.36 in the solid state. This solid-state fluorescence at such quantum yields is unprecedented in symmetric SQDs. These symmetric SQDs containing diaryl amine moieties can be synthesized from squaric acid and secondary arylamine in a one-step reaction with simple work up (Scheme 1) [26]. 2. Experimental section 2.1. Materials and methods N,N-dimethylformamide (DMF, Spectrosol®, DOJINDO Laboratories), ()-Quinine sulphate dehydrate (98%) and conc. H2SO4
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2.3. Estimation of fluorescence quantum yields
Scheme 1. Chemical structures of SQDs.
(Wako Pure Chemical Industries, Ltd., Japan) were used without further purification. Other solvents (Wako Pure Chemical Industries, Ltd., Japan) are used as received. The SQDs containing diaryl moieties, i.e. MePHA-SQD, DPhA-SQD, NpPhA-SQD, DTolA-SQD, and MeOPhTolA-SQD were synthesized and obtained as described in our earlier report [26].
2.2. Measurements Ultravioletevisible electronic absorption spectra were obtained using a V-670 UVevis spectrophotometer (JASCO Corporation) in a quartz glass cell with a 10 mm path length for ca. 1 106 M SQD solutions. Fluorescence spectra were obtained using an LS55 luminescence spectrometer (PerkinElmer Japan Co., Ltd.) in a quartz glass cell with a 10 mm path length for ca. 1 106 M SQD solutions. Fluorescence lifetimes in DMF and in the solid state were measured by a time-correlated single photon counting fluorescence spectroscopic method using a FluoroCube 1000U (HORIBA, Ltd.) equipped with a NonoLED-375L picosecond laser diode as an excitation light source (peak wavelength 378 nm, HORIBA, Ltd.). LUDOX® colloidal silica or glass plate were used as standards. Fluorescent lifetimes of SQDs were measured at their maximum fluorescence wavelengths (lmax) in solution or in the solid state. Fluorescence lifetime measurements in DMF were performed after N2 bubbling for 15 min. Lifetimes in the solid state were measured by placing SQD between sealed glass plates and fixing the plate edges with mending tape. Crystal structures were visualized from CIF data using Mercury 3.5.1 (Cambridge Crystallographic Data Centre).
Fluorescence quantum yields (Ff) in the solid state were determined using a C9920-02 absolute fluorescence quantum yield measurement system (Hamamatsu Photonics K.K.). Fluorescence quantum yields in solution were obtained relative to an aqueous quinine sulphate solution. Quantum yields were calculated using the following equation with a Fs value of 0.55 for quinine sulphate . in 0.5 M H2SO4 [27]:
Ff ¼ Fs ðI=Is Þ ðAs =AÞ n2 n2s ;
where I is the integration of the fluorescent intensity, A is the absorbance at the excitation wavelength, n is the refractive index of solvent and the subscript s refers to the quinine standard. 3. Results and discussions We already reported that a creation of symmetric SQDs containing diaryl amine moieties synthesized by only refluxing diaryl amine and squaric acid in a solution and the work up without chromatographic techniques yielded SQDs in good yield and purity (Scheme 1) [26]. These SQDs showed better electrochemical reversibility than required for optoelectronic device applications. Their crystal powders showed intense photoluminescence in the solid state upon UV light irradiation (Fig. 1). Typical SQDs show absorption in the visible light region in solution and emit in the red or near-IR regions if they are fluorescent; however, the diaryl amine-containing SQDs showed absorption near 400 nm in the blue-light region. Moreover, they showed fluorescence depending on their molecular structures in the solid state: light green for MePhA-SQD, orange for DPhA-SQD and NpPhA-SQD and green for DTolA-SQD and MeOPhTolA-SQD. Furthermore, the corresponding SQD solutions in DMF (10 mM) showed no photoluminescence to the naked eye (Fig. 1(c)). To elucidate the photophysical properties of SQDs, their fluorescence was measured in DMF and in the solid state, together with the corresponding fluorescence quantum yields (Ff) (Fig. 2, Table 1). Except for MePhA-SQD, the SQDs in solution showed weak fluorescence at ca. 510 nm. However, in the solid state, they showed fluorescence at longer wavelengths than in solution, consistent with the stabilizing effects of pconjugation in the solid (microcrystalline) state. Relative Ff
Fig. 1. Photographs of SQD powders; (a) SQDs under the ordinary fluorescent light, (b) SQDs under UV irradiation with black light (365 nm), (c) SQDs under UV irradiation a with laser pointer (405 nm), (d) 10 mM MeOPhTolA-SQD solution in DMF in a glass cell under UV irradiation (365 nm).
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values for SQDs in DMF were compared to quinine sulphate while absolute values were determined using the integrating sphere in the solid state. SQDs in DMF solution possess very low fluorescence intensities and Ff values < 0.01. In contrast, those in the solid state possess intense fluorescence and larger Ff values such as 0.36 for MeOPhTolA-SQD. These quantum yields are comparable to absolute Ff value for tris(8quinolinolato)aluminium(III) (Alq3) in solid thin films (Ff ¼ 0.20e0.25) [28], which is an excellent electrontransporting green-light emitter used in organic electroluminescent devices. The information about the excited species of SQDs in solution and in the solid state were obtained through fluorescence lifetime measurements (Tables 1 and 2, see also Supplementary data, Figs. S1 and S2 for decay curves). Estimated lifetimes demonstrated the same order of magnitude in solution and in the solid state (except for MePhA-SQD). However, we could not determine all the exited state observed in each state were identical from the lifetime. Comparable lifetimes were measured in both states for NpPhA-SQD (2.2 ns and 1.9 ns), DTolA-SQD (1.9 ns and 1.6 ns), and MeOPhTolA-SQD (5.4 ns and 5.9 ns) suggesting that the excited species in both states might be almost the same for each SQD. Moreover, the excited species emitting light in the solid state were almost quenched by some radiationless transitions in solution. In addition, the SQDs showed two lifetimes in each state and these results might suggest the existence of dual fluorescence originated from probable conformers of SQDs in both solution and the solid
Table 1 Absorption and fluorescence data of SQDs. Sample
lab./nm
log 3
lf soln./nm
Ff soln.
lf solid/nm
Ff solid
MePhA-SQD DPhA-SQD NpPhA-SQD DTolA-SQD MeOPhTolA-SQD
387 411 423 415 418
4.60 4.59 4.55 4.67 4.67
e 510 510 506 510
~0 <0.01 <0.01 <0.01 <0.01
560 563 602 525 534
0.12 0.051 0.16 0.23 0.36
lab.: maximum absorption wavelength [26a], 3: molecular extinction coefficient in DMF solution [26a], lf soln.: maximum fluorescence wavelength in DMF solution, lf solid: maximum fluorescence wavelength in the solid state, Ff soln.: Ff value in DMF solution relative to a quinine standard, Ff solid: absolute Ff value in the solid state. Fluorescence spectra were obtained at lab. values.
state [29]. Further study for understanding the photophysical properties of these SQDs is underway. Generally, fluorescence of organic dyes in the solid state is observed in the absence of quenching from efficient intermolecular pep interactions between dyes [4]. Reports have suggested that intramolecular charge transfer (CT) in the excited state of dyes played an important role in solid-state light emission [6]. Moreover, bulky substituents have been observed to contribute to the efficient fluorescence by preventing intermolecular pep interactions [6,8,9,11]. The calculated HOMO and LUMO of the new SQDs primarily covered the cyclobutane ring and adjacent N atoms and their CT contribution appeared low (see Supplementary data, Fig. S3) [26a]. In contrast, SQDs bearing diaryl amine moieties exhibited propeller-like conformers by crystal analysis, similar to p-conjugated compounds containing triphenylamine or diphenylamine moieties [30]. In crystal structures of DPhA-SQD and DTolA-SQD crystals obtained by recrystallization in CHCl3/n-hexane (Fig. 3), which showed light emission as seen in other SQD samples, the shortest distance between cyclobutane rings are ca. 6 Å (Fig. 3c and e) and there seems to be neither efficient stacking nor direct pep interaction between cyclobutane rings (other SQDs did not provide enough crystals for XRD analysis [31]). The emission enhancement of these SQDs in the solid state may be attributed to the reduced fluorescence quenching by the absence of efficient pep interactions between dyes due to introduction of propeller-like bulky substituents into SQDs. The Ff values in the solid state appeared to depend on the extent of p-conjugation (Table 1). In our previous study, MePhA-SQD, DPhA-SQD and NpPhA-SQD went through one-electron oxidation, whereas DTolA-SQD and MeOPhTolA-SQD underwent twoelectron oxidation because of their extended p-conjugation [26a]. This extended p-conjugation may stabilize the excited species, improving Ff. 4. Conclusions In conclusion, intense fluorescence was observed for symmetric SQDs containing diaryl amine moieties in the solid state. These dyes showed fluorescence quantum yields comparable to the excellent green-light emitter Alq3. The intense fluorescence in the solid state Table 2 Fluorescence lifetime of SQDs.
MePhA-SQD DPhA-SQD NpPhA-SQD DTolA-SQD MeOPhTolA-SQD Fig. 2. Electronic absorption and fluorescence spectra of SQDs. (a) DMF solution, (b) in the solid state.
DMF solution
Solid
e 1.5 2.2 1.9 2.4
0.5 0.3 1.1 0.4 1.8
ns ns ns ns
(9%), 3.5 ns (91%) (52%), 11.0 ns (48%) (17%), 3.4 ns (83%) (85%), 5.4 ns (15%)
ns ns ns ns ns
(33%), (24%), (21%), (32%), (47%),
1.3 2.0 1.9 1.6 5.9
ns ns ns ns ns
(68%) (76%) (79%) (68%) (53%)
Two values are shown when a two-exponential analysis was applied to the fluorescence decay curve to obtain a c2 value of 0.99e1.22. Values in parentheses correspond to the relative component amplitudes.
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Fig. 3. Molecular packing in the crystal state of SQDs {ball-and-stick representation (a, b) and wire frame representation (c, d)}; (a, c) DPhA-SQD, (b, d) DTolA-SQD obtained from CIF data [31].
may be attributed to the prevention of the quenching-causing pep interactions between SQD molecules. These results are expected to widen the implementation of SQDs to applications using emissive properties. Better SQD emitters with film-forming properties are now under investigation for optoelectronic device applications. Acknowledgements We would like to thank Nissan Chemical Industries for financial and technical support. We are grateful to Dr. Motoharu Kinugasa for his fruitful suggestions. We would like to thank Enago (www.enago. jp) for the English language review. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.dyepig.2015.06.025. References [1] For reviews: (a) Shimizu M, Hiyama T. Organic fluorophores exhibiting highly efficient photoluminescence in the solid state. Chem Asian J 2010;5:1516e31. (b) Hong YN, Lam JWY, Tang BZ. Aggregation-induced emission: phenomenon, mechanism and applications. Chem Commun 2009:4332e53. (c) Hong YN, Lam JWY, Tang BZ. Aggregation-induced emission. Chem Soc Rev 2011;40:5361e88. (d) Mei J, Hong YN, Lam JWY, Qin AJ, Tang YH, Tang BZ. Aggregation-induced emission: the whole is more brilliant than the parts. Adv Mater 2014;26: 5429e79. € th H, Linti G. The influence of packing effects on the [2] Langhals H, Poirawa T, No solid-state fluorescence of diketopyrrolopyrroles. Angew Chem Int 1989;28: 478e80. [3] Lupton JM, Hemingway LR, Samuel IDW, Burn PL. Electroluminescence from a new distyrylbenzene based triazine dendrimer. J Mater Chem 2000;10: 867e71.
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Contents lists available at ScienceDirect
Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig
A new family of light-emissive symmetric squarylium dyes in the solid state Yutaka Ohsedo a, *, Kowichiro Saruhashi b, Hisayuki Watanabe a, b a
Advanced Materials Research Laboratory, Collaborative Research Division, Art, Science and Technology Center for Cooperative Research, Kyushu University, 4-1 Kyudaishinmachi, Nishi-ku, Fukuoka 819-0388, Japan b Nissan Chemical Industries, Ltd., 2-10-1 Tsubonishi, Funabashi, Chiba 274-8507, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 April 2015 Received in revised form 15 June 2015 Accepted 16 June 2015 Available online 25 June 2015
High-intensity light emission was observed from symmetric squarylium dyes bearing diaryl amine moieties in the solid state unlike typical squarylium analogues. The enhanced light emission in the solid state may be ascribed to the propeller-like dye architectures, which sterically hinder the intermolecular p ep interactions rather than the intramolecular charge transfer. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Squarylium dyes Fluorescence Intramolecular charge transfer Radical cation Fluorescence quantum yield pep interaction
1. Introduction Recently, the structureeproperty relationship investigations of luminescent p-conjugated functional dyes have resulted in high performance dyes that find application as optoelectronic devices and medical imaging tools for affected and targeted regions. Moreover, certain dyes have exhibited higher luminescence upon aggregation or in the solid state rather than the typical fluorescence quenching [1]. This unusual strong light emission of some organic dyes in the solid state has prompted extensive efforts in designing and synthesizing new photoluminescent compounds [2e16]. Certain dyes have also demonstrated enhanced fluorescence upon selective sensing of biomolecules. This aggregation-induced emission can be used to diagnose specific health conditions [1a,1b,17]. Squarine or squarylium dyes (SQDs) are an extensively studied family of functional dyes [18]. These dyes are expected to be used as organic electroactive and photoactive devices such as electroluminescent devices [19], bulk heterojunction organic photovoltaic cells [20], and dye-sensitized solar cells [21]. In the field of medical science, SQDs have been studied as a sensitizer for photodynamic therapy [22] and fluorophores for sensors [23]. Furthermore, * Corresponding author. Tel.: þ81 92 400 4381; fax: þ81 92 400 4382. E-mail address: [email protected] (Y. Ohsedo). http://dx.doi.org/10.1016/j.dyepig.2015.06.025 0143-7208/© 2015 Elsevier Ltd. All rights reserved.
investigations of their use as supramolecular modifiers have revealed their long-lived durability, which is sufficient for medical imaging [24]. Despite extensive synthetic work and photophysical studies, there have been few reports on the solid-state light emission of these dyes. However, studies have reported SQDs bearing an indolenium moiety showed red fluorescence with a weak quantum yield of only 0.02 [25a], whereas indolenine-derived semi-squaric acid showed a quantum yield of 0.2 [25b]. Recently, a new family of asymmetric-type squarylium dyes showing larger fluorescence quantum yield ~0.36 in the solid state was reported [25c]. In this study, we report the enhanced fluorescence of symmetric SQDs. Fluorescence quantum yields increased from <0.01 in DMF to 0.05e0.36 in the solid state. This solid-state fluorescence at such quantum yields is unprecedented in symmetric SQDs. These symmetric SQDs containing diaryl amine moieties can be synthesized from squaric acid and secondary arylamine in a one-step reaction with simple work up (Scheme 1) [26]. 2. Experimental section 2.1. Materials and methods N,N-dimethylformamide (DMF, Spectrosol®, DOJINDO Laboratories), ()-Quinine sulphate dehydrate (98%) and conc. H2SO4
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2.3. Estimation of fluorescence quantum yields
Scheme 1. Chemical structures of SQDs.
(Wako Pure Chemical Industries, Ltd., Japan) were used without further purification. Other solvents (Wako Pure Chemical Industries, Ltd., Japan) are used as received. The SQDs containing diaryl moieties, i.e. MePHA-SQD, DPhA-SQD, NpPhA-SQD, DTolA-SQD, and MeOPhTolA-SQD were synthesized and obtained as described in our earlier report [26].
2.2. Measurements Ultravioletevisible electronic absorption spectra were obtained using a V-670 UVevis spectrophotometer (JASCO Corporation) in a quartz glass cell with a 10 mm path length for ca. 1 106 M SQD solutions. Fluorescence spectra were obtained using an LS55 luminescence spectrometer (PerkinElmer Japan Co., Ltd.) in a quartz glass cell with a 10 mm path length for ca. 1 106 M SQD solutions. Fluorescence lifetimes in DMF and in the solid state were measured by a time-correlated single photon counting fluorescence spectroscopic method using a FluoroCube 1000U (HORIBA, Ltd.) equipped with a NonoLED-375L picosecond laser diode as an excitation light source (peak wavelength 378 nm, HORIBA, Ltd.). LUDOX® colloidal silica or glass plate were used as standards. Fluorescent lifetimes of SQDs were measured at their maximum fluorescence wavelengths (lmax) in solution or in the solid state. Fluorescence lifetime measurements in DMF were performed after N2 bubbling for 15 min. Lifetimes in the solid state were measured by placing SQD between sealed glass plates and fixing the plate edges with mending tape. Crystal structures were visualized from CIF data using Mercury 3.5.1 (Cambridge Crystallographic Data Centre).
Fluorescence quantum yields (Ff) in the solid state were determined using a C9920-02 absolute fluorescence quantum yield measurement system (Hamamatsu Photonics K.K.). Fluorescence quantum yields in solution were obtained relative to an aqueous quinine sulphate solution. Quantum yields were calculated using the following equation with a Fs value of 0.55 for quinine sulphate . in 0.5 M H2SO4 [27]:
Ff ¼ Fs ðI=Is Þ ðAs =AÞ n2 n2s ;
where I is the integration of the fluorescent intensity, A is the absorbance at the excitation wavelength, n is the refractive index of solvent and the subscript s refers to the quinine standard. 3. Results and discussions We already reported that a creation of symmetric SQDs containing diaryl amine moieties synthesized by only refluxing diaryl amine and squaric acid in a solution and the work up without chromatographic techniques yielded SQDs in good yield and purity (Scheme 1) [26]. These SQDs showed better electrochemical reversibility than required for optoelectronic device applications. Their crystal powders showed intense photoluminescence in the solid state upon UV light irradiation (Fig. 1). Typical SQDs show absorption in the visible light region in solution and emit in the red or near-IR regions if they are fluorescent; however, the diaryl amine-containing SQDs showed absorption near 400 nm in the blue-light region. Moreover, they showed fluorescence depending on their molecular structures in the solid state: light green for MePhA-SQD, orange for DPhA-SQD and NpPhA-SQD and green for DTolA-SQD and MeOPhTolA-SQD. Furthermore, the corresponding SQD solutions in DMF (10 mM) showed no photoluminescence to the naked eye (Fig. 1(c)). To elucidate the photophysical properties of SQDs, their fluorescence was measured in DMF and in the solid state, together with the corresponding fluorescence quantum yields (Ff) (Fig. 2, Table 1). Except for MePhA-SQD, the SQDs in solution showed weak fluorescence at ca. 510 nm. However, in the solid state, they showed fluorescence at longer wavelengths than in solution, consistent with the stabilizing effects of pconjugation in the solid (microcrystalline) state. Relative Ff
Fig. 1. Photographs of SQD powders; (a) SQDs under the ordinary fluorescent light, (b) SQDs under UV irradiation with black light (365 nm), (c) SQDs under UV irradiation a with laser pointer (405 nm), (d) 10 mM MeOPhTolA-SQD solution in DMF in a glass cell under UV irradiation (365 nm).
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values for SQDs in DMF were compared to quinine sulphate while absolute values were determined using the integrating sphere in the solid state. SQDs in DMF solution possess very low fluorescence intensities and Ff values < 0.01. In contrast, those in the solid state possess intense fluorescence and larger Ff values such as 0.36 for MeOPhTolA-SQD. These quantum yields are comparable to absolute Ff value for tris(8quinolinolato)aluminium(III) (Alq3) in solid thin films (Ff ¼ 0.20e0.25) [28], which is an excellent electrontransporting green-light emitter used in organic electroluminescent devices. The information about the excited species of SQDs in solution and in the solid state were obtained through fluorescence lifetime measurements (Tables 1 and 2, see also Supplementary data, Figs. S1 and S2 for decay curves). Estimated lifetimes demonstrated the same order of magnitude in solution and in the solid state (except for MePhA-SQD). However, we could not determine all the exited state observed in each state were identical from the lifetime. Comparable lifetimes were measured in both states for NpPhA-SQD (2.2 ns and 1.9 ns), DTolA-SQD (1.9 ns and 1.6 ns), and MeOPhTolA-SQD (5.4 ns and 5.9 ns) suggesting that the excited species in both states might be almost the same for each SQD. Moreover, the excited species emitting light in the solid state were almost quenched by some radiationless transitions in solution. In addition, the SQDs showed two lifetimes in each state and these results might suggest the existence of dual fluorescence originated from probable conformers of SQDs in both solution and the solid
Table 1 Absorption and fluorescence data of SQDs. Sample
lab./nm
log 3
lf soln./nm
Ff soln.
lf solid/nm
Ff solid
MePhA-SQD DPhA-SQD NpPhA-SQD DTolA-SQD MeOPhTolA-SQD
387 411 423 415 418
4.60 4.59 4.55 4.67 4.67
e 510 510 506 510
~0 <0.01 <0.01 <0.01 <0.01
560 563 602 525 534
0.12 0.051 0.16 0.23 0.36
lab.: maximum absorption wavelength [26a], 3: molecular extinction coefficient in DMF solution [26a], lf soln.: maximum fluorescence wavelength in DMF solution, lf solid: maximum fluorescence wavelength in the solid state, Ff soln.: Ff value in DMF solution relative to a quinine standard, Ff solid: absolute Ff value in the solid state. Fluorescence spectra were obtained at lab. values.
state [29]. Further study for understanding the photophysical properties of these SQDs is underway. Generally, fluorescence of organic dyes in the solid state is observed in the absence of quenching from efficient intermolecular pep interactions between dyes [4]. Reports have suggested that intramolecular charge transfer (CT) in the excited state of dyes played an important role in solid-state light emission [6]. Moreover, bulky substituents have been observed to contribute to the efficient fluorescence by preventing intermolecular pep interactions [6,8,9,11]. The calculated HOMO and LUMO of the new SQDs primarily covered the cyclobutane ring and adjacent N atoms and their CT contribution appeared low (see Supplementary data, Fig. S3) [26a]. In contrast, SQDs bearing diaryl amine moieties exhibited propeller-like conformers by crystal analysis, similar to p-conjugated compounds containing triphenylamine or diphenylamine moieties [30]. In crystal structures of DPhA-SQD and DTolA-SQD crystals obtained by recrystallization in CHCl3/n-hexane (Fig. 3), which showed light emission as seen in other SQD samples, the shortest distance between cyclobutane rings are ca. 6 Å (Fig. 3c and e) and there seems to be neither efficient stacking nor direct pep interaction between cyclobutane rings (other SQDs did not provide enough crystals for XRD analysis [31]). The emission enhancement of these SQDs in the solid state may be attributed to the reduced fluorescence quenching by the absence of efficient pep interactions between dyes due to introduction of propeller-like bulky substituents into SQDs. The Ff values in the solid state appeared to depend on the extent of p-conjugation (Table 1). In our previous study, MePhA-SQD, DPhA-SQD and NpPhA-SQD went through one-electron oxidation, whereas DTolA-SQD and MeOPhTolA-SQD underwent twoelectron oxidation because of their extended p-conjugation [26a]. This extended p-conjugation may stabilize the excited species, improving Ff. 4. Conclusions In conclusion, intense fluorescence was observed for symmetric SQDs containing diaryl amine moieties in the solid state. These dyes showed fluorescence quantum yields comparable to the excellent green-light emitter Alq3. The intense fluorescence in the solid state Table 2 Fluorescence lifetime of SQDs.
MePhA-SQD DPhA-SQD NpPhA-SQD DTolA-SQD MeOPhTolA-SQD Fig. 2. Electronic absorption and fluorescence spectra of SQDs. (a) DMF solution, (b) in the solid state.
DMF solution
Solid
e 1.5 2.2 1.9 2.4
0.5 0.3 1.1 0.4 1.8
ns ns ns ns
(9%), 3.5 ns (91%) (52%), 11.0 ns (48%) (17%), 3.4 ns (83%) (85%), 5.4 ns (15%)
ns ns ns ns ns
(33%), (24%), (21%), (32%), (47%),
1.3 2.0 1.9 1.6 5.9
ns ns ns ns ns
(68%) (76%) (79%) (68%) (53%)
Two values are shown when a two-exponential analysis was applied to the fluorescence decay curve to obtain a c2 value of 0.99e1.22. Values in parentheses correspond to the relative component amplitudes.
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Fig. 3. Molecular packing in the crystal state of SQDs {ball-and-stick representation (a, b) and wire frame representation (c, d)}; (a, c) DPhA-SQD, (b, d) DTolA-SQD obtained from CIF data [31].
may be attributed to the prevention of the quenching-causing pep interactions between SQD molecules. These results are expected to widen the implementation of SQDs to applications using emissive properties. Better SQD emitters with film-forming properties are now under investigation for optoelectronic device applications. Acknowledgements We would like to thank Nissan Chemical Industries for financial and technical support. We are grateful to Dr. Motoharu Kinugasa for his fruitful suggestions. We would like to thank Enago (www.enago. jp) for the English language review. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.dyepig.2015.06.025. References [1] For reviews: (a) Shimizu M, Hiyama T. Organic fluorophores exhibiting highly efficient photoluminescence in the solid state. Chem Asian J 2010;5:1516e31. (b) Hong YN, Lam JWY, Tang BZ. Aggregation-induced emission: phenomenon, mechanism and applications. Chem Commun 2009:4332e53. (c) Hong YN, Lam JWY, Tang BZ. Aggregation-induced emission. Chem Soc Rev 2011;40:5361e88. (d) Mei J, Hong YN, Lam JWY, Qin AJ, Tang YH, Tang BZ. Aggregation-induced emission: the whole is more brilliant than the parts. Adv Mater 2014;26: 5429e79. € th H, Linti G. The influence of packing effects on the [2] Langhals H, Poirawa T, No solid-state fluorescence of diketopyrrolopyrroles. Angew Chem Int 1989;28: 478e80. [3] Lupton JM, Hemingway LR, Samuel IDW, Burn PL. Electroluminescence from a new distyrylbenzene based triazine dendrimer. J Mater Chem 2000;10: 867e71.
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