Introduction

Recently, optically active synthetic helical polymers have attracted much attention in the field of intelligent polymer architectures [1,2,3,4,5,6]. In particular, optically active helical π-conjugated polymers will promise interesting technological applications in combination with optical, electronic and magnetic properties. Helix-sense-selective polymerization (HSSP) using chiral catalysts or chiral initiators is a promising method for preparing optically active functional helical polymers because the monomers do not need a chiral moiety, allowing flexibility in the molecular design [7,8,9]. We have already succeeded in the HSSP of achiral phenylacetylenes using the catalyst [Rh(nbd)Cl]2 (nbd = 2,5-norbornadiene) in the presence of chiral 1-phenylethylamine (PEA), and 3,5-bis(hydroxymethyl)phenylacetylene derivatives (HPAs) gave the corresponding polymers (poly(HPA)s), whose rigid and one-handed helical conformation was kinetically stabilized in the solution by intramolecular hydrogen bonds among the hydroxyl groups in the poly(HPA)s (Scheme 1) [10,11,12,13,14,15,16,17]. We have recently reported that the HSSP of HPAs connected with a rigid π-conjugated substituent gave the corresponding optically active helical poly(HPA)s whose side groups were also arranged helically with the preferential helical sense [16, 17]. Some π-conjugated oligomers have been envisaged as materials for multicolor photoluminescence (PL) switching on the basis of structural perturbation [18,19,20,21]. In this study, we found that a biphenyl-substituted poly(HPA) (poly(DBHPA)) showed dual emission, whose intensity was tuned by the polar-solvent stimuli.

Scheme 1
scheme 1

HSSP of HPAs

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Results and discussion

DBHPA was polymerized in the presence of the catalyst [Rh(nbd)Cl]2 and chiral PEA by the same procedures as those used in our previous work [16, 17]. Yellow solid polymers were obtained from the polymerization mixtures by precipitation into methanol (Table 1). DBHPA gave the corresponding polymer with a high degree of polymerization, even though compared with 3,5-bis(hydroxymethyl)-4-dodecyloxyphenylacetylene (DHPA), DBHPA had a rigid and bulky biphenyl group substituted between two hydroxymethyl groups. The solubility of poly(DBHPA) was maintained due to the peripheral alkyl chain. The polymer dissolved well in common organic solvents, such as chloroform, tetrahydrofuran (THF) and toluene.

Table 1 Polymerization of HPAs using [Rh(nbd)Cl]2 in the presence of chiral PEAa
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The CD spectra of poly(HPA)s in THF exhibit split-type-induced CD signals at 300 nm and broad signals at 350–500 nm (Fig. 1). The isosbestic point of the CD signals for poly(HPA)s shifted bathochromically upon increasing the length of the π-conjugated side group. This fact indicates that the split-type-induced CD signals at 300 nm corresponded to the chromophore of the offset intramolecular stack structure including the π-conjugated side group. These CD patterns indicate that the static helical conformation is established by intramolecular hydrogen bonds because the polymers possess no chiral moieties except for helicity. The static stability of the helical conformation was diminished by the addition of polar solvents such as DMF and DMSO because the intramolecular hydrogen bonds were collapsed by the polar solvents, which was confirmed by the disappearance of CD signal upon adding polar solvents (Supplementary Fig. S1) [17].

Fig. 1
figure 1

CD and UV-vis absorption spectra of poly(DBHPA), poly(DPHPA), and poly(DHPA) in THF at 20 °C

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THF solutions of poly(DBHPA) and poly(DPHPA) exhibited a PL maximum at 390 and 370 nm, respectively, when excited at 300 nm (Fig. 2). The PL maximum wavelength of the former polymer was longer than that of the latter one by 20 nm. Moreover, the PL intensity of poly(DBHPA) was stronger than that of poly(DPHPA) at the same concentration. The corresponding monomers also exhibited the same tendency in their PL properties. Thus, the difference in PL between the two polymers is probably due to the difference in π-conjugation length between the chromophore side groups. This means that the PL emission upon excitation at 300 nm originates from the chromophore side groups.

Fig. 2
figure 2

UV-vis absorption and PL emission spectra of poly(DBHPA), poly(DPHPA), DBHPA, and DPHPA in THF at 20 °C (0.01 mM, λex = 300 nm). Spectral intensities are normalized at their maxima

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Quite interestingly, the PL emission of the polymers was highly variable depending on the excitation wavelength, while the corresponding monomers did not show such a dependency. As shown in Fig. 3, poly(DBHPA) excited at 280 nm emitted PL at 310 nm, which is quite different from its behavior upon excitation at 300 nm (Supplementary Fig. S2). On the other hand, when excited at 280 nm, the monomer DBHPA emitted PL at 370 nm, which is almost the same as that exhibited by the polymer when excited at 300 nm. This indicates that our polymers have at least two excited states. Where do these multiple excited species come from? They are closely related to intramolecular hydrogen bonding. To elucidate this idea, we examined the PL emission change accompanying the addition of polar solvent. Adding DMSO to the THF solution of poly(DBHPA) weakened the PL emission at 310 nm but significantly enhanced the PL emission at 390 nm (Fig. 3), as well as increasing the absorption at approximately 450 nm attributed to the extended π-conjugation of the polymer backbone (Supplementary Fig. S3). Similarly, according to a previous study, poly(DHPA), which has a shorter π-conjugated chromophore in its side group, emitted quite strong PL at 310 nm in THF solution when excited at 280 nm, while the emission disappeared in a polar solvent such as piperidine [22]. It was also found that the PL of poly(DHPA) originates from the excimer based on the intramolecular stack structure of the side phenyl rings which are locked tightly through intramolecular hydrogen bonding. In this respect, it was assumed that the intramolecular hydrogen bonding was broken in the presence of a polar solvent and that the intramolecular stack structure was then readily collapsed. This led to a significant PL quenching of the excimer emission. This is true in the case of poly(DBHPA), as described above. Consequently, the shorter wavelength emission band at 310 nm originates from the intramolecular excimer of the side phenyl rings, while the longer wavelength band at 390 nm comes from the π-conjugated chromophore of the side group. Although the side group of poly(DBHPA) consisted of the terphenyl group, the biphenyl moiety connected to the bis(hydroxymethyl)-substituted phenyl moiety would be highly twisted by the steric hindrance induced by the ortho-substituent. This highly twisted structure was supported by MO calculation and the blueshift of the absorption spectra compared to that of the simple alkoxyterphenylacetylene monomer (Supplementary Fig. S4 and Fig. S5) (see the Supplementary Information for more detailed experimental procedures). The torsional angle of the simple biphenyl structure is distributed around 45° [23, 24]. The torsional potential of DBHPA was calculated by AM1 to exhibit a minimum at 90°, which did not conflict with the theoretical calculations of other o-substituted biphenyls [25]. For DBHPA with a torsional angle of 90°, the HOMOs were isolated from each other at the twisted C–C bond. Therefore, similar to poly(DHPA), poly(DBHPA) with a torsional angle of 90° was excited at 280 nm in THF to give the excimer PL at 310 nm. When the intramolecular stack structure was collapsed by the addition of polar solvent, the moderately twisted excited terphenyl chromophore was formed by the rotation of the excited species with a highly twisted structure and/or by the energy transfer from the species excited at 280 nm instead of forming the excimer state, and the moderately twisted excited terphenyl chromophore emitted fluorescence at 390 nm (Fig. 4).

Fig. 3
figure 3

PL emission spectra of poly(DBHPA) (solid line) and DBHPA (dotted line) in THF/DMSO mixed solvent systems at 20 °C (0.01 mM, λex = 280 nm)

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Fig. 4
figure 4

Schematic diagram of the PL emission of poly(DBHPA)

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Conclusions

Optically active helical poly(DBHPA) has two chromophores: the phenyl moiety, which partially includes the π-conjugated backbone excited at 280 nm (Chromophore P), and the moderately twisted terphenyl moiety excited at 300 nm (Chromophore T). Photoexcitation of Chromophore T at 300 nm led to PL at 390 nm. On the other hand, photoexcitation of Chromophore P at 280 nm resulted in dual emission at 310 and 390 nm, whose intensities were tuned by the polar-solvent stimuli. That is, the photoexcitation at 280 nm of Chromophore P of optically active helical poly(DBHPA) in THF solution generated excimer PL at 310 nm based on the intramolecular stack structure which is locked tightly through intramolecular hydrogen bonding. Furthermore, the addition of a polar solvent induced the transformation and/or energy transfer from Chromophore P to Chromophore T, causing fluorescence emission at 390 nm accompanied with collapse of the intramolecular stack structure and optically active helical structure instead of the excimer PL. This solvent-tuned dual emission is a very unusual and unique phenomenon in optically active helical π-conjugated polymers. In other words, the excimer PL appeared at a shorter wavelength than another PL, while they were different species, and the change in the emission wavelength was accompanied by disappearance of the optical activity. This offers promise for more advanced functions and applications.