The effect of regioisomerism on the photophysical properties of alkylated-naphthalene liquids

B. Narayan a, K. Nagura a, T. Takaya b, K. Iwata b, A. Shinohara c, H. Shinmori c, H. Wang d, Q. Li d, X. Sun e, H. Li e, S. Ishihara a and T. Nakanishi *a
aFrontier Molecules Group, International Centre for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba 305-0044, Japan. E-mail: NAKANISHI.Takashi@nims.go.jp
bDepartment of Chemistry, Faculty of Science, Gakushuin University, 1-5-1 Mejiro, Toshima-ku, Tokyo 171-8588, Japan
cDepartment of Biotechnology, Interdisciplinary Graduate School of Medicine and Engineering, University of Yamanashi, 4-4-37 Takeda, Kofu 400-8510, Japan
dLiquid Crystal Institute, Kent State University, 1425 University Esplanade, Kent, OH 44242-0001, USA
eLanzhou Institute of Chemical Physics, Chinese Academy of Sciences, No. 18 Tianshui Middle Road, Lanzhou 730000, P. R. China

Received 16th August 2017 , Accepted 11th September 2017

First published on 11th September 2017


Novel regioisomeric alkylated-naphthalene liquids were designed and synthesized. In the solvent-free liquid state, 1-alkyloxy regioisomers showed excimeric luminescence, whereas 2-alkyloxy analogues exhibited monomer-rich luminescence features. Correlations among the molecular structures and the photophysical, calorimetric, and rheological properties are presented, demonstrating the impact of regioisomerism on the alkylated-chromophore liquid systems.


Functional π-conjugated chromophores1,2 have become an integral part of day-to-day utilities such as organic electronics and biomarkers. However, the attractive interactions between the π-conjugated units often result in the formation of π–π-stacked aggregates that are solids and barely soluble in common solvents.3 Moreover, the intermolecular π–π interactions lead to optical properties that differ from that of the innate chromophore, for example, in terms of the wavelengths and intensities of the absorption and emission maxima. Because of these uncontrollable optical features, it is difficult to predict the performance of optical devices fabricated with thin films of chromophores at high densities. The direct utilization of the intrinsic optical properties of chromophores in their solvent-free condensed phases is consequently a challenging task.

In this regard, room-temperature functional molecular liquids (FMLs) are attractive alternatives.4–7 Their nonvolatile, free-flowing, and deformable nature permits them to act as solvents that can accommodate dopant molecules.8–12 The resulting fluid mixtures display efficient energy transfer and can finely control their luminescence colour8–10 or their generated photocurrent.11,12 Tethering of multiple bulky alkyl chains at the periphery of the π-conjugated core unit can lead to isolation of the π-core and suppression of photoinduced side reactions, such as dimerization or oxidation.9 More importantly, this π-core isolation strategy of chemical functionalization with multiple alkyl chains leads to stabilization of the nonaggregated monomeric species in the solvent-free bulk phase.13 However, realization of such molecular designs requires challenging and laborious multistep syntheses. Furthermore, because of the larger volume of the peripheral bulky alkyl chains, the density of functional π-cores is somewhat reduced.

Another effective strategy for creating FMLs involves monoalkylation of the π-conjugated chromophore. One branched alkyl chain substituent on the periphery of a small dye unit, for instance, a pyrene13–16 or carbazole moiety,17 can produce the corresponding solvent-free liquid at room temperature. However, the resulting liquids form random chromophore–chromophore aggregates, as demonstrated by the appearance of shifts in their absorptions and excimer emissions. The chemistry of FMLs would be more meaningful and attractive if the liquids could be synthesized in fewer steps and by much simpler methods, while maintaining the inherent monomeric functions (optical properties) of the π-conjugated chromophore in their solvent-free state. Even pristine benzene, the smallest aromatic core that exists as a liquid at room temperature, has a tendency to form excimers under the solvent-free neat conditions.18,19 Therefore, in an attempt to create FMLs with predictable properties, we chose a naphthalene chromophore. Naphthalene and its derivatives have a relatively low association constant of self-organization,20,21 and therefore their monomeric or excimeric luminescence features will reflect the extent of naphthalene–naphthalene interactions. Another advantage of using naphthalene is the ease with which it can be regioselectively mono-alkylated at the 1- or 2-position.22,23 The position of the alkyl-chain substituent can alter the electronic structure and molecular packing of the naphthalene unit, thereby permitting a systematic study of correlations pertaining to the molecular structure, in particular the effects of regioisomerism on the photophysical, calorimetric, and rheological properties.

We designed the 1- and 2-alkylated naphthalenes 1–6 (Fig. 1a), with three branched alkyl chains of different lengths, which we planned to synthesize by O-alkylation of 1- or 2-naphthol, respectively. We have chosen Guerbet alcohols24–26 in this regard, which are well-known for their unique branching pattern, oxidative stability and liquidity of the alkylated products. Three different Guerbet alcohol-based branched alkyl chains, 2-butyloctyl (2-C4C8), 2-hexyldecyl (2-C6C10) and 2-octyldodecyl (2-C8C12), were chosen. First, the alcohols were converted into the corresponding tosylates, and then attached to 1- or 2-naphthol (see ESI, Scheme S1) to give the corresponding final compounds in relatively high yields (∼70%). The products were purified by silica-gel column chromatography and unambiguously characterized by means of 1H and 13C NMR spectroscopy and MALDI-TOF mass spectrometry (Fig. S1–S18, ESI). All 1–6 were colourless liquids under daylight (Fig. 1b and Video S1, ESI) and showed blue luminescence features under UV irradiation (λex = 254 nm; Fig. 1c and Video S2, ESI) at room temperature. In addition, the 1H NMR spectra and thermogravimetric analyses (TGA, Fig. S19, ESI) confirmed the absence of residual solvent in the neat samples.


image file: c7cp05584f-f1.tif
Fig. 1 (a) Chemical structures of regioisomeric liquid naphthalenes (1–6). The odd series represent the 1-regioisomers and the even series is the 2-regioisomers. The chain lengths chosen for this study are C4C8, C6C10 and C8C12. Photographs of 1 dropping down through the PTFE needle under ambient light (b) and UV (254 nm) light (c).

The physical nature of alkylated-naphthalenes was investigated by means of rheology measurements. The viscous loss modulus (G′′) was higher than the storage elastic modulus (G′) throughout the range of applied angular frequencies (ω), indicating an innate liquid behaviour (Fig. S20, ESI). The complex viscosities (η*) were in the range of 38.4 to 67.0 mPa s at 25 °C (ω = 10 rad s−1) (Fig. S21, ESI and Table 1); these values are much lower than those of the previously reported alkylated π-FMLs from our laboratory, such as alkylated oligo(p-phenylenevinylenes) (0.64–34.6 Pa s),8 fullerenes (0.26–128 kPa s),27–29 pyrene (2.3–58.0 Pa s),13 or anthracenes (0.28–84.0 Pa s).9 This smaller value of η* might be explained by the smaller size of molecules 1–6.30 The η* values increased upon increasing the length of the alkyl chains in 1–6 (and thereby the size of the whole molecule). The amorphous liquid nature was evaluated by small- and wide-angle X-ray scattering (SWAXS) analysis. All samples showed only two broad halos, revealing an amorphous nature for 16 (Fig. 2a and b). The halo at a higher q value indicated a molten state of the alkyl chain, whereas the diffuse halo at a lower q of ∼4 nm−1 corresponded to the average naphthalene–naphthalene distance, which ranges from 1.08 to 1.78 nm. The breadth of the halo indicated that various conformations/orientations of the naphthalene unit were present. Note that the halo at the lower q value shifted gradually to a smaller value of q with increasing chain length. We assume that an attached longer (and therefore bulkier) branched-alkyl chain increases the average distance between naphthalene units (Table 1). In addition, although the derivatives bearing 2-C8C12 chains (5 and 6) are exceptions, the first halo for the 1-regioisomers generally occurred at a slightly higher q value than that for the 2-regioisomers. This is presumably due to the presence of smaller but relatively strong naphthalene–naphthalene interactions in the 1-isomers compared with the 2-isomers.

Table 1 Thermal, rheological, and structural properties of alkylated naphthalene liquids 1–6
Compound T d[thin space (1/6-em)]95% /°C T g /°C η* [thin space (1/6-em)] /mPa s Ave. dπ–πd/nm
a Decomposition temperature at 95% of TG. b Determined by the second heating scan. The onset of glass transition in the heating trace is taken. Scan rate: 10 °C min−1 under N2. c Determined at an angular frequency ω = 10 rad s−1 at 25 °C. d Average distance taken from the aromatic halo top position in SWAXS results.
1 201.6 −73.1 38.4 1.08
2 214.4 −70.4 46.2 1.31
3 233.8 −75.9 49.8 1.44
4 241.7 −74.8 59.4 1.50
5 270.8 −75.6 63.9 1.78
6 271.1 −73.3 67.0 1.53



image file: c7cp05584f-f2.tif
Fig. 2 SWAXS profiles for (a) the 1-regioisomers (1, 3, and 5) and (b) the 2-regioisomers (2, 4, and 6). POM images of 1 (c) and 2 (d), respectively. The region marked with asterisk denotes the sample. (e) DSC cooling and heating traces of 1 and 2. Scan rate is 10 °C min−1.

Polarizing optical microscopy (POM) observations showed no birefringence. In conjunction with the SWAXS results, these suggest that all the compounds lack a long-range ordered phase, i.e., they are amorphous (Fig. 2c, d, and Fig. S22, ESI). Thermogravimetric analysis (TGA) indicated that the liquid naphthalenes are thermally stable up to 201.6–271.1 °C (Table 1 and Fig. S19, ESI) and that the thermal stability increases with increasing chain length. Differential scanning calorimetry (DSC) analysis of the cooling process showed that a liquid-to-glass phase transition occurred at an onset temperature of about −75 °C (Fig. 2e and Fig. S23, ESI). This transition did not appear to be complete at −100 °C (the cooling limit of the instrument). This wide temperature range for the glass transition might have resulted from contributions from the many different conformations of the molecules. Similarly, upon heating from the glass state, all compounds exhibited only a glass-to-liquid phase transition at around −73 °C (Table 1). We therefore conclude that 1–6 are amorphous and stable liquids at room temperature.

However, the conventional characterization methods described above are insufficiently powerful to provide nanoscale/nanodomain structural information on amorphous matter. As an alternative, we carried out optical spectroscopy investigations by comparing the compounds in their dilute-solution and solvent-free liquid states. Compounds 1 and 2, bearing shorter C4C8 alkyl chains, were chosen as representative examples. A dilute solution (0.1 mM) of 1 in n-hexane exhibited structureless absorption features with a peak maximum at 294 nm (Fig. 3a), whereas the corresponding solution of 2 showed more vibronic features in its absorption spectrum (Fig. 3b), which was basically distinct from that of other naphthalene regioisomers.31 This suggests that regioisomers 1 and 2 have intrinsically different electronic structures (see Fig. S24, ESI). As we expected, the emission spectra of the dilute solutions of 1 and 2 revealed sharp monomeric fluorescence features. The absolute fluorescence quantum yield of 2 in solution was slightly higher (ΦFL = 0.09) than that of 1 (ΦFL = 0.06) (Table 2). The fluorescence lifetime (τ) of 6.2 ns for 1 and 5.3 ns for 2 corresponded well with mono-exponential decay, indicating that the fluorescence originated from monomeric components (Table 2).32 The average τ-value of 1–6 in the solution state was determined to be around 6 ns, which is in good agreement with the reported values in the literature.33 Spectroscopic investigations in the solvent-free liquid state permitted an examination of the inherent effect of regioisomerism on the bulk optical properties of the liquid naphthalenes. Liquid samples of both 1 and 2 showed absorption spectra red-shifted by only a few nanometres compared with the corresponding solutions. In addition, the peak shapes were basically similar, indicating very weak intermolecular interactions in the ground state (Fig. 3). Note that the fluorescence spectrum of 1 showed features corresponding to the monomer mixed with broad excimer-like emissions, whereas monomer-enriched emissions with much smaller excimer features were apparent in the case of 2. Furthermore, the contribution of monomer emissions in the solvent-free phase of the 1-substituted regioisomers increased upon lengthening the branched alkyl chain (Fig. S24, S25 and Table S1, ESI). This confirmed the liquefying and bulky nature of the branched alkyl chains, which disperse and/or dilute the π-aromatic cores, thereby weakening the intermolecular interactions between neighbouring naphthalene units.


image file: c7cp05584f-f3.tif
Fig. 3 Normalized UV-visible absorption (solid lines) and fluorescence (dotted lines) spectra of 1 (a) and 2 (b) in the n-hexane solution ([c] = 0.1 mM, red lines) and in the solvent-free liquid state (blue lines), respectively. Arrows have been provided to guide the eye.
Table 2 Photophysical properties of 1–6 in the n-hexane solution ([c] = 0.1 mM) and in the solvent-free liquid state
Compound Solution Solvent-free liquid
Φ FL τ /ns k r /107 s−1 k nr /108 s−1 Φ FL τ 1 /ns

τ 2 /ns

R
a Absolute fluorescence quantum yield determined by a calibrated integral sphere system. b Fluorescence lifetime at 350 nm. c Radiative decay rate constants: kr = ΦFL/τ. d Non-radiative decay rate constants: knr = (1 − ΦFL)/τ. e Obtained by the global least-squares fitting analysis from 330 to 580 nm. f Defined by eqn (1) in the main text.
1 0.06 6.2 0.97 1.52 0.16 0.72

11

0.83
2 0.09 5.3 1.70 1.72 0.15 1.4

15

0.80
3 0.09 5.6 1.61 1.63 0.16 0.84

10

0.84
4 0.15 6.2 2.42 1.37 0.22 1.8

15

0.79
5 0.12 6.1 1.97 1.44 0.18 1.1

11

0.80
6 0.14 5.5 2.55 1.56 0.26 2.2

16

0.78


Interestingly, the fluorescence quantum yields (ΦFL) in the solvent-free liquid state were higher than those in n-hexane solutions (Table 2). The ΦFL values for the 2-substituted regioisomers were similar to those for the 1-substituted series for the C4C8 derivatives (12), whereas their values were slightly higher for derivatives with longer alkyl chains (3 < 4; 5 < 6). The difference became more pronounced upon increasing the chain length. This might be related to the efficient dispersion of the naphthalene core unit, as lengthening the substituent chains should reduce the probability of self-quenching.8,34

The most important finding in these luminescence properties of the alkylated-naphthalene liquids is that the 1-substituted regioisomers showed excimeric features, whereas the 2-substituted isomers maintained monomeric luminescence features in the solvent-free liquid state. To obtain deeper insights into the reasons underlying these different luminescence features in the liquid state, we determined the radiative (kr) and nonradiative (knr) decay constants. The values of kr and knr for the monomeric species in dilute solutions were calculated from the ΦFL and τ data by fitting to equations kr = ΦFL/τ and knr = (1 − ΦFL)/τ (Table 2). Although the knr values were comparable for all the derivatives, the values of kr for the 2-substituted derivatives were larger than those for the 1-substituted ones. This suggests that the monomer-emission decay process of the 2-substituted isomers is intrinsically faster than that of the 1-substituted isomers.

Picosecond time-resolved fluorescence measurements of the solvent-free liquid naphthalenes revealed their excimer formation dynamics (Fig. S26, ESI). The fluorescence intensity in the visible region was underestimated in the picosecond fluorescence measurements because the spectrometer was optimized for the detection of fluorescence in the 330–380 nm region, where both the spectrograph and the detector had relatively low sensitivities. The results showed a rapid decay in the monomer fluorescence in the 330–380 nm region, and a rise of excimer fluorescence in the 380–500 nm region, with an isoemissive point of between 0 and 2–4 ns for all the compounds (Fig. S27, ESI). This behaviour was assigned to direct formation of excimers by the electronically excited and electronically unexcited monomers. The monomer and excimer fluorescence signals decayed simultaneously after 4 ns. This simultaneous decay strongly suggests that the excimer reaches equilibrium with the monomers after the excimer-formation process. The equilibrium between the monomers and the excimer is probably caused by the long and bulky side chains, which weaken the π–π interactions between the stacked naphthalene units.

The fluorescence-decay traces between 330 and 580 nm were well reproduced by a global least-squares fitting analysis with biexponential functions with two common time constants (Table 2). The τ1 value indicates the time constant to attain equilibrium between the monomers and excimers, because the decay-associated spectrum with a time constant of τ1 shows a decay in monomer fluorescence at 330–380 nm and a rise in excimer fluorescence at 380–500 nm (Fig. S28, ESI). The value of τ1 ranged from 0.72 to 1.1 ns for the 1-substituted regioisomers, and from 1.4 to 2.2 ns for the 2-substituted ones. A positive correlation was found between the equilibration time constant and the viscosity, with the equilibration time constant increasing (Table 2) with decreasing viscosity (Table 1). This correlation suggests that excimer formation is controlled by the translational and/or rotational diffusion of the molecules.

The value of τ2 is an indicator of the lifetime of the excited states, because the decay-associated spectrum with a time constant of τ2 shows a simultaneous decay of both monomer and excimer fluorescence. The values of τ2 were estimated to be 10–11 and 15–16 ns for the 1- and 2-substituted regioisomers, respectively (Table 2). The difference in the τ2 values between the two types of regioisomers is significant, whereas the influence of the length of the side chain is negligible. The lifetimes of the excited states in the solvent-free liquid state are therefore predominantly determined by the position of the substituent, as well as by the lifetimes of the monomers in solution. The relative intensity (R) of the first decay component compared with the second decay component at the time origin, which represents the ratio of the number of initially excited monomer molecules that are converted to the excimer to the total number of initially excited monomers, is given by the following expression.

 
image file: c7cp05584f-t1.tif(1)
Here, I0mi (i = 1, 2) is the pre-exponential factor of the i-th exponential function. The obtained R values are shown in Table 2. Although the 1-substituted regioisomers showed larger excimer yields than did the 2-substituted ones, the difference was not significant. However, both the steady-state (Fig. 3) and the time-resolved fluorescence spectra (Fig. S28, ESI) showed much stronger excimer fluorescence intensities for the 1-substituted regioisomers. This apparent discrepancy between the calculated excimer yield and the observed fluorescence intensity of the excimer can be explained in the following manner. For the steady-state fluorescence spectra, the intensity ratio of excimeric fluorescence to monomeric one ρ is given by the ratio of their quantum yields:
 
image file: c7cp05584f-t2.tif(2)
Here m and e denote monomers and excimers, respectively. The ratio of the fluorescence lifetimes, τ2/τ1, can be calculated from the values in Table 2. It is shown from the calculation that the 1-substituted regioisomers have larger τ2/τ1 values than the 2-substituted ones. If it is assumed that the 2-substituted regioisomers have larger kmr values than the 1-substituted ones in the neat liquid state as well as in solutions while the two regioisomers have almost identical ker values to each other, then eqn (2) gives a greater ratio, ρ, for the 1-substituted isomers. Although a large spectral overlap of the monomer and excimer fluorescence hampers an experimental estimation of the ker values, the above assumption is plausible as long as the two regioisomers form excimers with a similar π–π stacking structure.

To understand the electronic effects of alkyloxy substituents on the naphthalene moiety, we performed theoretical calculations for 1 and 2 (Tables S2–S4, ESI). The regioisomerism of the 1- and 2-alkyloxy substituents perturbs their frontier molecular orbitals in a distinct manner (Fig. S29 and S30, ESI). Accordingly, the dipole moments of the energy-minimized structures of alkylated-naphthalenes showed a slight difference between that of 1 (1.2421 D) and that of 2 (1.0843 D) (Fig. 4). As diamagnetic organic molecules tend to cancel their dipole moments to remain in an equilibrium state in the absence of an external electric field, we speculate that 1 shows enhanced π-stacking interactions compared with 2. The molecular organization in 1 is expected to have closer stacking of the naphthalene units and greater face-to-face overlaps with opposite orientations of the longitudinal axis.


image file: c7cp05584f-f4.tif
Fig. 4 Dipole moments (blue arrows) and electrostatic potential maps of liquid naphthalenes 1 and 2, calculated at the MP2/6-31G(d)//B3LYP/6-311G(d,p) level.

Conclusions

Retention or emphasis of intrinsic molecular functions in the solvent-free bulk liquid state of alkylated-naphthalenes was made possible by rational design of regioisomeric substitution. Detailed investigations into the luminescence features and liquid physical properties indicated the presence of intricate differences in the 1- or 2-regioisomeric series of liquid naphthalenes. This simple molecular-design strategy might be applicable to other alkylated-π liquid systems and could pave the way to direct exploitation of their inherent chromophoric nature in various practical applications, for example in inks or in printed electronics technologies.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was partially supported by the Grants-in-Aid for Scientific Research (JSPS KAKENHI Grant Numbers JP25104011, JP15H03801, JP16H00850 and JP16H00782), the NIMS Molecule & Material Synthesis Platform in the “Nanotechnology Platform Project”, the MEXT-Supported Program for the Strategic Research Foundation at Private Universities, 2015–2019, from MEXT, Japan, and the TIA collaborative research promotion program “KAKEHASHI”. We also acknowledge the NIMS Internship Program.

Notes and references

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Footnote

Electronic supplementary information (ESI) available: Experimental including synthesis, TGA, rheology, POM, DSC, UV-vis, steady state FL, picosecond TRFL, global least-squares fitting analysis and theoretical calculations. See DOI: 10.1039/c7cp05584f

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