Chromic properties of dibenzo[j,l]fluoranthenes exhibiting different resonance contributions

Abstract

Chromic molecules change colour in response to external stimuli and are utilized in applications such as food additive detection, light dimmers, and biological probes. One of the common design strategies for organic chromic molecules is based on changes in the π-conjugation. We have hypothesized that non-alternant polyaromatic hydrocarbon (PAH) skeletons can be used as backbones for chromic molecules. Herein, we synthesized hydroxy-substituted dibenzo[j,l]fluoranthenes, a class of non-alternant PAHs, as novel chromic compounds and evaluated their halochromic properties by UV-vis and fluorescence spectroscopy. Under basic conditions, the 1-hydroxy derivatives show a hyperchromic shift, whereas the 9-hydroxy derivatives show a bathochromic shift and fluorescence although the skeleton of the chromophore is the same. Density functional theory calculations indicated that the different chromic properties are attributed to the differences in their resonance structures.

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1. Introduction

Chromism is the reversible colour change that a compound undergoes induced by external stimuli (pH, solvent, heat, light, mechanical stimuli, etc.) and is used for food additive detection,¹ light dimmers,² and biological probes.³ The design strategies of organic chromic molecules are usually based on changes in the π-conjugation^3a,4 or molecular aggregation of long linearly conjugated molecules.⁵ Polycyclic aromatic hydrocarbons (PAHs) have also attracted considerable attention as chromophores because of their expected absorption in the visible and near-infrared regions.⁶ Fluoranthenes, whose skeletons contain benzene and naphthalene units connected by pentagonal carbocycles, represent a class of non-alternant PAHs. Fluoranthenes are found not only in incomplete combustion products as pollutants,⁷ but also in nature as biologically active compounds.⁸ The fluoranthene core is also utilised as a chromophore in materials science. Fluoranthene derivatives have been developed for use in fluorescent sensors,⁹ optoelectronic devices,¹⁰ and dye-sensitised solar cells.¹¹

Several efforts for the synthesis of fluoranthene derivatives have been reported,¹² and recently we have also achieved the synthesis of π-extended fluoranthenes by a domino reaction (Scheme 1).¹³ Thus, a reaction of biaryl compound 1a bearing acyl and naphthylalkenyl moieties with KHMDS afforded 1-hydroxydibenzo[j,l]fluoranthene 2avia a (2 + 2) cycloaddition–S_NAr–O,C migration–retro (2 + 2) cycloaddition sequence.

Scheme 1 A domino reaction giving dibenzofluoranthene 2a.

During the course of our study, we found that the UV-vis absorbance of 2a changed reversibly depending on the pH of the solution (halochromism). The chromic property of 2a was analysed by UV-vis and fluorescence spectroscopy under neutral and basic conditions in CH₂Cl₂ solution (Fig. 1). The absorption spectrum of 2a exhibited major bands at 400 nm (moderate) and 420–500 nm (broad, weak) under neutral conditions (Fig. 1a, black line). When 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) was added to 2a, the absorption at 450 nm gradually increased and became saturated with the addition of 1.0 equivalent of DBU (Fig. 1a, red line, basic conditions). In contrast, the absorption spectrum was not changed even when an excess amount of trifluoroacetic acid (TFA) was added (acidic conditions, see the ESI, Fig. S3†). The emission spectrum of 2a shows yellow-green fluorescence at 543 nm (λ_ex = 381 nm) under neutral conditions (Fig. 1b, black line). When DBU was added to the solution of 2a, the fluorescence was dramatically strengthened at almost the same wavelength (Fig. 1b, red line). For 2a, the obtained fluorescence quantum yield was 3.9% under neutral conditions, and it increased to 8.7% when DBU was added. Reversible photophysical changes in 2a with DBU were observed upon the addition of TFA. The colour change was accompanied by isosbestic points. This means that the resonance system of the dibenzofluoranthene skeleton, which is composed of naphthalene and phenanthrene rings, is largely recombined by the change in pH.

Fig. 1 (a) UV-vis absorption and (b) fluorescence spectra of 2a in CH₂Cl₂, with added DBU at 296 K (λ_ex = 381 nm). (c) The colour change of 2a under daylight (above) and 365 nm-UV irradiation (bottom) with addition of DBU.

We envisaged that the introduction of an auxochrome, such as a hydroxy group, at the appropriate position in the chromophore would induce characteristic chromic properties depending on the resulting resonance hybrid. We designed two types of hydroxy-substituted dibenzo[j,l]fluoranthenes, with substitutions at positions 1 and 9, viz. 2 and 3, respectively. We expected that the resonance contribution could be controlled by adjusting the position of the hydroxy substituent. Thus, the contribution of the fluorenyl-type structure increased in the anion of 2, whereas that of the quinoid-type resonance increased in the anion of 3 (Fig. 2). Herein, we report the syntheses and photophysical properties of dibenzo[j,l]fluoranthenes bearing a hydroxy group, which exhibit halochromism in their UV-vis absorption and fluorescence spectra based on changes in their π-conjugation states.

Fig. 2 Typical resonance structures of dibenzofluoranthene and the designed dibenzo[j,l]fluoranthenes 2a–c and 3a–c.

2. Results and discussion

2.1 Synthesis of hydroxydibenzofluoranthenes

We designed 1-hydroxy-9-methoxydibenzo[j,l]fluoranthene (2b) as a common synthetic intermediate for the synthesis of 2c and 3a–c (Scheme 2). In line with our previous report,¹³ the treatment of biaryl ketone 1b with KHMDS afforded 1-hydroxy-9-methoxy derivative 2b in 62% yield. Subsequent triflation of 2b produced compound 5 in 86% yield, which, upon Pd-catalysed reduction in the presence of Et₃SiH and subsequent demethylation using BBr₃, led to the formation of 9-hydroxyfluoranthene 3a in 70% yield (two steps). Pd-catalysed cyanation of 5, followed by demethylation using a thiolate anion, afforded 1-cyano-9-hydroxy derivative 3c in 62% yield (two steps). 9-Hydroxy-1-methoxy 3b was synthesized from 2b in two steps. Methylation of 2b gave 1,9-dimethoxy derivative 6 in 91% yield. When 6 was treated with a slightly excess amount of BBr₃ (1.34 equiv.) at 20 °C, demethylation at the less hindered methyl ether mainly proceeded to give 9-hydroxy-1-methoxy 3b in 51% yield. 9-Cyano-1-hydroxy derivative 2c was also prepared from 3b, following the synthetic route for 3c from 2b. The control compound, non-hydroxy derivative 4, was synthesized from 2a in 98% yield.

Scheme 2 Preparation of 2b–c, 3a–c and 4.

Single crystals of 2b suitable for X-ray crystallographic analysis were successfully grown by slow diffusion of hexane into the solution of 2b in ethyl acetate (Fig. 3). The dibenzofluoranthene skeleton of 2b is completely planar.

Fig. 3 X-ray crystallographic structure of 2b (CCDC 2251568).†

2.2 Photophysical characterization of 2 and 3

Initially, to validate our working hypothesis, the photophysical properties of non-hydroxy derivative 4 were investigated (Fig. 4a). Unlike compound 2a, almost identical UV-vis absorption spectra were observed for 4 in the absence or presence of DBU. This observation strongly suggests that the chromic behaviour of 2a is induced by the deprotonation of its phenolic proton. Next, we explored the substitution effect at the 9 position of 1-hydroxyfluoranthene 2. The series of compounds 2 exhibited similar absorption and emission spectra and hyperchromic properties, regardless of whether the substituent was an electron-donating (2b) or electron-withdrawing (2c) group (Fig. 4b–f). The fluorescence quantum yields (Φ) of 2 in CH₂Cl₂ were weak (Table 1). Notably, the Φ values of 2 under basic conditions (in the presence of DBU) were higher than those under neutral conditions, with 2c showing the highest quantum yield (10%) under basic conditions. The fluorescence lifetimes (τ) of 2 under neutral and basic conditions were determined using the time-correlated single photon counting method in CH₂Cl₂. As the result, it was found that the radiative rate constants (k_F = Φ/τ) of 2a increased with the addition of DBU (1.29 × 10⁷ s⁻¹ under neutral conditions; 3.11 × 10⁷ s⁻¹ under basic conditions).

Table 1 Photophysical properties of the 1-hydroxydibenzofluoranthenes 2 in CH₂Cl₂ ^a

Compounds	Neutral form (no additive)					Anionic form (in the presence of DBU)^b
	λ Abs [nm] (ε [10^3 L (mol cm)^−1])	λ em [nm]	Φ [%]	τ [ns]	k F [10^7 s^−1]	λ Abs [nm] (ε [10^3 L (mol cm)^−1])	λ em [nm]	Φ [%]	τ [ns]	k F [10^7 s^−1]
a Both absorption and emission spectra were recorded in solution (c = 5.0 × 10^−5 M) at 296 K. b Each hydroxy-dibenzofluoranthene (anionic form) was studied in the presence of DBU (for 2a–c: 1.0 eq.). c Quinine sulfate was used as a reference dye (λex = 366 nm, Φ = 55%).^14 d The radiative rate constants were calculated from the experimentally measured fluorescence quantum yields and lifetimes as kF = Φ/τ.
2a	400 (8.84), 451 (1.92)	543	3.9	3.03	1.29	452 (12.1)	537	8.7	2.80	3.11
2b	402 (6.40), 461 (2.00)	565	3.9	2.85	1.37	458 (9.00)	554	8.9	4.20	2.12
2c	404 (11.1), 450 (2.44)	535	6.3	2.84	1.64	478 (16.7)	511	10	1.60	6.25

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Fig. 4 (a) UV-vis absorption of (a) 4, (b) 2b and (d) 2c and fluorescence spectra of (c) 2b and (e) 2c in CH₂Cl₂, with added DBU at 296 K. (f) The colour change of 2b (left) and 2c (right) under daylight (top) and 365 nm-UV irradiation (bottom) with addition of DBU.

Interestingly, the chromic properties of 9-hydroxy derivatives 3 differed from those of 1-hydroxy derivatives 2 (Fig. 5 and Table 2). Under neutral conditions, 3a exhibited major bands at 400 nm (moderate) and 410–530 nm (broad, weak) (Fig. 5a, black line). However, upon addition of DBU, the absorption at longer wavelengths red-shifted to 420–630 nm (Fig. 5a, red line), and the bathochromic shift reached saturation with the addition of 30 equivalents of DBU. Under basic conditions, the colour of the solution turned red (Fig. 5c, top). The emission spectrum of 3a showed a peak at 571 nm under neutral conditions (λ_ex = 381 nm, Fig. 5b). When 30 equivalents of DBU were added,‡ emission at a longer wavelength of 715 nm was observed, although it was weak (red line). The Φ value of 3a was very low, 2.7% under neutral conditions and 0.92% under basic conditions. Additionally, the radiative rate constant (k_F) of 3a decreased with the addition of DBU (1.13 × 10⁷ s⁻¹ under neutral conditions, 0.37 × 10⁷ s⁻¹ under basic conditions).

Fig. 5 UV-vis absorption of (a) 3a, (d) 3b and (g) 3c and fluorescence spectra of (b) 3a, (e) 3b and (h) 3c in CH₂Cl₂, with added DBU at 296 K. The colour change of (c) 3a, (f) 3b and (i) 3c under daylight (top) and 365 nm-UV irradiation (bottom) with addition of DBU.

Table 2 Photophysical properties of the hydroxy-dibenzofluoranthenes 3 in CH₂Cl₂^a

Compounds	Neutral form (no additive)					Anionic form (in the presence of DBU)^b
	λ Abs [nm] (ε [10^3 L (mol cm)^−1])	λ em [nm]	Φ [%]	τ [ns]	k F [10^7 s^−1]	λ Abs [nm] (ε [10^3 L (mol cm)^−1])	λ em [nm]	Φ [%]	τ [ns]	k F [10^7 s^−1]
a Both absorption and emission spectra were recorded in solution (c = 5.0 × 10^−5 M) at 296 K. b Each hydroxy-dibenzofluoranthene (anionic form) was studied in the presence of DBU (for 3a: 30 eq.; 3b: 100 eq.; 3c: 40 eq.). c Quinine sulfate was used as a reference dye (λex = 366 nm, Φ = 55%).^14 d The radiative rate constants were calculated from the experimentally measured fluorescence quantum yields and lifetimes as kF = Φ/τ.
3a	400 (3.84), 442 (2.68)	571	2.7	2.40	1.13	475 (3.18)	715	0.92	2.50	0.37
3b	407 (9.16), 440 (3.34)	572	2.8	2.11	1.33	462 (3.66)	725	1.5	2.37	0.63
3c	418 (3.08), 480 (1.70)	640	0.57	—	—	570 (2.78)	764	0.30	—	—

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The absorption and fluorescence spectra of 3b were similar to those of 3a, indicating that the introduction of an electron-donating group at the 1 position barely affected the photophysical properties of 3 (Fig. 5d–f). In contrast, substitution with an electron-withdrawing group at the 1 position (3c) resulted in absorption at longer wavelengths than both 3a and 3b under neutral conditions (Fig. 5g). A bathochromic shift was observed for 3a–c upon the addition of DBU. Notably, 1-cyano-9-hydroxy 3c exhibited a red-shifted absorption of more than 100 nm and near-infrared (NIR) fluorescence (λ_em = 764 nm, λ_ex = 365 nm) under basic conditions (Fig. 5h). When comparing the donor–π-acceptor type molecules 2c and 3c, the introduction of the CN group at the 9 position in 1-hydroxydibenzofluoranthene (2c) had little effect on the absorption and fluorescence. However, the introduction of this electron-withdrawing group at the 1 position of the 9-hydroxy group (3c) resulted in a significant bathochromic shift.

2.3 Solvatochromic properties of 2c and 3c

The effect of the solvent on the chromic properties was also examined. The UV-vis and fluorescence spectra of 2c and 3c in DMSO, an aprotic polar solvent, are shown in Fig. 6. Similar to the observation in CH₂Cl₂, a hyperchromic shift of 2c was noted in DMSO under basic conditions. However, unlike in CH₂Cl₂, the addition of TFA reduced the longer wavelength absorption of 2c. These results indicate that DMSO may partially deprotonate the phenolic proton of 2c, even under neutral conditions. When comparing the absorption wavelengths (λ_Abs) in DMSO with those in CH₂Cl₂, it was found that 2c exhibited a longer λ_Abs under both neutral and basic conditions.

Fig. 6 UV-vis absorption of (a) 2c and (b) 3c in DMSO, with added DBU or TFA at 296 K. (c) The colour change of 2c (top) and 3c (bottom) under daylight in neutral or basic conditions.

In contrast, under neutral conditions the UV-vis absorption of 3c in DMSO was similar to that in CH₂Cl₂, but λ_Abs was red-shifted by 110 nm under basic conditions. Surprisingly, 3c in DMSO exhibited an absorbance wavelength of 680 nm under basic conditions, reaching the NIR region.

2.4 Computational study

To gain insight into the photophysical properties of hydroxydibenzofluoranthenes, time-dependent density functional theory (TD-DFT) calculations using the B3LYP/6-31+G(d,p) basis set with a CH₂Cl₂ solvation model were performed. A comparison of the crystal structure of compound 2b with all of the bond distances obtained from the calculations confirms their validity (see the ESI, Fig. S2†). The frontier molecular energy levels, electron density distributions of the highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs), and oscillator strengths (f) for 2a, 2c, 3a, and 3c, along with their corresponding anions, are summarized in Fig. 7a (for 2b and 3b, see the ESI, Fig. S10†). In the neutral forms of compounds 2 and 3, the electron density in the HOMO orbitals is localized along the long axis of the phenanthrene (ABC) and naphthalene (EF) rings. Conversely, their LUMO orbitals are aligned parallel to the short axis of these rings, with a high density concentrated on the central pentagon (D). Deprotonation of both hydroxydibenzofluoranthenes 2 and 3 significantly alters the electron density distribution in the HOMO orbital. For the anionic forms of 2 and 3, the electron density of the HOMO orbital delocalizes across the ABDEF rings, indicating cross-conjugation of the phenanthrene and naphthalene rings under basic conditions. However, the densities of the LUMO orbitals of 2(anion) and 3(anion) remain quite similar to those of 2(neutral) and 3(neutral), respectively. The HOMO–LUMO energy gaps (ΔE_HOMO–LUMO, eV) and the oscillator strengths (f) are in good agreement with observed trends in UV-vis absorption. For the series of 1-hydroxyfluoranthenes 2 exhibiting a hyperchromic shift, the respective ΔE_HOMO–LUMO values for the neutral and anionic species are comparable [2a(neutral) 3.30 eV vs.2a(anion) 3.15 eV, 2c(neutral) 3.27 eV vs.2c(anion) 3.04 eV], and the f values of the anionic species are significantly higher than those of the neutral species [2a(neutral) 0.092 vs.2a(anion) 0.239, 2c(neutral) 0.132 vs.2c(anion) 0.433]. These calculations also explain the observed bathochromic shifts of 9-hydroxyfluoranthenes 3. ΔE_HOMO–LUMO and f values significantly decreased and increased, respectively, upon transformation of 3(neutral) into 3(anion). Those changes suggest that the absorptions of 3(anion) would be observed at longer wavelengths and would be stronger than those of 3(neutral).

Fig. 7 (a) Calculated frontier molecular orbitals, energy levels (ΔE_HOMO–LUMO) and oscillator strengths (f) of 2a(neutral, anion), 2c(neutral, anion), 3a(neutral, anion) and 3c(neutral, anion) at the B3LYP/6-31+G(d,p)-CPCM(DCM) level of theory. The contribution of the HOMO to LUMO transition is shown in parentheses below the f value. (b) NICS(1) values of 2a(neutral, anion), 2c(neutral, anion), 3a(neutral, anion) and 3c(neutral, anion) at the GIAO/B3LYP/6-31+G(d,p) level of theory.

Nucleus-independent chemical shifts (NICS) were calculated to estimate the local aromaticity of each ring in both the neutral and anionic forms of 2 and 3 (Fig. 7b; for 2b and 3b, see the ESI Tables S19, 20, 25 and 26†). For the neutral forms of 2 and 3, the NICS(1) values for rings A, B, C, E and F ranged between −7.79 and −10.0, while the NICS(1) value for ring D varied from +3.20 to −0.20. These results indicate that, under neutral conditions, the phenanthrene (ABC) and naphthalene (EF) rings exhibit almost independent aromatic characteristics, and the central pentagon is non-aromatic or weakly anti-aromatic. The HOMA values for compound 2b, whose X-ray crystal structure was obtained, are consistent with the aromaticity predictions derived from the NICS(1) values (see the ESI, Table S3†). For the anionic species of 2, the NICS(1) values of ring F increase significantly, whereas that of ring D decreases compared to their neutral counterparts. This suggests that the anion forms of 2 make a fluorenyl-anion-type resonance structure, as illustrated in Fig. 1, with a large contribution. In contrast, for 3(anion), the NICS(1) values of rings A, B and F are considerably higher than those of 3(neutral), indicating a weakening in the aromatic character of those rings in the anion forms of 3. Additionally, the local anti-aromatic character of ring D slightly intensifies under basic conditions, implying an increased contribution of quinoid resonance. These calculation studies effectively elucidate that the distinctive chromic properties of 2 and 3 result from different resonance hybrids.

3. Conclusion

In this study, we synthesised new chromic fluoranthene derivatives and found that even though the same chromophore skeleton was employed, altering the positions of the auxochromes resulted in contrasting halochromic properties under basic conditions. Notably, 9-hydroxydibenzofluoranthenes 3 exhibit emissions red-shifted by almost 200 nm compared to 1-hydroxy compounds 2. TD-DFT and NICS calculations supported the idea that the characteristic chromic properties of fluoranthenes are based on changes in the resonance structures of the cross-conjugated system.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by JSPS KAKENHI (Grant Numbers 23K18183 and 23K27295) and MEXT KAKNHI (Grant Number JP21H05211) in Digi-TOS and BINDS from AMED (Grant Numbers 22ama121042j0001 and 22ama121034j0001). K. K. acknowledges support from Sasakawa Scientific Research Grant and JST SPRING (JPMJSP2110).

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 2251568. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4ob00750f

‡ The amount of base required for complete deprotonation depends on the pK_a value of 2a (12.9 ± 0.2 in DMSO) and 3a (14.2 ± 0.3 in DMSO).

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