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. 2025 Oct 16;10(42):50326–50339. doi: 10.1021/acsomega.5c07385

Impact of Azobenzene Side Chains on the Ultraviolet–Visible and Fluorescence Properties of Coumarin Derivatives

Yasemin Akdis 1, Akin Akdag 1,*
PMCID: PMC12573160  PMID: 41179125

Abstract

The effect of photoswitchable compounds on the light-emitting properties of nanoparticles has been drawing increasing attention. To investigate the effect of photoswitchable azobenzene units on the photophysical properties of coumarin, a biologically relevant fluorophore, photoresponsive azobenzene-coumarin derivatives were synthesized and characterized. The investigation of the effect of azobenzene isomerization on the ultraviolet (UV)–visible absorption and fluorescence properties of coumarin was explored. The azobenzene unit, attached to the coumarin chromophore at different positions via 3 and 8 carbon linkers, exhibited significant trans –cis isomerization upon UV irradiation at 365 nm, which affected both the absorption and fluorescence spectra of the coumarin part. The study demonstrated that the trans-to-cis transformation of the azobenzene moiety influences fluorescence intensity, with an increase observed in 7-hydroxycoumarin-derived compounds. However, for derivatives of 7-amino coumarin, the fluorescence intensity decreased. Density functional theory (DFT) and time-dependent DFT calculations suggested that the observed fluorescence changes are unrelated to the electronic coupling of the azobenzene and coumarin units. The increase in fluorescence is rather related to the absorption of excitation photons by trans azobenzene, and the decrease in fluorescence is due to the emitted photons absorbed by the azobenzene’s n–π* region.

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Introduction

Photoresponsive chromophores have attracted considerable attention in molecular design due to their ability to modulate photophysical and photochemical properties in response to light. Azobenzene, a well-known photoisomerizable moiety, is a compound in which two aromatic moieties are linked via –NN– bonding (Figure ). This chromophore has been studied intensively for its photoresponsive character − due to its reversible trans – cis isomerization, making it a versatile tool in supramolecular chemistry, materials science, and bioresponsive systems. The azo-compounds are thermodynamically more stable in their trans configuration than their cis configuration. , As the trans – cis configuration change is usually attained by light, the cis configuration is converted back to trans thermally or photolytically. − This configuration change has been exploited for studying the molecular motion. −

1.

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Azobenzene.

Exposure of trans-azobenzene to light between 320 and 380 nm is known to yield trans – cis isomerization, whereas exposure of the cis isomer between 400 and 450 nm has been observed to promote cis – trans isomerization (Figure ). By irradiation with light of a specific wavelength, it is possible to control the configuration of each of the trans – cis photostable states.

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Trans – cis isomerization of the azobenzene.

Studies have demonstrated that the isomerization of azobenzene can influence nearby fluorophores through either conformational changes or electronic communications. In another study, azobenzene derivatives coupled with fluorophores such as cyanine dyes displayed near-infrared (NIR) fluorescence control through supramolecular encapsulation and isomerization, indicating potential in advanced imaging applications. The promise of azobenzene in the design of light-regulated fluorescence materials is further demonstrated by azobenzene-functionalized conjugated polymers, in which the fluorescence is modified as a result of changes in polymer conformation upon isomerization. Similarly, azobenzene-containing methacrylate polymers have been investigated for their dipole alignment and optical responses, indicating possible uses in optoelectronics. Furthermore, hybrid systems comprising porphyrins and azobenzene have shown adjustable photophysical characteristics, with photoisomerization influencing both emission and absorption patterns. When combined, these investigations demonstrate that azobenzene functions as a modulator of photophysical activity, specifically fluorescence, in addition to being a photoswitch.

Coumarin, on the other hand, is a well-known fluorophore in chemical sensing and bioimaging due to its inherent fluorescence, high quantum yields, and sensitivity to changes in the environment (Figure ).

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Structure and numbering of the coumarin.

Coumarin is a highly versatile chromophore that has been utilized to detect many species through its fluorescence properties. − Furthermore, the UV–vis absorption profile of the coumarin unit can be tuned by installing different substituents on it. − With its easy accessibility and being a good sensorial material, these qualities make coumarin derivatives useful for various applications. − Studies using coumarin in luminescent probes and chromophoric materials have demonstrated that coumarin units can be successfully integrated into supramolecular structures and dye systems to investigate environment-sensitive fluorescence.

Additionally, coumarin and/or azobenzene-containing systems, like dye–polymer systems or conjugated conducting polymers, show improved photostability and emission control, especially when exposed to different polarities or pH levels. , Further demonstrating their versatility in multifunctional materials, the azobenzene moieties have also been coupled to electroactive platforms, where electronic influences have modified their fluorescence response. ,

Although azobenzene and coumarin have been extensively studied independently, there are still only a few investigations that combine both chromophores in a single system. Several investigations have incorporated coumarin and azobenzene units into conjugated or polymeric structures, frequently emphasizing electrochemical or structural characteristics while just passingly discussing fluorescence activity. ,, Specifically, the modulation of coumarin fluorescence by azobenzene photoisomerization within a covalently coupled molecular system has remained unexplored. Moreover, azobenzene, being a nonfluorescent modulator, and coumarin, being a responsive emitter, in a covalently bonded molecular system, have not been studied.

On the contrary to the molecular systems, nanoparticles were decorated with photoswitchable organic compounds to modulate their fluorescence properties. Toward this goal, gold nanoparticles were covered with azobenzene units. This study showed that when nanoparticles covered with azobenzene in the trans form were excited, the NIR fluorescence of the gold nanoparticles increased. On the other hand, the cis form decreased the NIR fluorescence. The enhancement in fluorescence for the trans was attributed to energy transfer from azobenzene to the nanoparticle. In another study, silicon nanoparticles were covalently attached to azobenzene. The fluorescence properties of the silicon nanoparticles did not change upon E to Z isomerization of the pendant azobenzene units.

We have initiated a study of coumarin, focusing on its capacity to detect environmental changes, specifically conformation and solvation. , A natural legitimate question that emerged from this research is whether the trans –cis isomerization of the azobenzene side chain influences the UV–vis and fluorescence properties of the coumarin unit inspired by nanoparticle studies. In this regard, we synthesized new coumarin derivatives that incorporate azobenzene as a side chain and conducted a series of experiments to explore the properties of these compounds.

In this study, we aimed to address this untapped area of research by examining a unique chemical structure in which azobenzene and coumarin units are covalently linked and azobenzene isomerization directly affects coumarin fluorescence.

Results and Discussion

The backbone of all synthesized compounds, 4-hydroxyazobenzene (1), was produced in high yield and analyzed using 1H NMR and IR spectroscopy in this investigation. Compounds 1a and 1b were produced by synthesizing two derivatives with hydroxyl-terminated alkyl chains using 3-bromo-1-propanol and 8-chloro-1-octanol to facilitate conjugation with coumarin. The purpose of these flexible linkers was to evaluate the impact of the spacer length on photophysical characteristics. Compounds 2a and 2b were created by tosylating the hydroxyl groups of 1a and 1b to enable nucleophilic substitution. In the meantime, the coumarin core, ethyl 7-hydroxycoumarin-3-carboxylate (C1), was created by Knoevenagel condensation. FC1 and FC2 were produced by coupling the tosylated azo derivatives to C1’s 7-position.

To identify the impact of positional variation, the 7-hydroxy group of compound C1 was capped with butyl, and the 3-carboxylic ester was hydrolyzed to carboxylic acid (C1a), allowing EDC·HCl-mediated coupling with aliphatic azo-alcohols (1a and 1b) to afford FC3 and FC4. In order to examine electron-donating effects, ethyl 7-(diethylamino)­coumarin-3-carboxylate (C2) was synthesized and hydrolyzed; its derivatives FC5 and FC6 were obtained by coupling with compounds 1a and 1b. The characterization of each compound was conducted by using a combination of nuclear magnetic resonance (NMR), infrared (IR) spectroscopy, and high-resolution mass spectrometry (HRMS).

Synthesis

The 4-hydroxyazobenzene moiety, 1, which is present in the backbone of all target compounds, was obtained in 90% yield from the reaction of the corresponding diazonium salt obtained from aniline and phenol. It was characterized by 1H NMR and IR spectroscopy, and is consistent with the literature. As seen in Scheme , the compound 1 was further alkylated to get compounds 1a and 1b. These compounds were tosylated to form compounds 2a and 2b. These compounds (1a, 1b, 2a, and 2b) will be used to obtain target compounds.

1. Synthesis of the Azobenzene Backbone.

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The synthesis of ethyl 7-hydroxycoumarin-3-carboxylate (C1) was accomplished through a Knoevenagel condensation, in which 2,4-dihydroxybenzaldehyde and diethyl malonate were reacted in the presence of piperidine as a base and acetic acid as a catalyst, under reflux conditions in ethanol. The same procedure was applied to synthesize C2 as depicted in Scheme . The compound C1 was butylated and subjected to saponification to get compound C1a. These compounds are used as reporters for our fluorescence studies.

2. Synthesis of the Coumarin Derivatives.

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Photoresponsive azobenzene side chains were introduced to the 7-position of the coumarin unit by treating 2a/2b with C1. This reaction successfully furnished compounds FC1 and FC2 (Scheme ). To attach the azobenzene unit to the 3-position of coumarin units C2 and C1a, EDC couplings were facilitated to bond C2 and C1a with 1a and 1b. These reactions yielded FC3, FC4, FC5, and FC6 as shown in Scheme .

3. Synthesis of Final Compounds.

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UV–Vis and Fluorescence Studies

UV–vis spectroscopy was used to probe the trans–cis transformation and vice versa. Since the trans-azobenzene derivatives have extended conjugation, π–π* transitions absorb light in longer wavelengths than their cis form. ,, In order to be certain of the conversion of our synthesized aliphatic azo-alcohol, which is an azobenzene derivative, we performed a proof and control experiment with substance 1a as shown in Figure . After measuring the sample prepared in DCM without exposure to light, it was exposed to 365 nm UV light for 10 min and then measured again. Immediately afterward, without intervening time, the same sample was immediately re-exposed to UV light for another 10 min and then measured again. The time expressions given throughout the paper are cumulative.

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UV–vis absorption spectra of (E)-3-(4-(phenyldiazenyl)­phenoxy)­propan-1-ol (1a) in DCM and after its exposure to light at 365 nm for the corresponding time (10–5 M).

As seen for (E)-3-(4-(phenyldiazenyl)­phenoxy)­propan-1-ol (1a), the absorption λmax = 320 nm decreases while compound 1a (in trans configuration) is irradiated at 365 nm; that is, compound 1a in the trans form is transformed into the cis form. The irradiating light source spectra overlap with aliphatic azo-alcohol 1a in a small region; therefore, the rapid conversion from the trans to cis form is not observed. That is why even after 20 min of irradiation, trans azobenzene was present in the solution as it can be inferred from Figure .

Before the final compounds were studied, the UV–vis spectra of 1a and C1a were also measured to determine the absorption profile of each chromophore. As seen in Figure , there is significant overlap between the absorptions of 1a and C1a. Also, it should be noted that irradiation of the 7-butoxy coumarin C1a unit with a 365 nm wavelength did not affect the UV–vis spectrum of the compound after the irradiation.

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UV–vis spectra of compound 1a and ethyl-C1a in DCM (10–6 M).

With these results in hand, we started our studies with FC1, which has an azobenzene side chain connected to the seventh position of the coumarin unit. The compound FC1 was studied with the UV–vis spectrum showing λmax = 348, with a shoulder at around 315 nm. After irradiation with 365 nm for 10, 20, and 30 min, the absorption decreased. Expectedly, this shows that the trans azobenzene unit transformed into the cis azobenzene unit (Figure ). To investigate if coumarin’s fluorescence properties are changing during trans–cis isomerization, the emission and excitation spectra of the FC1 with exposure to the light (λ = 365 nm) were recorded. Although λmax of excitation was similar to that in the UV–vis spectra, as the compound was exposed to light, the excitation intensity increased. As to fluorescence spectra, a similar trend to excitation spectra was observed (Figure ). Surprisingly, since the solution concentration did not change, this observation was unexpected.

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UV–vis and fluorescence spectra of FC1 in DCM (10–5 M) before and after irradiation.

With this surprising observation in hand, to conclude if this observed phenomenon occurred due to the charge transfer, we ran the same experiments within the different solvents, i.e., THF, MeCN, and MeOH. Charge transfer is known to be affected by solvent polarity. The same phenomena as those in DCM were observed for these different solvents (Supporting Information). This excludes the possibility of photoinduced charge transfer affecting the fluorescence spectrum to some extent. If photoinduced charge transfer is a factor in these observations, then the distance between azobenzene and coumarin units should also be affected.

To test this approach, a derivative called FC2 was constructed, in which an 8-carbon linker separated the azobenzene and coumarin units. The purpose of the longer chain was to test whether the fluorescence behavior is changed by the greater separation between the coumarin core and the photoresponsive azobenzene unit. Through this alteration, we were able to investigate whether the azobenzene group’s proximity promotes or inhibits charge transfer processes or any conformational changes that affect the molecule’s photophysical characteristics. We compared the optical responses of FC1 and FC2 to acquire a better understanding of how the relative position of the azobenzene side chain influences the coumarin fluorescence. For this purpose, we applied the same procedure, in which the 365 nm light irradiation was performed for 10, 20, and 30 min, and UV–vis and fluorescence studies operated in DCM (Figure ). Analyzing the excitation and emission spectra of FC2 revealed a rise in fluorescence intensity upon azobenzene isomerization, which is consistent with the trend reported in FC1, the 3-carbon-linked analog. To check the solvent dependency, we also performed these analyses again in THF, MeCN, and MeOH, and the results were consistent with those in DCM (Supporting Information).

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UV–vis and fluorescence spectra of FC2 in DCM (10–5 M) before and after irradiation.

In order to further explore the positional effects of substituents on fluorescence enhancement, two new derivatives were synthesized: FC3 and FC4. In these compounds, the seventh position of the coumarin core was capped with a butyl group, effectively removing any potential interaction at this position. This design serves to isolate the role of the azobenzene-coumarin interaction at a singular position, thereby facilitating a more comprehensive understanding of its influence. This approach enabled us to concentrate specifically on the impact of azobenzene side chains attached to the 3-position via linkers of varying lengths (3-carbon and 8-carbon aliphatic azo-alcohols). With this, FC3 and FC4 dissolved in DCM and UV–vis (λmax = 350 nm for both), and a fluorescence study was performed in the same manner as that mentioned above. Results are shown in Figure , and the pattern was observed as it was observed for FC1 and FC2. The results demonstrated a consistent enhancement in fluorescence intensity upon the trans-to-cis isomerization of the azobenzene unit across other solvents: THF, MeCN, and MeOH, each with varying polarity (Supporting Information).

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UV–vis and fluorescence spectra of FC3 and FC4 in DCM (10–5 M) before and after irradiation.

With all of these results in mind, it was decided that the coumarin absorption should be shifted to the bathochromic region compared to the azobenzene absorption region. This shift, in turn, creates an absorption spectrum on which the chromophore units will have separated absorptions. With this, we could control trans–cis isomerization better without disturbing the coumarin unit’s absorption region so that trans–cis isomerization interfering with coumarin unit absorption will have a minimum value. For this purpose, we synthesized FC5 and FC6. Installing the diethylamino group at the seventh position of the coumarin shifted the absorption of the coumarin unit to the red region. This choice separated the coumarin chromophore from the azobenzene chromophore in the UV–vis spectrum (Figure ). Irradiating the compound in DCM at 365 nm led to a disappearance of the absorption at 340 nm (azobenzene region). This shows that the trans-azobenzene side chain converted into its cis form. The same pattern was observed for different solvents (Supporting Information). As to the emission properties of these compounds with azobenzene in the trans form and cis form, the spectra were recorded without and after irradiation. When compounds were excited with light of λmax = 398 nm, we observed fluorescence at around 455 nm. Contrary to previous observations, azobenzene in the cis form caused a decrease in the fluorescence intensity (Figure ).

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UV–vis and fluorescence spectra of FC5 and FC6 in DCM (10–5 M) before and after irradiation.

Theoretical Calculations

To explain this observed abnormal effect of trans–cis isomerization of these compounds, theoretical calculations were employed. The geometries of all the above-mentioned compounds were optimized at the B3LYP/6-31g­(d) level of theory as implemented in Gaussian 09. The cis forms of these compounds were also optimized at the same level. In order to shine light on the fluorescence properties observed above, we also performed population analysis at the same level of theory and time-dependent DFT calculations at the CAM-B3LYP/6-31g­(d) level of theory. To minimize the computation time for 7-butoxycoumarin compounds (FC3 and FC4), the butyl group at the seventh position of these derivatives was replaced with the methyl group. Optimization shows that the compounds did not aggregate within themselves. This shows that there is no force to stack the coumarin unit with the azobenzene unit. The basic assumption in our calculations is that the octyl linker will not affect the photophysical properties. It was given that no interaction between the azo and coumarin units was observed for the calculations of the propyl linker unit. Furthermore, the frontier orbitals for the compounds with an octyl linker (FC2, FC4, and FC6) show the exact same ordering as with the compounds with a propyl linker. This is consistent with the experimental observations where the octyl linker and the propyl linker compounds’ photophysical responses are the same.

Population analysis was done at the B3LYP/6-31g­(d) level for these optimized geometries. The visualization of molecular orbitals was done by using Jmol. For the azobenzene unit at its trans form, attached to the seventh and third (Figure ) position of coumarin, this resulted in the same frontier orbitals; that is, HOMO–1 is located on nonbonding orbitals of azo units, the HOMO is located on the π MO of the azo unit, the LUMO is located on coumarin, and LUMO+1 is located on the π* MO of the azo unit. It is more clearly seen that the frontier molecular orbitals of FC1 and FC3, where the coumarin of the azobenzene unit is located at the seventh and third position, respectively, are not only located at the same places but also have similar orbital energies (Figure ).

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Calculated frontier orbitals of FC1 and FC3 in their trans form.

For the cis form of these compounds (FC1 and FC3), the HOMO, LUMO, and LUMO+1 stayed in the same order, while HOMO–1 is now located on coumarin for both. This could be easily understood due to the conjugation being disrupted for the cis form on the azobenzene unit (Figure ). The calculated frontier orbital energy resemblance for the trans form was also observed in the cis form of these substances, expectedly.

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Calculated frontier orbitals of FC1 and FC3 in their cis form.

For the 7-(diethylamino)­coumarin derivative compound FC5, the coumarin unit has more contribution to the frontier molecular orbitals that the HOMO and LUMO+1 are located on coumarin, while HOMO–1 and LUMO are located on the azobenzene unit for the trans form (Figure ). In the cis form, the contribution of chromophores to the molecular orbitals exchanged; that is, HOMO–1 and LUMO are located on coumarin while the HOMO and LUMO+1 are located on the azobenzene unit.

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Calculated frontier orbitals of FC5 in its trans and cis form.

Time-dependent DFT calculations (CAM-B3LYP/6-31g­(d)) were carried out for these compounds. The results for the corresponding transitions are tabulated in Table . Due to the computational cost, we calculated only three vertical excitations to compare the results qualitatively. Vertical transitions show that the transitions are isolated; that means that transitions from the azobenzene unit to the coumarin unit or vice versa are not observed. This is consistent with the experimental observations. As we irradiated trans-azobenzene compounds FC1, FC3, and FC5 with 365 nm UV light to convert them into the cis form, there is no change on the UV–vis spectrum for the corresponding coumarin region. The same is valid for the amino compounds’ coumarin region. These results imply no effect of cis and trans isomerizations on fluorescence.

1. Calculated Vertical Transitions .

  trans
cis
compound λ (nm) f (oscillator strength) transition λ (nm) f (oscillator strength) transition
FC1 448 0.0000 122–126 467 0.0415 124–126
320 1.2631 124–126 302 0.6403 123–125
302 0.3789 123–125 280 0.0010 118–125
FC3 476 0.0000 119–122 485 0.0585 120–122
383 0.0006 120–121 415 0.0001 120–121
351 0.0000 119–121 328 0.0010 114–121
FC5 497 0.0000 130–133 510 0.0491 132–134
354 1.4709 132–133 348 0.1285 132–133
346 0.2389 131–134 347 0.6462 131–133
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a

In FC1, 124 is the HOMO, and 125 is the LUMO; in FC3, 120 is the HOMO, and 121 is the HOMO; in FC5, 132 is the HOMO, and 133 is the LUMO.

With experimental and theoretical results in mind, there was no obvious explanation for the fluorescence intensity increase in FC1, FC2, FC3, and FC4 and the fluorescence decrease for FC5 and FC6. Theoretical calculations show that the trans–cis azobenzene conversion does not have any effect on vertical excitations and thus on fluorescence. When we re-examined our experimental results for FC1–4, we realized that, in the realm of excitation wavelengths, the trans-azobenzene unit has an absorption. In the cis form, that absorption vanishes; thus, more photons are available for exciting coumarin units, which in turn resulted in a fluorescence intensity increase. On the other hand, for the trans form, the number of photons reaching to the coumarin moiety is less than that of the cis form, which leads to a lower fluorescence intensity compared to the cis form. As a result, as the trans-azobenzene converted to the cis form, a fluorescence increase was observed for FC1, FC2, FC3, and FC4. Furthermore, we have taken the excitation and emission spectra of FC3 in three different concentrations (Supporting Information). We observed that fluorescence increases in the cis form in all concentrations.

As for FC5 and FC6, a fluorescence intensity decrease should be explained differently because the excitation wavelength is not in the region of the azobenzene absorption band. As we converted the trans-azobenzene unit of FC5 into the cis form, the n → π∗ band (around 410–450 nm) increased. That region falls into the emitting region of 7-(diethylamino)­coumarin units. This shows that in coumarin fluorescence, the n → π∗ region absorbs these photons. This, in turn, leads to a decrease in the fluorescence of FC5 and FC6.

Conclusions

In this study, six new coumarin-conjugated azobenzene derivatives were successfully synthesized to explore their optical properties. The structural adjustments were focused on the positional variations of the azobenzene at the 3- and 7-positions and the distance between the coumarin core and azobenzene unit. Their effects on fluorescence were studied experimentally and theoretically. A key finding was that the substitution of amino or butoxy units at the seventh position of the coumarin core led to distinct fluorescence behaviors, that is, as trans–cis isomerization occurred, fluorescence enhancement was observed when the seventh position of the coumarin unit was capped with butoxy, while a decrease in fluorescence was observed in 7-(diethylamino)­coumarin derivatives. Theoretical studies and experiments in various solvents allowed us to conclude that there is no charge transfer and that chromophoric units act independently. Additionally, computational studies showed that fluorescence is not directly impacted by the trans–cis isomerization of azobenzene. Experimental findings revealed that azobenzene absorbs at the excitation wavelength in the trans form, lowering the amount of photons that reach the coumarin unit; in contrast, this absorption disappears in the cis form, increasing the fluorescence for FC1–FC2 and FC3–FC4. In contrast, compounds FC5 and FC6 undergo a fluorescence intensity decrease as a result of the trans–cis conversion. This decrease was attributed to increases in the n → π* absorption for the cis form in the emission region of the 7-(diethylamino)­coumarin unit. In light of these findings, it can be concluded that the utilization of azobenzene-coumarin derivatives, whose absorption regions intersect, can result in an increase in fluorescence. Conversely, the separation of the absorption regions through the use of an electron-rich system can lead to a decrease in fluorescence. In regard to these considerations, it can be stated that the development of new systems that can be easily obtained synthetically could enable the control of fluorescence.

Experimental Section

The experimental conditions and the instruments used for the measurements are given in the Supporting Information.

Synthesis of (E)-4-(Phenyldiazenyl)­phenol (1)

Aniline (5 g, 53.68 mmol) was dissolved in 20 mL of concentrated HCl and 20 mL of distilled water in a round-bottom flask placed in an ice bath. A solution of sodium nitrite (5 g, 72.47 mmol) in 20 mL of water was added to the former stirring solution for diazotization to occur. In another flask, a solution of phenol (5.8 g, 61.63 mmol) in 50 mL of 10% NaOH was prepared. This solution also was cooled to 5 °C by immersion in an ice bath. After 30 min, the phenol solution was strongly stirred along with the slow addition of cold diazonium salt solution. A dark orange-yellow solid product soon appeared and precipitated. The mixture was filtered through a funnel, and the solid product on filter paper was washed several times with cold water and dried for isolation (8.97 g, 90%). IR: 1470 cm–1 (NN), 2800–3200 cm–1 (OH).

1H NMR (400 MHz, CDCl3): δ 7.88 (d, J = 4.6 Hz, 2H), 7.86 (d, J = 3.0 Hz, 2H), 7.50 (m, 2H), 7.44 (m, 1H), 6.94 (d, J = 8.77 Hz, 2H).

Synthesis of (E)-3-(4-(Phenyldiazenyl)­phenoxy)­propan-1-ol (1a)

4-Hydroxy azobenzene (1) (1.98 g, 9.98 mmol) and 3-bromo-1-propanol (1.08 mL, 12.01 mmol) were dissolved in 30 mL of DMF. K2CO3 (2.07 g, 14.97 mmol) was added to this solution, and the solution was stirred at 75 °C for 12 h. After cooling to the RT, the solution was poured into 50 mL of cold water and extracted with 50 mL of chloroform. The organic phase was washed with 30 mL of 1 M HCl and 30 mL of brine solution and then dried over anhydrous MgSO4. The solvent evaporated under vacuum, and the orange solid product isolated (silica, EtOAc/Hex 1:3–1) (1.90 g, 74%). IR: 1494 cm–1 (NN), 2800–3300 cm–1 (OH).

1H NMR (100 MHz, CDCl3): δ 7.90 (dd, J = 9.08, 8.9 Hz, 4H), 7.50 (t, J = 7.5 Hz, 2H), 7.44 (t, J = 7.2, 1H), 7.02 (d, J = 9.1, 2H), 4.20 (t, J = 5.9 Hz, 2H), 3.90 (t, J = 5.2, 2H), 2.09 (m, 2H), 1.64 (s, 1H).

Synthesis of (E)-8-(4-(Phenyldiazenyl)­phenoxy)­octan-1-ol (1b)

The same procedure with the synthesis of (E)-3-(4-(phenyldiazenyl)­phenoxy)­propan-1-ol (1a) was applied with 8-chloro-1-octanol as the starting material. The orange solid product was obtained (silica, EtOAc/Hex 1:3–1) (0.67 g, 68%) (m.p.: 91.3 °C). IR: 1472 cm–1 (NN), 2800–3280 cm–1 (OH).

1H NMR (400 MHz, CDCl3): δ 7.91 (dd, J = 8.9, 2.2 Hz, 2H), 7.85 (m, 2H), 7.50 (td, J = 8.0 Hz, 2H), 7.43 (m, 1H), 7.00 (dd, J = 9.1, 2.0 Hz, 2H), 4.05 (t, J = 6.32 Hz, 2H), 3.65 (q, J = 6.3, 2H), 1.8 (m, 2H), 1.5 (m, 2H), 1.37 (m, 8H).

13C NMR (100 MHz, CDCl3): δ 160.38, 151.42, 145.57, 128.97, 127.70, 123.39, 121.20, 113.33, 31.49, 27.98, 27.81, 24.60, 24.33.

Synthesis of (E)-3-(4-(Phenyldiazenyl)­phenoxy)­propyl 4-Methylbenzenesulfonate (2a)

1a (0,82 g, 3.19 mmol) was dissolved in 5 mL of pyridine and cooled to 0 °C in an ice bath for 30 min, and then, tosyl chloride (0.7 g, 3.67 mmol) was added slowly to the stirring solution. The solution was kept in an ice bath for 3 h while reaction was proceeding. After 3 h, 30 mL of ice–water was added to quench the reaction. The precipitated solid was filtered and washed several times with cold water. Afterward, the solid was collected on the filter paper dissolved in DCM and extracted with 30 mL of 0.1 M H2SO4. The compound was dried over MgSO4, and the solvent evaporated under vacuum. A brownish-orange solid product was isolated purely (0.95 g, 73%). IR: 1466 cm–1 (NN), 1746 cm–1 (CO), 2939 cm–1 (methyl group of p-toluene from the tosyl group).

1H NMR (400 MHz, CDCl3): δ 7.80 (m, 3H), 7.45 (m, 3H), 7.23 (d, J = 7.52 Hz, 2H), 6.85 (d, J = 8.84, 2H), 4.26 (t, J = 6.48 Hz, 2H), 4.02 (t, J = 5.56, 2H), 2.35 (s, 3H), 2.15 (t, J = 5.36, 2H), 1.37 (m, 8H).

Synthesis of (E)-8-(4-(Phenyldiazenyl)­phenoxy)­octyl 4-Methylbenzenesulfonate (2b)

The same procedure with the synthesis of (E)-3-(4-(phenyldiazenyl)­phenoxy)­propyl 4-methylbenzenesulfonate was applied with (E)-8-(4-(phenyldiazenyl)­phenoxy)­octan-1-ol (1b) as a starting material (0.71 g, 71%) (m.p.: 101.8 °C). IR: 1463 cm–1 (NN), 1603 cm–1 (CO), 2939 cm–1 (methyl group of p-toluene from the tosyl group).

1H NMR (400 MHz, CDCl3): δ 7.90 (m, 4H), 7.80 (d, J = 8.16, 2H), 7.49 (m, 4H), 7.35 (d, J = 8.72, 1H), 7.00 (d, J = 9.12 Hz, 2H), 4.02 (t, J = 6.50, 4H), 2.95 (s, 3H), 1.80 (m, 3H), 1.30 (m, 10H).

13C NMR (100 MHz, CDCl3): δ 161.66, 152.73, 146.80, 144.66, 133.25, 130.29, 129.73, 129.15, 127.91, 124.60, 122.43, 114.57, 70.61, 68.19, 29.12, 28.84, 28.81, 25.87, 25.25, 21.65.

HRMS (ESI/MS) m/z: [M + H]+ calcd for C27H33N2O4S+, 481.2161; found, 481.2161.

Synthesis of Ethyl 7-Hydroxy-2-oxo-2H-chromene-3-carboxylate (C1)

2,4-Dihydroxybenzaldehyde (1.21 g, 8.77 mmol) and diethyl malonate (1.46 mL, 9.57 mmol) were dissolved in 15 mL of EtOH. Piperidine (0.37 mL) and acetic acid (0.15 mL) were added to this stirring solution, and reaction was carried out under reflux conditions for 18 h. EtOH was evaporated under vacuum, and an oily residue was poured into an ice–water mixture. The precipitated solid was filtered and recrystallized from EtOH. White crystals were dried and isolated (1.80 g, 88%). 1H NMR (400 MHz, DMSO-d 6): δ 8.57 (s, 1H), 7.66 (d, J = 8.20, 2H), 6.63–6.75 (m, 2H), 4.15 (q, J = 7.88, 2H), 1.20 (t, J = 6.20, 3H).

Synthesis of FC1

Ethyl 7-hydroxy-2-oxo-2H-chromene-3-carboxylate (C1) (0.50 g, 2.20 mmol) and K2CO3 (0.35 g, 2.53 mmol) were dissolved in 25 mL of DMF under slow heating to 60 °C. After an hour, (E)-3-(4-(phenyldiazenyl)­phenoxy)­propyl 4-methylbenzenesulfonate (2a) (0.90 g, 2.20 mmol) and KI (0.28 g, 1.68 mmol) were added to the mixture and heat rose to 75 °C. After 18 h, the reaction mixture was cooled to RT and then poured into 50 mL of cold water. After precipitation finished, it was filtered and the solid was washed several times with cold water. The compound was recrystallized from EtOH, and a dark orange solid was dried and isolated (0.42 g, 40%) (m.p.: 107.9 °C). IR: 1468 cm–1 (NN), 1750 cm–1 (CO), 2881 cm–1 (C–H).

1H NMR (400 MHz, CDCl3): δ 8.50 (s, 1H), 7.89 (dd, J = 9.32, 7.56, 4H), 7.50 (m, 4H), 7.02 (d, J = 8.74, 2H), 6.91 (dd, J = 1.84 Hz, 1H), 6.85 (s, 1H), 4.40 (q, J = 7.28, 6.84, 2H), 4.27 (q, J = 7.92, 8.28, 4H), 2.35 (m, 2H), 1.40 (t, J = 6.6, 3H).

13C NMR (100 MHz, CDCl3): δ 188.28, 164.33, 163.46, 161.15, 157.53, 152.75, 148.97, 147.09, 130.80, 130.44, 129.02, 124.81, 122.57, 114.76, 113.81, 111.70, 101.08, 65.18, 64.29, 61.73, 28.87, 14.37.

HRMS (TOF/MS) m/z: [M + H]+ calcd for C27H25N2O6 +, 473.1714; found, 473.1713.

Synthesis of FC2

The same procedure with the synthesis of FC1 was applied with (E)-8-(4-(phenyldiazenyl)­phenoxy)­octyl 4-methylbenzenesulfonate (2b) as a starting material (0.095 g, 32%) (m.p.: 114.6 °C). IR: 1473 cm–1 (NN), 1745 cm–1 (CO), 2853 and 2936 cm–1 (C–H).

1H NMR (400 MHz, CDCl3): δ 8.50 (s, 1H), 7.89 (dd, J = 7.68, 8.92, 4H), 7.48 (m, 4H), 7.00 (d, J = 8.96, 2H), 6.87 (m, 1H), 6.79 (d, J = 2.12, 1H), 4.39 (q, J = 7.08, 7.06, 2H), 4.04 (m, 4H), 1.83 (m, 4H), 1.41 (m, 11H).

13C NMR (100 MHz, CDCl3): δ 164.73, 163.51, 161.64, 157.60, 157.25, 152.76, 149.04, 146.86, 130.66, 130.32, 129.02, 124.74, 122.52, 114.68, 114.03, 113.88, 111.47, 100.76, 68.90, 68.25, 61.69, 29.24, 29.21, 29.15, 28.85, 25.94, 25.86, 14.29.

HRMS (TOF/MS) m/z: [M + H]+ calcd for C32H35N2O6 +, 543.2495; found, 543.2495.

Synthesis of 7-Butoxy-2-oxo-2H-chromene-3-carboxylic Acid (C1a)

Ethyl 7-hydroxy-2-oxo-2H-chromene-3-carboxylate (C1) (0.23 g, 1.00 mmol) and K2CO3 (0.20 g, 1.50 mmol) were dissolved in 7 mL of dry DMF and allowed to stir for an hour with heating to 45 °C (the temperature should not be above 60 °C). After an hour, 1-bromobutane (0.13 mL, 1.20 mmol) was added portionwise to the reaction medium. The reaction was allowed to proceed for 12 h, and after controlling with TLC (EtOAc/Hex 1:1), the reaction was finished and cooled to RT. After cooling, the reaction mixture was poured into an ice–water mixture and filtered, washed with water several times, and dried. The obtained white solid (0.24 g, 0.82 mmol, 82%) was dissolved in 5 mL of 60% EtOH-water solution with KOH (0.08 g, 1.38 mmol). The reaction was allowed to proceed for 5 h at 50 °C and then an additional 1 h at RT. After completion of the reaction, EtOH was removed under vacuum, and the residual solution was acidified to pH = 1 with dilute HCl solution in an ice bath. The precipitated solid was filtered and dried to provide a white solid product (0.21 g, 96%). 1H NMR (400 MHz, DMSO-d 6): δ 8.72 (s, 1H), 7.82 (d, J = 8.60, 1H), 6.98–7.01 (m, 2H), 4.11 (t, J = 6.48, 2H), 1.76 (m, 2H), 1.44 (m, 2H), 0.93 (t, J = 7.28, 3H).

Synthesis of FC3

DMAP (0.09 g, 0.76 mmol) and EDC.HCl (0.55 g, 2.85 mmol) were successively added to a stirring solution of 7-butoxy-2-oxo-2H-chromene-3-carboxylic acid (C1a) (0.50 g, 1.90 mmol) and (E)-3-(4-(phenyldiazenyl)­phenoxy)­propan-1-ol (1a) (0.48 g, 1.90 mmol) in 20 mL of anhydrous DCM. The mixture was stirred at RT for 24 h under an argon atmosphere. After that, in DCM, extraction was performed with saturated NaHCO3 solution, water, and brine and Na2SO4 was used as a drying agent. The solvent was removed under vacuum, and the solid product was recrystallized from EtOH. Yellow crystals were isolated (0.42 g, 45%) (m.p.: 98.0 °C). IR: 1471 cm–1 (NN), 1724 cm–1 (CO), 2872 and 2959 cm–1 (C–H).

1H NMR (400 MHz, CDCl3): δ 8.50 (s, 1H), 7.90 (m, 4H), 7.50 (m, 4H), 7.04 (d, J = 9.04 Hz, 2H), 6.86 (dd, J = 6.32, 3.14 Hz, 1H), 6.79 (d, J = 2.40, 1H), 4.56 (t, J = 6.20 Hz 2H), 4.26 (t, J = 5.88 Hz 2H), 4.04 (t, J = 6.64 Hz 2H), 2.30 (m, 2H), 1.80 (m, 2H), 1.50 (m, 2H), 0.99 (t, J = 7.12, 3H).

13C NMR (100 MHz, CDCl3): δ 164.90, 163.70, 161.30, 157.69, 157.14, 152.76, 149.31, 147.05, 130.75, 130.36, 129.03, 124.78, 123.14, 122.75, 122.56, 114.79, 114.06, 113.61, 111.43, 100.80, 68.69, 64.74, 62.33, 30.88, 28.58, 19.14, 13.77.

HRMS (TOF/MS) m/z: [M + H]+ calcd for C29H29N2O6 +, 501.2026; found, 501.2025.

Synthesis of FC4

The same procedure with the synthesis of FC3 was applied with the difference of usage of (E)-8-(4-(phenyldiazenyl)­phenoxy)­octan-1-ol (2b) as a starting material (0.40 g, 44%) (m.p.: 82 °C). IR: 1472 cm–1 (NN), 1748 cm–1 (CO), 2855 and 2939 cm–1 (C–H).

1H NMR (400 MHz, CDCl3): δ 8.49 (s, 1H), 7.90 (m, 4H), 7.50 (m, 4H), 7.00 (d, J = 9.05 Hz, 2H), 6.86 (dd, J = 6.44, 2.56 Hz, 1H), 6.78 (d, J = 2.40, 1H), 4.33 (t, J = 6.60 Hz 2H), 4.04 (m, 4H), 1.79 (m, 5H), 1.45 (m, 11H), 0.98 (t, J = 7.32, 3H).

13C NMR (100 MHz, CDCl3): δ 164.77, 163.65, 161.70, 157.62, 157.19, 152.80, 148.95, 146.86, 130.64, 130.28, 129.01, 124.74, 122.53, 114.70, 114.00, 111.46, 100.79, 68.67, 68.31, 65.74, 30.88, 29.19, 29.15, 29.12, 28.60, 25.92, 25.82, 19.13, 14.12, 13.76.

HRMS (TOF/MS) m/z: [M + H]+ calcd for C34H39N2O6 +, 571.2808; found, 501.2807.

Synthesis of 7-(Diethylamino)-2-oxo-2H-chromene-3-carboxylic Acid (C2)

4-Diethylaminosalicylaldehyde (0.96 g, 5.00 mmol) was dissolved in 3.3 mL of EtOH. Diethyl malonate (1 mL, 6.5 mmol) and 0.4 mL of piperidine were added to this solution, and the reaction mixture was refluxed for 3 h. After evaporation of EtOH, reaction medium was diluted with 50 mL of water and extracted with EtOAc (50 mL × 3). It was dried over MgSO4, and the solvent was removed under vacuum. The viscous product was purified with column chromatography (silica, EtOAc/Hex 1:5–1), and yellow crystals were obtained (0.75 g, 52%). Afterward, to a solution of this ester product (0.29 g, 1.0 mmol) in 10 mL of EtOH was added 10 mL of 0.5 M NaOH. The reaction was allowed to proceed at RT for 12 h, and after the completion of the reaction, EtOH was evaporated. After adding 5 mL of water to the remaining solution, acidification was performed with 1 M HCl until pH = 4. Then, with the completion of the precipitation, the solid was filtered and washed with water for a few times. The orange solid product was isolated (0.1 g, 40%). 1H NMR (400 MHz, CDCl3): δ 8.66 (s, 1H), 7.45 (d, J = 9.0 Hz, 1H), 6.71 (dd, J = 6.58, 4.76, 1H), 6.53 (d, J = 2.6 Hz, 1H), 3.49 (q, J = 7.12, 4H), 1.26 (t, J = 6.68, 6H).

Synthesis of FC5

EDC·HCl (0.15 g, 0.76 mmol) and DMAP (0.03 g, 0.25 mmol) were added to a stirring solution of 7-(diethylamino)-2-oxo-2H-chromene-3-carboxylic acid (C2) (0.14 g, 0.51 mmol) and (E)-3-(4-(phenyldiazenyl)­phenoxy)­propan-1-ol (1a) (0.13 g, 0.51 mmol) in 10 mL of anhydrous DCM. The mixture was stirred at RT for 24 h under an argon atmosphere. After that, in DCM, extraction was performed with saturated NaHCO3 solution, water, and brine and Na2SO4 was used as a drying agent. The solvent was removed under vacuum, and the yellow solid product was isolated after column chromatography (silica, EtOAc/Hex 1:3–1) (0.12 g, 45%) (m.p.: 115.0 °C). IR: 1214 cm–1 (C–N (aromatic)), 1189 cm–1 (C–N (aliphatic)), 1465 cm–1 (NN), 1725 cm–1 (CO).

1H NMR (400 MHz, CDCl3): δ 8.40 (s, 1H), 7.88 (m, 4H), 7.45 (m, 3H), 7.33 (d, J = 9.32 Hz, 1H), 6.58 (dd, J = 6.56, 2.32 Hz, 1H), 6.45 (d, J = 2.36, 1H), 4.53 (t, J = 5.60 Hz, 2H), 4.26 (t, J = 6.32 Hz, 2H), 3.43 (q, J = 6.96, 7.16 Hz, 4H), 2.30 (m, 2H), 1.23 (m, 6H).

13C NMR (100 MHz, CDCl3): δ 164.30, 161.27, 158.45, 158.04, 152.84, 152.69, 149.23, 146.92, 130.97, 130.16, 128.87, 124.64, 122.42, 114.71, 109.43, 108.63, 107.59, 96.64, 64.82, 61.70, 44.97, 28.58, 12.31.

HRMS (TOF/MS) m/z: [M + H]+ calcd for C29H30N3O5 +, 500.2185; found, 500.2185.

Synthesis of FC6

The same procedure with the synthesis of FC5 was applied with the difference of usage of (E)-8-(4-(phenyldiazenyl)­phenoxy)­octan-1-ol (1b) as a starting material (0.11 g, 38%). M.p.: 84.3 °C. IR: 1215 cm–1 (C–N (aromatic)), 1190 cm–1 (C–N (aliphatic)), 1471 cm–1 (NN), 1747 cm–1 (CO).

1H NMR (400 MHz, CDCl3): δ 8.42 (s, 1H), 7.90 (m, 4H), 7.46 (m, 3H), 7.34 (d, J = 8.84 Hz, 1H), 7.00 (d, J = 8.82 Hz, 2H), 6.58 (dd, J = 6.36, 2.36, 1H), 6.45 (d, J = 2.2 Hz 1H), 4.31 (t, J = 6.48 Hz 2H), 4.04 (t, J = 6.56 Hz 2H), 3.43 (q, J = 7.12, 7.24, 4H), 1.80 (m, 4H), 1.44 (m, 4H), 1.23 (m, 6H).

13C NMR (100 MHz, CDCl3): δ 164.39, 161.72, 158.48, 158.29, 152.84, 152.79, 149.15, 146.83, 131.02, 130.29, 129.03, 124.75, 122.53, 114.71, 109.47, 109.02, 107.68, 96.73, 68.33, 65.27, 45.09, 29.22, 29.17, 28.69, 25.93, 25.87, 12.44.

HRMS (TOF/MS) m/z: [M + H]+ calcd for C34H40N3O5 +, 570.2968; found, 570.2968.

Supplementary Material

ao5c07385_si_001.pdf (5.1MB, pdf)

Acknowledgments

We would like to thank the Department of Chemistry at the Middle East Technical University.

The data supporting this article have been included as part of the Supporting Information.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c07385.

  • Experiments, characterization raw data, UV–vis spectra, fluorescence spectra, XYZ coordinates of the optimized structures, wavelength profile of the irradiation source, and 1H NMR of the FC3 before and after irradiation (PDF)

Yasemin Akdiş: Investigation, writing, review, and editing. Akin Akdag: Investigation, writing, review, editing, supervision, and methodologies.

The authors declare no competing financial interest.

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