1. Introduction

2. Materials and Methods

3. Results and Discussion

3.1. Synthesis

A series of six dansyl derivatives L1-L6 were prepared (Scheme 1, Scheme 2 and Scheme 3), and their photophysical properties were examined in current investigation. According to databases, five of them are new. Only compound L1 was obtained earlier, in a publication concerning the development of new method for the synthesis of sulfonamides from benzylic alcohols [13]. However, the photophysical properties of this compound were not subjected to further studies.

The synthesis of four intermediates 4, 5 and 8 from commercially available reagents was performed (Scheme 1). These intermediate products were obtained quantitatively and used in the next steps for the synthesis of the target compounds. Compound 4 was prepared under basic conditions, through a reaction between anthranilic acid (1) and thiomorpholine in the presence of coupling reagent TBTU. Amide 5 was prepared in a similar manner from 1 and amine 3. Amide 8 was obtained also by selective acylation of diamine 6 with sulfochloride 7 (Scheme 3). The coupling reactions occur selectively towards the aliphatic amino group, which is not unusual [14]. Thus, due to stronger nucleophilicity of aliphatic amino groups in reagents 2, 3 and 6 there is no need for protection of the aromatic groups of 1 and 6. Amides 4, 5 and 8 were observed as single-step reaction products.

Scheme 1. Synthetic approach to the preparation of intermediates 4, 5 and 8.

Scheme 1. Synthetic approach to the preparation of intermediates 4, 5 and 8.

Intriguingly, the preparation and structural characterization of 4 [15] and 5 [14,16,17,18,19] was already described in some studies. However, different synthetic methods were employed – aminolysis of isatoic anhydride with amines 2 and 3, respectively. The preparation of intermediate 12 starting from commercially available dansyl chloride (9) and diamine 6 (Scheme 3) was performed by implementation of an existing protocol [20].

Scheme 2. Synthetic approach to the preparation of the target compounds L1-L4 and L6.

Scheme 2. Synthetic approach to the preparation of the target compounds L1-L4 and L6.

The synthesis of amide L1 (Scheme 2) was accomplished by one step acylation of amine 3 with 9 in the presence of DIPEA. Other target compounds (L2-L4 and L6) were prepared by acylation of the above-mentioned intermediates (4, 5 and 8) or the commercially available amine 10 with dansyl chloride (9) in dry pyridine. The reaction outcome was monitored using TLC until starting reagents exhausted. Compound L5 was prepared by addition of methyl isothiocyanate to intermediate 12 in refluxing methanol (Scheme 3), as Ambati et al. described for similar compounds [21]. All target compounds L1-L6 were obtained quantitatevely. Due to steric hindrance (as in the case of compound L2) or possible tautomerism (in the case of L5), NMR spectra of both compounds at room temperature were not enough informative. Thus, they were performed at 353 K aiming to obtain narrow signals.

Scheme 3. Synthesis of intermediate 12 and target compound L5.

Scheme 3. Synthesis of intermediate 12 and target compound L5.

3.2. Photophysical studies

The absorption and emission spectra of all six dansyl derivatives L1-L6 were studied in five different solvents, namely, DMSO, CH₃CN, EtOH, THF and CHCl₃ at 298 K (Figures S8–S13). Additionally, the emission solid state spectra were also obtained for these derivatives. The main results are summarized in Figure 1 and Table 1.

Selecting acetonitrile as a representative solvent, all compounds exhibited an absorption band at 341 nm, 345 nm, 320 nm, 345 nm, 337 nm, 333 nm, from L1 to L6, characteristic of the π - π* transition of the dansyl chromophores. These compounds appear colorless to the naked eye. However, upon emission, they display a pronounced yellow emission, with maximum bands at 519, 528, 535, 530, 520 and 528 nm, from L1 to L6, respectively. Specifically, L2, L3, L4 and L6, which have a benzene unit directly attached to the dansyl unit, do not exhibit significant changes were observed in the excited state, with maximum emission value ca. 528-530 nm. The same was not verified in the ground-state where the increase of donor atoms and side chain length promotes a red shift in the maximum bands, ranging from 320 nm to 345 nm. In contrast, L1 and L5, which have a spacer between the dansyl chromophores, showed a blue shift of ca. 10 nm in the emission bands (ca. 519-520 nm), but no significant changes in the absorption.

For solid-state emissions, L4 and L5 produced emission bands at a longer wavelength (ca. 500-505 nm) than the other compounds (ca. 478-486 nm), probably due to their higher number of donor atoms. L1 and L5 demonstrate the highest fluorescence quantum yield (φ =ca. 35-36%), suggesting that the introduction of a spacer might stabilize these molecules in their excited state.

An intriguing observation was the solvatochromic effect in various solvents. As solvent polarity increased, a change in emission color from yellow to green was noted. This change was due to a red shift in the maximum emission bands, varying from 496 to 522 nm for L1, 499 nm to 540 nm for L2, 503 to 536 nm for L3, 504 to 533 nm for L4, 500 nm to 524 nm to L4 and 497 nm to 537 nm for L6, respectively. In the absorption no significant correlation is identified.

Figure 1. (A) Chemical structures of dansyl-derivatives L1-L6. (B) Images of L1-L6 under a UV light lamp. Polarity decreasing from DMSO to CHCl₃.

Figure 1. (A) Chemical structures of dansyl-derivatives L1-L6. (B) Images of L1-L6 under a UV light lamp. Polarity decreasing from DMSO to CHCl₃.

Based on previous studies from our group it is evident that solvents with greater polarity significantly influence the stability of molecules. From the observed shifts, all compounds display a pronounced positive solvatochromic effect, suggesting enhanced stabilization of their excited state in highly polar solvents. This behavior provides an opportunity to stimulate the properties of the compound’s microenvironment, offering potential applications in determining the local polarity in membranes, proteins, DNA [22]. Furthermore, this phenomenon can be valuable in understanding the characteristics of ionic liquids or mixed solvents and hold importance in the textile industry [23].

Table 2. Photophysical characterization data of dansyl derivatives L1-L6 in the different solvents.

Table 2. Photophysical characterization data of dansyl derivatives L1-L6 in the different solvents.

Cpd.	Solv.	λabs [nm]	λem [nm]	ε [103 cm-1 M-1]	Stokes shift [cm-1]	λem.solid nm]	Φ (%)	Brightness (ε X ϕ) [cm-1 M-1]	t[ns]
L1	DMSO	337	522	5.765	54054	484	22	1291	17
	CH3CN	341	519	5.531	56179		35	1941	10
	EtOH	335	516	4.873	55248		31	1506	13
	THF	334	498	4.969	60975		21	1048	12
	CHCl3	345	496	4.721	66225		35	1671	14
L2	DMSO	353	540	5.950	53475	478	11	666	13
	CH3CN	345	528	5.519	54644		29	1606	12
	EtOH	346	520	4.983	57471		38	1913	13
	THF	334	504	5.201	58823		39	200	13
	CHCl3	345	499	5.204	64935		37	1951	15
L3	DMSO	340	536	5.397	51020	486	10	550	13
	CH3CN	320	535	5.454	46511		28	1538	11
	EtOH	340	531	5.052	52356		32	1596	11
	THF	339	511	4.921	58139		32	1555	13
	CHCl3	346	503	4.057	63694		32	1282	15
L4	DMSO	343	533	4.254	52631	505	21	914	15
	CH3CN	345	530	4.471	54054		27	1211	10
	EtOH	345	522	3.780	56497		17	646	13
	THF	343	502	4.653	62893		27	1261	13
	CHCl3	345	504	4.482	62893		29	1313	15
L5	DMSO	339	524	4.640	54054	500	30	1378	17
	CH3CN	337	520	5.674	54644		36	2048	11
	EtOH	338	515	3.968	56497		22	877	13
	THF	337	497	4.940	62500		26	1304	13
	CHCl3	343	500	0.3593	63694		34	1210	15
L6	DMSO	347	537	4.279	52631	486	17	740	14
	CH3CN	333	528	4.817	51282		28	1354	11
	EtOH	329	525	7.164	51020		17	1189	13
	THF	341	501	4.370	62500		29	1280	13
	CHCl3	341	497	4.782	64102		34	1616	14

To quantitatively understand the interactions between solute and solvent, a multiparametric fit using the Kamlet-Taft equation was carried out (Equation 1). This equation enabled the determination of certain solute-dependent parameters which relate to solvent properties such as hydrogen bond donating (HBD) acidity (α), hydrogen bond accepting (HBA) basicity (β), and the solvent's dipolarity/polarizability (π^∗).

$$\upsilon ={\upsilon }_0+a\alpha +b\beta +p{\pi }^{*}$$

(1)

here ${\upsilon }_0$ is the value of emission in a reference solvent. (see Table 1) [23].

Table 3 shows the fitted parameters (υ₀, a, b and p), the slope and correlation coefficients based on the fitting linear plots of υ_exp. versus υ_calc.

Analysis of the results in Table 3, clearly evidences that the solvatochromic effect is essential due to higher sensitivity to H-bond acceptor (or electron donor) strength of the solvents, as can be observed in the b values, with one-order of magnitude higher than a, respectively. However, the values indicate an even stronger influence of the solvent’s dipolarity/polarizability with p values with one and two orders of magnitude higher than a. Thus, electronic interactions (polarity and polarizability) and hydrogen bond accepting behavior of the solvent play a dominant role compared to its hydrogen bond donating character.

4. Conclusions

References

1. Sivaraman, G.; Iniya, M.; Anand, T.; Kotla, N.G.; Sunnapu, O.; Singaravadivel, S.; Gulyani, A.; Chellappa, D. Chemically Diverse Small Molecule Fluorescent Chemosensors for Copper Ion. Coord Chem Rev 2018, 357, 50–104. [Google Scholar] [CrossRef]
2. Oliveira, E.; Bértolo, E.; Núñez, C.; Pilla, V.; Santos, H.M.; Fernández-Lodeiro, J.; Fernández-Lodeiro, A.; Djafari, J.; Capelo, J.L.; Lodeiro, C. Green and Red Fluorescent Dyes for Translational Applications in Imaging and Sensing Analytes: A Dual-Color Flag. ChemistryOpen 2017. [Google Scholar] [CrossRef]
3. Qin, X.; Yang, X.; Du, L.; Li, M. Polarity-Based Fluorescence Probes: Properties and Applications. RSC Med Chem 2021, 12, 1826–1838. [Google Scholar] [CrossRef] [PubMed]
4. Wei, P.; Xiao, L.; Gou, Y.; He, F.; Zhou, D.; Liu, Y.; Xu, B.; Wang, P.; Zhou, Y. Fluorescent “on–off–on” Probe Based on Copper Peptide Backbone for Specific Detection of Cu(II) and Hydrogen Sulfide and Its Applications in Cell Imaging, Real Water Samples and Test Strips. Microchemical Journal 2022, 182, 107848. [Google Scholar] [CrossRef]
5. Duarte, F.; Dobrikov, G.; Kurutos, A.; Santos, H.M.; Fernández-Lodeiro, J.; Capelo-Martinez, J.L.; Oliveira, E.; Lodeiro, C. Enhancing Water Sensing via Aggregation-Induced Emission (AIE) and Solvatofluorochromic Studies Using Two New Dansyl Derivatives Containing a Disulfide Bound: Pollutant Metal Ions Detection and Preparation of Water-Soluble Fluorescent Polymeric Particles. Dyes and Pigments 2023, 218, 111428. [Google Scholar] [CrossRef]
6. Duarte, F.; Dobrikov, G.; Kurutos, A.; Capelo-Martinez, J.L.; Santos, H.M.; Oliveira, E.; Lodeiro, C. Development of Fluorochromic Polymer Doped Materials as Platforms for Temperature Sensing Using Three Dansyl Derivatives Bearing a Sulfur Bridge. J Photochem Photobiol A Chem 2023, 445, 115033. [Google Scholar] [CrossRef]
7. Métivier, R.; Leray, I.; Lebeau, B.; Valeur, B. A Mesoporous Silica Functionalized by a Covalently Bound Calixarene-Based Fluoroionophore for Selective Optical Sensing of Mercury(Ii) in Water. J Mater Chem 2005, 15, 2965. [Google Scholar] [CrossRef]
8. Algethami, J.S. A Review on Recent Progress in Organic Fluorimetric and Colorimetric Chemosensors for the Detection of Cr ^3+/6+ Ions. Crit Rev Anal Chem 2022, 1–21. [Google Scholar] [CrossRef] [PubMed]
9. Adhikari, B.; Majumdar, S. Polymers in Sensor Applications. Prog Polym Sci 2004, 29, 699–766. [Google Scholar] [CrossRef]
10. GANS, P.; SABATINI, A.; VACCA, A. Investigation of Equilibria in Solution. Determination of Equilibrium Constants with the HYPERQUAD Suite of Programs. Talanta 1996, 43, 1739–1753. [Google Scholar] [CrossRef]
11. Berlman, I.B. Handbook of Fluorescence Spectra of Aromatic Molecules, 2nd ed.; Press, A., Ed.; Academic Press: New York, NY, USA, 1971. [Google Scholar]
12. Montalti, M.; Credi, A.; Prodi, L.; Gandolfi, M.T. Handbook of Photochemistry, 3rd ed.; Taylor & Francis; BOCA: Boca Raton, FL, USA, 2006. [Google Scholar]
13. Shi, F.; Tse, M.K.; Zhou, S.; Pohl, M.-M.; Radnik, J.; Hübner, S.; Jähnisch, K.; Brückner, A.; Beller, M. Green and Efficient Synthesis of Sulfonamides Catalyzed by Nano-Ru/Fe ₃ O ₄. J Am Chem Soc 2009, 131, 1775–1779. [Google Scholar] [CrossRef] [PubMed]
14. Yang, Y.; Yu, X.; He, N.; Huang, X.; Song, X.; Chen, J.; Lin, J.; Jin, Y. FeCl ₃ -Catalyzed Oxidative Amidation of Benzylic C–H Bonds Enabled by a Photogenerated Chlorine-Radical. Chemical Communications 2023, 59, 10299–10302. [Google Scholar] [CrossRef] [PubMed]
15. Huang, F.-Q.; Dong, X.; Qi, L.-W.; Zhang, B. Visible-Light Photocatalytic α-Amino C(Sp3)–H Activation through Radical Translocation: A Novel and Metal-Free Approach to α-Alkoxybenzamides. Tetrahedron Lett 2016, 57, 1600–1604. [Google Scholar] [CrossRef]
16. Ye, W.; Liu, Y.; Ren, Q.; Liao, T.; Chen, Y.; Chen, D.; Wang, S.; Yao, L.; Jia, Y.; Zhao, C.; et al. Design, Synthesis and Biological Evaluation of Novel Triazoloquinazolinone and Imidazoquinazolinone Derivatives as Allosteric Inhibitors of SHP2 Phosphatase. J Enzyme Inhib Med Chem 2022, 37, 1495–1513. [Google Scholar] [CrossRef] [PubMed]
17. Tang, B.-D.; Zhang, J.-Y.; Ma, H.-X.; Wang, N.; An, X.; Li, G.-M.; Zhou, Z. SYNTHESIS, CRYSTAL STRUCTURE, AND DFT STUDY OF 1-(PYRROLIDIN-1- YL-METHYL)-4-(THIOPHEN-2-YL-METHYL)- [1,2,4]TRIAZOLO[4,3-a]QUINAZOLIN-5(4H)-ONE. Journal of Structural Chemistry 2022, 63, 19–25. [Google Scholar] [CrossRef]
18. Zaytsev, V.P.; Revutskaya, E.L.; Kuz´menko, M.G.; Novikov, R.A.; Zubkov, F.I.; Sorokina, E.A.; Nikitina, E. V.; Toze, F.A.A.; Varlamov, A. V. Synthesis of Furyl-, Furylvinyl-, Thienyl-, Pyrrolinylquinazolines and Isoindolo[2,1-a]Quinazolines. Russian Chemical Bulletin 2015, 64, 1345–1353. [Google Scholar] [CrossRef]
19. Jang, Y.; Lee, S.B.; Hong, J.; Chun, S.; Lee, J.; Hong, S. Synthesis of 2-Aryl Quinazolinones via Iron-Catalyzed Cross-Dehydrogenative Coupling (CDC) between N–H and C–H Bonds. Org Biomol Chem 2020, 18, 5435–5441. [Google Scholar] [CrossRef]
20. Sanmartín-Matalobos, J.; Bermejo-Barrera, P.; Pérez-Juste, I.; Fondo, M.; García-Deibe, A.M.; Alves-Iglesias, Y. Experimental and Computational Studies on the Interaction of a Dansyl-Based Fluorescent Schiff Base Ligand with Cu2+ Ions and CuO NPs. Int J Mol Sci 2022, 23, 11565. [Google Scholar] [CrossRef]
21. Ambati, N.B.; Anand, V.; Hanumanthu, P. A Facile Synthesis of 2-N(Methyl Amino) Benzothiazoles. Synth Commun 1997, 27, 1487–1493. [Google Scholar] [CrossRef]
22. Aliaga-Alcalde, N.; Rodríguez, L. Solvatochromic Studies of a Novel Cd2+–Anthracene-Based Curcuminoid and Related Complexes. Inorganica Chim Acta 2012, 380, 187–193. [Google Scholar] [CrossRef]
23. Oliveira, E.; Baptista, R.M.; Costa, S.P.; Raposo MM, M.; Lodeiro, C. Synthesis and Solvatochromism Studies of Novel Bis(Indolyl)Methanes Bearing Functionalized Arylthiophene Groups as New Colored Materials. Photochemical & Photobiological Sciences 2014, 13, 492–498. [Google Scholar] [CrossRef]
24. Marcelo, G.A.; Pires, S.M.G.; Faustino, M.A.F.; Simões, M.M.Q.; Neves, M.G.P.M.S.; Santos, H.M.; Capelo, J.L.; Mota, J.P.; Lodeiro, C.; Oliveira, E. New Dual Colorimetric/Fluorimetric Probes for Hg2+ Detection & Extraction Based on Mesoporous SBA-16 Nanoparticles Containing Porphyrin or Rhodamine Chromophores. Dyes and Pigments 2019, 161, 427–437. [Google Scholar] [CrossRef]
25. Gonc, A.C.; Luis, J.; Lodeiro, C.; Dos, A.A. Sensors and Actuators B : Chemical A Seleno-Pyrene Selective Probe for Hg 2 + Detection in Either Aqueous or Aprotic Systems. 2017, 239, 311–318. [CrossRef]
26. Pinheiro, D.; De Castro, C.S.; Seixas De Melo, J.S.; Oliveira, E.; Nu??ez, C.; Fern??ndez-Lodeiro, A.; Capelo, J.L.; Lodeiro, C. From Yellow to Pink Using a Fluorimetric and Colorimetric Pyrene Derivative and Mercury (II) Ions. Dyes and Pigments 2014, 110, 152–158. [Google Scholar] [CrossRef]