Spectral and Photophysical Behavior of Cytisine in n-Hexane

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Spectral and Photophysical Behavior of Cytisine in NHexane. Experimental Evidence for the S(n,#*)#S Fluorescence 1

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Ewa Krystkowiak, Anna K. Przybyl, Malgorzata Bayda, Julia Józkowiak, and Andrzej Maciejewski J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b02280 • Publication Date (Web): 12 Jul 2017 Downloaded from http://pubs.acs.org on July 16, 2017

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Spectral and Photophysical Behavior of Cytisine in n-Hexane. Experimental Evidence for the S1(n,π π*)→ →S0 Fluorescence Ewa Krystkowiak*[a], Anna K. Przybył[a], Małgorzata Bayda[a], Julia Józkowiak[a], and Andrzej Maciejewski[a,b] [a] Faculty of Chemistry, Adam Mickiewicz University in Poznań, Umultowska 89b, 61-614 Poznań, Poland, E-mail: [email protected] (Ewa Krystkowiak) [b] Center for Ultrafast Laser Spectroscopy, Adam Mickiewicz University in Poznań, Umultowska 85, 61-614 Poznań, Poland

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Abstract: Spectral and photophysical properties of (−)-cytisine (the compound used as a smoking cessation aid and a potential drug in Alzheimer’s and Parkinson’s diseases) were investigated. The two conformers of cytisine, whose presence in the S0 state has been earlier proved by the NMR and IR methods as well as in theoretical calculation, in non-polar n-hexane show a rarely observed prompt fluorescence from the S1(n, π*) excited state. This observation is unambiguously evidenced by very small radiative rate constants of these two emitting conformers, kF= 7.4×105 s-1 and 3.0×105 s-1. Their lifetimes in the S1(n, π*) state are relatively long, ߬ୗభ = 1.9 ns and 6.7 ns, therefore their fluorescence quantum yield is relatively high φF∼ 10-3. The long-wavelength band in the cytisine absorption originates from the excitation to the S2(π,π*) state, while the S1(n, π*) state is not observed in this spectrum. Thus, the excited state S2(π,π*) is manifested only in the absorption spectrum, while the excited state S1(n, π*) – only in the fluorescence spectrum, so cytisine in n-hexane is characterized by close lying (n, π*) and (π,π*) excited singlet states.

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Introduction The fluorescence from singlet (π,π*) or intramolecular charge-transfer (ICT) electronic excited states, often with high fluorescence quantum yield is commonly observed, but prompt emission from excited singlet (n, π*) state of aromatic compounds has been relatively rarely observed.1-12 The S1(n, π*)→S0 fluorescence is characterized by very low quantum yields, φF, due to low values of the radiative rate constants, kF∼ 105−106 s-1, and relatively short S1-state lifetimes, ߬ୗభ (Table 1). More often the thermally activated fluorescence from S1(n, π*) state caused by small energy difference between the lowest singlet S1 and triplet T1 excited states is observed.13-18 In this case the S1(n, π*)→S0 fluorescence can be relatively intense because of a thermally activated T1→S1 pathway in the luminescent process from a long-lived T1-state. The emission spectra including fluorescence from S1(n, π*) state, have been measured for xanthone and quinoline in vapor phase, at different temperatures and pressures.13-14 This emission has not been observed upon decreasing temperature.15-18 (−)-Cytisine (see Fig. 1 for the structure) is a naturally occurring quinolizidine alkaloid extracted from seeds of Laburnum anagyroides and other Leguminosae plants.19 From the biological point of view, cytisine belongs to the family of antagonists of nicotinic acetylcholine receptors (nAChRs).20-21 It has been used as a smoking cessation aid (Tabex, Desmoxan) and is also a very promising compound for the development of new drugs for the treatment of central nervous system disorders, particularly Alzheimer’s and Parkinson’s diseases.22 It was shown that in solid state, cytisine prefers the chair conformation with the equatorially positioned hydrogen atom in the imine group of piperidine moiety.2326

Two conformers, axial (endo) and equatorial (exo) of the chair conformation of cytisine were found in

solution

25, 27-33

and in gas phase.31-34 DFT calculation performed for these two most stable cytisine

conformers reproduce very well the molecule in solution phase. The energy barrier for the NH inversion, calculated at the DFT level (3.6 kcal×mol-1), confirms coexistence of the two conformers (equatorial and axial) in the gas phase or apolar solvents. Their abundances are almost the same in the mixture (ca. 40 and 60%, respectively). This enables to distinguish the component FT-IR bands for both conformers in solution.31 Despite wide discussion of cytisine structure and computational study concerning ACS Paragon Plus Environment

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prediction of its bioavailability and physicochemical properties,32, 35 there are no literature data on UVVIS absorption and emission spectra as well as on photophysical study, except one absorption spectrum measured in water.36 Because of cytisine biological activity, such study seems to be of great importance for understanding the mechanism of its interactions with the environment. It is worth to note that a weak fluorescence in n-hexane solution (φF = 10-3 – 10-5) was previously reported for two alkaloids from lupine class whose structures are similar to that of cytisine (see Table 1).37 In the present work, we discuss the results of spectral and photophysical study of (−)-cytisine in nhexane, which disclose a rarely observed fluorescence from S1(n, π*) excited state of two conformers.

Table 1. Spectral and photophysical properties of the selected aromatic and quasi aromatic compounds containing carbonyl group and displaying the S1(n, π*)→S0 prompt fluorescence.

Compound

Solvent

εmax/ M-1 × cm-1

߬ୗభ / ns

φF

kF/ s-1

knr/ s-1

Ref.

coumarin

cyclohexane

5 700

≤0.10

3×10-4

≥3×106

≥1×1010

1

3-chlorocoumarin

cyclohexane

7 200

≤0.10

4×10-4

≥4×106

1×1010

1

khelin

cyclohexane

1.2

2×10-4

2×105

8×108

2

lupanine

n-hexane

8 200

S0 Fluorescence in Trans-Azobenzene. Chem. Phys. Lett. 1979, 68, 267-271. (10). Ghosh, S.; Chowdhury, M., S1 (N,Π*), T1 (N,Π*) and S2(N,Π*) Emissions in 3,6-Diphenyl-STetrazine. Chem. Phys. Lett. 1982, 85, 233-238. (11). Itoh, T., Fluorescence and Phosphorescence from Higher Excited States of Organic Molecules. Chem. Rev. 2012, 112, 4541-4568. (12). Itoh, T., Intramolecular Mechanisms for the Occurence of Fluorescence from Upper Excited States of Aromatic Molecules and Linear Polyenes. In Reviews in Fluorescence 2015, Geddes, C. D., Ed. Spronger International Publishing: Switzerland, 2016; Vol. 8, pp 213-234. (13). Itoh, T., S1(N, Π*), 1a2 Fluorescence and T1(N, Π*), 3a2 Phosphorescence Spectra of Xanthone Vapor. Chem. Phys. Lett. 2014, 599, 12-14. (14). Itoh, T., Fluorescence and Phosphorescence Emission of Quinoline Vapor. J. Mol. Spectrosc. 2014, 301, 9-12. (15). Szymański, M.; Maciejewski, A.; Kozłowski, J.; Koput, J., Photophysics of the S2 State of Thiocoumarin: A Vibrationally Unequilibrated Luminophore. J. Phys. Chem. A 1998, 102, 677-683. (16). Li, J.; Zhang, Q.; Nomura, H.; Miyazaki, H.; Adachi, C., Thermally Activated Delayed Fluorescence from 3nπ* to 1nπ* up-Conversion and Its Application to Organic Light-Emitting Diodes. Appl. Phys. Lett. 2014, 105, 013301. ACS Paragon Plus Environment

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(17). Itoh, T.; Hashimoto, R., Temperature Dependence of the Emission Spectra of P-Benzoquinone in a P-Dichlorobenzene Matrix. J. Lumin. 2012, 132, 236-239. (18). Parker, C. A., Advances in Photochemistry; John Wiley & Sons, Inc., 1964; Vol. 2, p 305. (19). Rouden, J.; Lasne, M.-C.; Blanchet, J.; Baudoux, J., (−)-Cytisine and Derivatives: Synthesis, Reactivity, and Applications. Chem. Rev. 2014, 114, 712-778. (20). Tasso, B.; Boido, C. C.; Terranova, E.; Gotti, C.; Riganti, L.; Clementi, F.; Artali, R.; Bombieri, G.; Meneghatti, F.; Sparatore, F., Synthesis, Binding, and Modeling Studies of New Cytisine Derivatives, as Ligands for Neuronal Nicotinic Acetylcholine Receptor Subtypes. J. Med. Chem. 2009, 52, 4345-4357. (21). Pérez, E. G.; Méndez-Gálvez, C.; Cassels, B. K., Cytisine: A Natural Product Lead for the Development of Drugs Acting at Nicotinic Acetylcholine Receptors. Nat. Prod. Rep. 2012, 29, 555-567. (22). Boido, C. C.; Tasso, B.; Boido, V.; Sparatore, F., Cytisine Derivatives as Ligands for Neuronal Nicotine Receptors and with Various Pharmacological Activities. Farmaco 2003, 58, 265-277. (23). Freer, A. A.; Robins, D. J.; Sheldrake, G. N., Structures of (–)-Cytisine and (–)-NMethylcitisine: Tricyclic Quinolizidine Alkaloids. Acta Cryst. C 1987, 43, 1119-1122. (24). Barlow, R. B.; Johnson, O., Relations between Structure and Nicotine-Like Activity: X-Ray Crystal Structrure Analysis of (–)-Cytisine and (–)-Lobeline Hydrochloride and a Comparison with (–)Nicotine and Other Nicotine-Like Compounds. Br. J. Pharmacol. 1989, 98, 799-808. (25). Mascagni, P.; Christodoulou, M.; Gibbons, W. A.; Asres, K.; Phillipson, J. D.; Niccolai, N.; Mangani, S., Solution and Crystal Structure of Cytisine, a Quinolizidine Alkaloid. J. Chem. Soc. Perkin Trans. 2 1987, 1159-1165. (26). Górnicka, E.; Rode, J. E.; Raczyńska, E. D.; Dasiewicz, B.; Dobrowolski, J. C., Experimental (Ft-Ir and Raman) and Theoretical (Dft) Studies on the Vibrational Dynamics in Cytisine. Vib. Spectrosc. 2004, 36, 105-115. (27). Liu, Z.; Yang, L.; Jia, Z.; Chen, J., Stereochemical Study of Anagyrine-Type Quinolizidine Alkaloids by 15n and 2d Nmr Spectroscopy. Mag. Res. Chem. 1992, 30, 511-514. (28). Brukwicki, T.; Wysocka, W., Geometry of Tricyclic Quinolizidine-Piperidine Alkaloids in Solution by Nmr Spectroscopy. J. Mol. Struct. 1999, 474, 215-222. (29). Mikhova, B.; Duddeck, H., 13c Nmr Spectroscopy of Tri- and Tetracyclic Quinolizidine Alkaloids: Compilation and Discussion. Mag. Res. Chem. 1998, 36, 779-796. (30). Górnicka, E.; Raczyńska, E. D., Ft-Ir Spectroscopic, Am1 and Pm3 Computational Studies of Conformation of Natural Products: Cytisine. Talanta 2002, 57, 609-616. (31). Rode, J. E.; Raczyńska, E. D.; Górnicka, E.; Dobrowolski, J. C., Low Inversion Energy Barrier of Cytisine Nh Group - an Explanation for the Ft-Ir Bands Splitting. J. Mol. Struct. 2005, 749, 51-59. (32). Raczyńska, E. D.; Makowski, M.; Górnicka, E.; Darowska, M., Ab Initio Studies on the Preferred Site of Protonation in Cytisine in the Gas Phase and Water. Int. J. Mol. Sci. 2005, 6, 143-156. (33). Galasso, V.; Przybył, A. K.; Christov, V.; Kovač, B.; Asaro, F.; Zangrando, E., Theoretical and Experimental Studies on the Molecular and Electronic Structures of Cytisine and Unsaturated KetoSparteines. Chem. Phys. 2006, 325, 365-377. (34). Górnicka, E.; Makowski, M.; Darowska, M.; Raczyńska, E. D., Conformational Preference in Isolated Neutral Cytisine. Polish J. Chem. 2001, 75, 1483-1491. (35). Pieńko, T.; Grudzień, M.; Taciak, P. P.; Mazurek, A. P., Cytisine Basicity, Solvation, Logp, and Logd Theoretical Determination as Tool for Bioavailability Prediction. J. Mol. Graph. Model. 2016, 63, 15-21. (36). Przybył, A. K.; Kubicki, M.; Jatrząb, R., Complexing Ability (-)-Cytisine - Synthesis, Spectroscopy and Crystal Structures of the New Copper and Zinc Compexes. J. Inorg. Biochem. 2014, 138, 47-55. (37). Halpern, A. M.; Falck, C.; Legenza, M. W., The Photophysical Properties of Some C15 Lupine Alkaloids: Sparteine, Lupanine, Thermospine, and α-Diplospartyrine. Photochem. Photobiol. 1989, 49, 33-36.

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(38). Marrière, E.; Rouden, J.; Tadino, V.; Lasne, M.-C., Synthesis of Analogues of (−)-Cytisine for in Vivo Studies of Nicotinic Receptors Using Positron Emission Tomography. Org. Lett. 2000, 2, 11211124. (39). Przybył, A. K.; Kubicki, M., A Compartive Study of Dynamic Nmr Spectroscopy in Analysis of Selected N-Alkyl-, N-Aacyl-, and Halogenated Cytisine Derivatives. J. Mol. Struct. 2011, 985, 157166. (40). Suzuki, K.; Kobayashi, A.; Kaneko, S.; Takehira, K.; Yoshihara, T.; Ishida, H.; Shiina, Y.; Oishi, S.; Tobita, S., Reevaluation of Absolute Luminescence Quantum Yields of Standard Solutions Using a Spectrometer with an Integrating Sphere and a Back-Thinned Ccd Detector. Phys. Chem. Chem. Phys. 2009, 11, 9850-9860. (41). Ciesielska, B.; Łukaszewicz, A.; Celewicz, L.; Maciejewski, A.; Kubicki, J., Method of Determination of Emission Properties of Very Weakly Emitting Species Ensuring Elimination of Emission of Impurities. Appl. Spectrosc. 2007, 61, 102-109. (42). Ciesielska, B.; Kubicki, J.; Maciejewski, A.; Paszyc, S., Emission Properties of Gossypol in Solution. Chem. Phys. Lett. 2002, 353, 69-76. (43). Krystkowiak, E.; Bachorz, R. A.; Maciejewski, A., Hydrogen-Bonded 4h-1-Benzopyrane-4Thione–Water Complexes Properties in the S2 Excited State: The Mechanism and Dynamics of Deactivation. Phys. Chem. Chem. Phys. 2016, 18, 492-502. (44). Lakowicz, J. R., Principles of Fluorescence Spectroscopy, Second ed.; Kluwer Academic/Plenum Publisher: New York, 1999. (45). Klessinger, M.; Michl, J., Excited States and Photochemistry of Organic Molecules; VCH Publishers, Inc.: New York, 1995. (46). Turro, N. J.; Ramamurthy, V.; Scaiano, J. C., Modern Molecular Photochemistry of Organic Molecules; University Science Books: Sausalito, California, 2010. (47). Michl, J.; Bonačić-Koutecký, V., Electronic Aspects of Organic Photochemistry; John Wiley & Sons, Inc.: New York, 1990. (48). Grabowski, Z. R.; Rotkiewicz, K.; Rettig, W., Structural Changes Accompanying Intramolecular Electron Transfer: Focus on Twisted Intramolecular Charge-Transfer States and Structures. Chem. Rev. 2003, 103, 3899-4031. (49). Rettig, W., Charge Separation in Excited States of Decoupled Systems—Tict Compounds and Implications Regarding the Development of New Laser Dyes and the Primary Process of Vision and Photosynthesis. Angew. Chem. Int. Ed. 1986, 25, 971-988. (50). Prigogine, I.; Rice, S. A.; Lippert, E.; Rettig, W.; Bonačić-Koutecký, V.; Heisel, F.; Miehé, J. A., Photophysics of Internal Twisting. In Advances in Chemical Physics, Prigogine, I.; Rice, S. A., Eds. John Wiley & Sons, Inc.: 1987; Vol. 68, pp 1-174. (51). Suppan, P.; Ghoneim, N., Solvatochromism; The Royal Society of Chemistry, Information Services, 1997.

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TOC Graphic

S1(n,π π*)

20000

S0

25000

S0

30000

S2(π π,π π*)

35000 ν / cm

-1

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