Competition between solvent quenching and indole quenching of 9-fluorenone: A spectroscopic and computational study

doi:10.1016/j.saa.2009.11.028

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

Volume 75, Issue 2, February 2010, Pages 624-628

https://doi.org/10.1016/j.saa.2009.11.028 Get rights and content

Abstract

The interaction between 9-fluorenone, various indoles and solvents has been studied using steady-state fluorescence spectroscopy and quantum chemical calculations. It was determined that polar protic solvents such as methanol and ethanol significantly quenched the fluorescence of 9-fluorenone but various indoles reversed the solvent quenching. The effect of various solvents on the 9-fluorenone carbonyl vibration was investigated using infrared spectroscopy. Ab initio calculations using Gaussian03 were also carried out in order to determine the minimum energy conformations of these systems along with binding energies.

Introduction

Fluorenones are aromatic carbonyl compounds whose photophysical characteristics are influenced by the solvent. 9-Fluorenone (9F) is a rigid, planar molecule and the carbonyl group introduces non-bonding electrons and consequently, n–π^* bands appear in the electronic spectrum. The energy difference between the excited states S₁, which has nπ^* character and S₂, which has ππ^* character, is small and solvent-dependent. Solvent-induced shifts resulting from the addition of polar solvents cause these electronic states to interchange [1]. This is also accompanied by a change in fluorescence intensity. 9F forms an intermolecular hydrogen-bonded complex with polar protic solvents such as alcohols [2], [3], [4], [5], [6], [7], [8], [9]. As a result, the hydrogen bond can act as an effective acceptor for radiationless deactivation processes [2]. This can be observed through fluorescence quenching experiments where the emission intensity is compared across a series of solvents of varying polarity and H-bond forming ability.

Fluorenones also experience fluorescence quenching in presence of various indoles. It has been reported that 5-hydroxyindole (5HI) can quench 9F efficiently in acetonitrile over a wide range of indole concentrations [10]. In this system, 5HI acts as a donor which participates in a highly exothermic electron transfer reaction with the excited 9F. As the emission wavelength of 9F is around 500 nm where 5HI does not absorb, this type of quenching is not a result of Forster's energy transfer (FRET). Depending on the choice of solvent an interesting scenario may arise, whereby the solvent and indole derivative are both quenching the fluorescence of 9F or the indole derivative is undoing the effect of solvent quenching by disrupting the hydrogen bonding between the fluorenone and the solvent.

Fluorenones have generated a lot of interest and have been widely studied for the above-stated reasons. Several groups have also studied various substituted fluorenones from a conformational context [11], [12], [13], [14], [15], [16]. The underlying motivation for this work is our group's interest in a specific substituted fluorenone, 9-fluorenone-2-carboxylic Acid (9F2C). 9F2C has been used as a photoactivator to cross-link proteins such as bovine serum albumin (BSA) at aromatic residues such as tryptophan, via a multiphoton excitation process [17]. The resulting cross-links can be used for creating scaffolds or devices suitable for microfluidic applications such as drug delivery [17], [18], [19], [20]. We are interested in measuring the minimum distance between 9F2C and various proteins. To successfully carry out those experiments, a solvent system suitable for experiments involving proteins as well as 9F2C is required. One major consideration is the effect various solvents may have on 9F2C fluorescence as well as protein function. 9F and 5HI were chosen for this work as they are ideal precursors to 9F2C and tryptophan.

Section snippets

Experimental

The indoles (5HI, 1-methyl indole, tryptophan and melatonin), 9F and spectroscopic grade methanol and acetonitrile were purchased from Sigma–Aldrich and used without further purification. The concentration of 9F in all experiments was 0.7 mM [10]. The indole concentration ranged from 0.0 mM to 3.5 mM. The excitation wavelength of 9F in each solvent was determined using a Cary 4000 UV–vis Spectrophotometer. Fluorescence experiments were conducted using a PerkinElmer LS-50B Luminescence

Fluorescence quenching of 9F by 5HI in acetonitrile

Fig. 1 shows the fluorescence quenching of 9F by 5HI in 100% acetonitrile, a polar aprotic solvent. The trend is consistent with the results reported by Misra et al. [10]. The quenching is not accompanied by any significant change in the shape of the emission peak but there is a blue-shift as the concentration of the indole is increased. The fluorescence intensity decreases by approximately 54% when the concentration of 5HI is increased from 0.0 mM to 3.5 mM. The results were independent of

Conclusion

In conclusion, indoles known to quench the fluorescence of 9F in acetonitrile are reversing solvent quenching of 9F by alcohols. Infrared experiments showed minor changes to the OH stretching region of methanol and C triple bond N stretching region of acetonitrile but no change to the C double bond O region of 9F. Calculations also showed that certain solvents bind both 9F and 9F/5HI strongly and the binding is stronger at the NH group of 5HI. These results indicate that the quenching of 9F by indoles is highly

Acknowledgements

We thank Susquehanna University for providing funding for this project. We also thank Dr. Brian Williams (Bucknell University) and Dr. Haribabu Arthanari (Harvard Medical School) for helpful discussions.

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Competition between solvent quenching and indole quenching of 9-fluorenone: A spectroscopic and computational study

Abstract

Introduction

Section snippets

Experimental

Fluorescence quenching of 9F by 5HI in acetonitrile

Conclusion

Acknowledgements

Chem. Phys. Lett.

Chem. Phys.

Chem. Phys.

J. Elec. Spec. Rel. Phen.

Anal. Biochem.

J. Mol. Struct.

Chem. Phys. Lett.

Spectrochim. Acta A

J. Fluorosc.

J. Phys. Chem. A

J. Am. Chem. Soc.

Bull. Chem. Soc. Jpn.

J. Phys. Chem.