Do DNA and Guanine Quench Fluorescence of Conjugated Cationic Polymers by Induced Aggregation?

Matthew L. Davies,^1,2 Peter Douglas,¹ Hugh D. Burrows, ² M. Graça Miguel,^2,* Alastair Douglas³

¹ Chemistry Group, School of Engineering, Swansea University, Singleton Park Swansea, SA2 8PP, UK

² Departamento de Química da Universidade de Coimbra, Rua Larga, 3004-535 Coimbra, Portugal

³ AD Technology Consulting Limited, Swansea, SA2 7UZ, UK

 

Received 14 May 2009; accepted 21 May 2009

 

Abstract

DNA and guanine are efficient fluorescence quenchers of the cationic conjugated polymer, poly {9,9-bis[N,N-(trimethylammonium)hexyl] fluorene-co-l,4-phenylene} (CCP). Studies with CCPs, of average chain length ~6, 12 and 100 repeat units, with single strand (ss) DNA, double strand (ds) DNA, and guanine, in 25/75 acetonitrile/water (v/v) mixtures result in Stern-Volmer quenching plots that show upward curvature. Initial Stern-Volmer constants, k_SV, are in the range ≈ 3-20 x 10⁷ M^-1 which is much higher than possible by diffusional encounter quenching. Aggregation studies in acetonitrile/water mixtures show that aggregation is also an effective quencher of CCP fluorescence, and we note that both aggregation and quenching by DNA or guanine is accompanied by a reduction in solution absorbance at 380 nm. Comparison of the relationship between changes in absorbance and changes in emission intensity suggest that both solvent and chemical induced fluorescence quenching are due to aggregation. We interpret the correlated changes in absorption and emission, high quenching constants, and upward curving Stern-Volmer plots as evidence that the dominant mechanism for fluorescence quenching by DNA or guanine is via induced aggregation of the polymer. The upward curvature of Stern- Volmer plots and high k_SV values for DNA and guanine are indicative of “aggregate energy migration quenching” in which CCP aggregates around a DNA or guanine molecule to form an aggregate complex in which excitation energy migrates between and along the polymer chains until it is quenched at an aggregate trap.

Keywords: cationic conjugated polyelectrolytes, DNA, guanine, quenching, aggregation, energy migration.

 

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^*Corresponding author.:burrows@ci.uc.pt