Ultrafast dynamics of C30 in solution and within CDs and HSA protein

Steady-state UV-visible absorption and emission together with femto to nanosecond time-resolved emission techniques have been applied to study the dynamics of 3-(2-N-methylbenzimidazolyl)-7-(N,N-diethylamino)coumarin (C30) in neat solvents, as well as in the presence of chemical (β-CD and DM-β-CD) and biological (HSA protein) cavities. The formation of inclusion complexes inside the hydrophobic CDs gives 1:1 and 1:2 guest:host complexes, whereas with the HSA protein, the formed 1:1 inclusion complexes are more robust. The picosecond experiments show the importance of the interactions of C30 with the medium, as well as the intramolecular events in the excited-state relaxation as evidenced by the increase in the global emission lifetime from ∼0.5 ns in MeOH/H2O mixtures to 2.5 ns in THF, and to 1-3 ns when the dye is trapped within CDs and HSA cavities. Time-resolved anisotropy (r(t)) results indicate the involvement of ultrafast depolarization processes, whereas in the MeOH/H2O mixtures r(0) = 0.27, in DM-β-CD, r(0) = 0.35. The rotational time decays clearly show the robustness of the formed complexes with CDs and HSA protein: ∼170 ps in MeOH/H2O solvent mixtures, ∼850 ps due to 1:1 and 1:2 β-CD complexes, and 28 ns for HSA complexes. The femtosecond time-resolved emission experiments reveal the significant changes of the dynamics with the encapsulation of C30 by CDs (from approximately τ1 = 0.3 and τ2 = 2 ps in THF to approximately τ1 = 1.0 and τ2 = 7.5 ps in the MeOH/H2O binary mixture, and to approximately τ1 = 3 and τ2 = 30 ps in the CD complexes). The change is explained in terms of how the water molecules modulate the intramolecular charge transfer (ICT) time (τ1) and how the restriction of the environment modifies the torsional process (τ2). In the case of trapped C30 within the HSA protein the intermolecular interactions with the amino acid residues are revealed, giving rise to a complex photodynamical behavior due to the hydrophobic, H-bonding, electrostatic, and polar nature of the heterogeneous environment inside the protein. The protein confinement does not allow the occurrence of twisting motion in the trapped C30, and we observed a very fast (less than 100 fs) and slower (∼13 ps) ICT processes. We believe that the reported findings bring new knowledge for a better understanding of the photobehavior of coumarins in solution and trapped within hydrophobic pockets. The results can be applied to design better coumarin-based fluorescent labels for biological applications.