Exciplex Formation Dynamics of Photoexcited Copper(II) Tetrakis(4-N-methylpyridyl)porphyrin with Synthetic Polynucleotides Probed by Transient Absorption and Raman Spectroscopic Techniques

Abstract

To elucidate the interaction mechanism of water-soluble copper(II) tetrakis(4-N-methylpyridyl)porphyrin (CuII(TMpy-P4)) with synthetic polynucleotides such as poly(dA-dT)2 and poly(dG-dC)2, the exciplex formation dynamics of photoexcited CuII(TMpy-P4) with added polynucleotides have been investigated by using femtosecond transient absorption as well as transient Raman spectroscopic methods. The nanosecond transient Raman spectra of CuII(TMpy-P4) mixed with poly(dA-dT)2 clearly demonstrate the exciplex formation between photoexcited CuII(TMpy-P4) and poly(dA-dT)2. On the other hand, the exciplex formation of photoexcited CuII(TMpy-P4) with poly(dG-dC)2 is not so manifest as compared with that mixed with poly(dA-dT)2. The transient absorption of CuII(TMpy-P4) mixed with poly(dG-dC)2 exhibits a rise component of 1.3 ps in addition to the very slow decay component (τ ∼ 22 ns). This observation is quite different from that of CuII(TMpy-P4) in aqueous solution or CuII(TMpy-P4) mixed with poly(dA-dT)2, because the exciplex formed in poly(dG-dC)2, if any, is so short-lived that it returns quickly back to four-coordinate CuII(TMpy-P4) intercalated at poly(dG-dC)2, which blocks the axial coordination by water molecules and consequently gives rise to a long lifetime (τ ∼ 22 ns).

Mechanism of exciplex formation between Cu-porphyrin and calf-thymus DNA as revealed by saturation resonance Raman spectroscopy

“…The calculated dependences were found to be insensitive to minor variations of the exciplex building time ( TE AT ), while the value of the exciplex lifetime ES AT is more critical for the minimization of the deviation between the experimental and the theoretical dependences. Since it was found that the time constant D 2.3 ns, reported by Jeoung et al 23 for the Cu(TMpy-P4)-poly(dA-dT) 2 exciplex, gave a better fit than the value D 3.2 ns reported by Kruglik et al, 17 the former value was used in the calculations.…”

Solvent Effects on Nonradiative Relaxation Dynamics of Low-Energy Ligand-Field Excited States: A CuCl42– Complex

“…34 To summarize, we note that associative exciplex formation mechanism was previously invoked to explain quenching of excited states of Cu(II) porphyrins in coordinating solvents (including water). 19,20 Furthermore, the deactivation of several copper(I) complexes in MLCT states, in which the copper center can be formally regarded as Cu(II), was also thought to occur via exciplex formation. 50,51 Our experiments on the d-d state of CuCl 4 2− quite possibly suggest the generality of this exciplex associative mechanism.…”

Elucidating the Ultrafast Dynamics of Photoinduced Charge Separation in Metalloporphyrin-Fullerene Dyads Across the Electromagnetic Spectrum

“…In nitrogen-containing Lewis base solvents, such as bzn, it is instead quenched within about a hundred of picoseconds. [30][31][32][33][34] Figure5b shows the kinetics at 465 nm, which can be fitted with a single-exponential decay lifetime of 396 ± 5 ps. The trends in lifetime reduction have been convincingly explained by solvent-dependant energetics of the CT states.…”

“…For Cu II P, such CT states have usually eluded unambiguous assignment through optical and Raman spectroscopies. [27][28][29][30][31][32][33][34][35][36][37] Being element-specific and applicable to any state of matter, time-resolved X-ray absorption spectroscopy (TR-XAS) is a rapidly maturing technique capable of delivering unique information about the electronic and geometric structures of metastable photoexcited states with picosecond and femtosecond temporal resolution. [38][39][40][41][42][43][44][45][46] Recently, it has been applied as a complementary analytical tool to investigate the decay pathways in laser-excited MPs and the early steps of CO and NO ligation/deligation in hemelike porphyrins relevant for respiration.…”

“…This can be traced back to the fact that the photoinduced dynamics are complicated by the participation of (d,π), (π,d), or (d,d) charge-transfer (CT) states that have been proposed at the origin of the prompt deactivation undergone by the photoactive state. For Cu II P, such CT states have usually eluded unambiguous assignment through optical and Raman spectroscopies. − Being element-specific and applicable to any state of matter, time-resolved X-ray absorption spectroscopy (TR-XAS) is a rapidly maturing technique capable of delivering unique information about the electronic and geometric structures of metastable photoexcited states with picosecond and femtosecond temporal resolution. − Recently, it has been applied as a complementary analytical tool to investigate the decay pathways in laser-excited MPs and the early steps of CO and NO ligation/deligation in heme-like porphyrins relevant for respiration. , In the present work, TR-XAS is employed in combination with ultrafast time-resolved optical absorption spectroscopy (TR-OAS) in the near-infrared and the visible regions in order to clarify the ultrafast dynamics within a prototypical Cu II P following light absorption and to elucidate several unexpected aspects of the photoinduced intramolecular ET in a related Cu II P–C 60 dyad.…”