The present work deals with singlet excitation energy transfer (EET) occurring in contact ion pairs (CIPs) of several anionic oxonol analogues (acting as EE donors) and cationic cyanines (acting as acceptors) characterized by off resonance individual transitions. Combining conductometric and spectroscopic measurements with decreasing solvent polarity, we were able to observe a progressive ion pairing leading first to solventseparated ion pairs (SSIPs) and then to CIPs. Analysis of the absorption spectra of three selected salts (A2, C1, A2, C2, and A1, C4) in chloroformtoluene mixtures showed that the transformation of SSIP into CIP involves the appearance of a certain exciton coupling, the extent of which decreases regularly with increasing gap between the local excitation energies. Fluorescence excitation spectra showed that EET occurs in CIP, and EET efficiencies were evaluated with a procedure expressly devised for weakly emitting donors. These were between 0.2 and 0.65 for the examined ion pairs involving anions A1 and A2. The spectroscopic study was complemented by a theoretical investigation aimed at establishing the dynamic regime of the observed EET. From classical MD simulations and local full geometry optimizations, A2, C1 and A2, C2 were found to form rather stable sandwichtype CIP structures with interchromophore distances (R) of about 0.450.50 nm. The donoracceptor electronic coupling was calculated in terms of Coulombic interactions between atomic transition charges. For CIP, the electronic coupling was decidedly beyond the limit of the weak coupling required for an incoherent Forstertype mechanism. Thus, we tried to arrange the EET dynamics within the theory developed by Kimura, Kakitani, and Yamato (J. Phys. Chem. B 2000, 104, 9276) for the intermediate coupling case, which provides analytical expressions of time dependent occupation probability, EET rate, and coherency in terms of two basic quantities: the electronic coupling and a correlation time related to the FranckCondon factor. The latter was shown to be primarily modulated by Forster's spectral overlap integral (related in turn to the excitation energy gap). Calculations were carried out for the three sample systems using three values of the electronic coupling roughly corresponding to CIP, 1.0, and 2.0 nm interchromophore distances. At the CIP distance, EET in both A2, C1 and A2, C2 was predicted to occur with a partial exciton mechanism, very short transfer times (about 10 fs), and high degree of coherence. In A1, C4 (having the largest energy gap), EET was found to occur with a hottransfer mechanism. More or less hottransfer dynamics appeared to be retained by all three systems at R=1.0 nm. Fully incoherent EET appeared to become operative only at distances larger than 2.0 nm.
Excitation energy transfer in ion pairs of polymethine cyanine dyes: Efficiency and dynamics / PONTERINI, Glauco; M., Fiorini; VANOSSI, Davide; A. S., Tatikolov; MOMICCHIOLI, Fabio.  In: JOURNAL OF PHYSICAL CHEMISTRY. A, MOLECULES, SPECTROSCOPY, KINETICS, ENVIRONMENT, & GENERAL THEORY.  ISSN 10895639.  STAMPA.  110:24(2006), pp. 75277538. [10.1021/jp0600211]
Excitation energy transfer in ion pairs of polymethine cyanine dyes: Efficiency and dynamics
PONTERINI, Glauco;VANOSSI, Davide;MOMICCHIOLI, Fabio
20060101
Abstract
The present work deals with singlet excitation energy transfer (EET) occurring in contact ion pairs (CIPs) of several anionic oxonol analogues (acting as EE donors) and cationic cyanines (acting as acceptors) characterized by off resonance individual transitions. Combining conductometric and spectroscopic measurements with decreasing solvent polarity, we were able to observe a progressive ion pairing leading first to solventseparated ion pairs (SSIPs) and then to CIPs. Analysis of the absorption spectra of three selected salts (A2, C1, A2, C2, and A1, C4) in chloroformtoluene mixtures showed that the transformation of SSIP into CIP involves the appearance of a certain exciton coupling, the extent of which decreases regularly with increasing gap between the local excitation energies. Fluorescence excitation spectra showed that EET occurs in CIP, and EET efficiencies were evaluated with a procedure expressly devised for weakly emitting donors. These were between 0.2 and 0.65 for the examined ion pairs involving anions A1 and A2. The spectroscopic study was complemented by a theoretical investigation aimed at establishing the dynamic regime of the observed EET. From classical MD simulations and local full geometry optimizations, A2, C1 and A2, C2 were found to form rather stable sandwichtype CIP structures with interchromophore distances (R) of about 0.450.50 nm. The donoracceptor electronic coupling was calculated in terms of Coulombic interactions between atomic transition charges. For CIP, the electronic coupling was decidedly beyond the limit of the weak coupling required for an incoherent Forstertype mechanism. Thus, we tried to arrange the EET dynamics within the theory developed by Kimura, Kakitani, and Yamato (J. Phys. Chem. B 2000, 104, 9276) for the intermediate coupling case, which provides analytical expressions of time dependent occupation probability, EET rate, and coherency in terms of two basic quantities: the electronic coupling and a correlation time related to the FranckCondon factor. The latter was shown to be primarily modulated by Forster's spectral overlap integral (related in turn to the excitation energy gap). Calculations were carried out for the three sample systems using three values of the electronic coupling roughly corresponding to CIP, 1.0, and 2.0 nm interchromophore distances. At the CIP distance, EET in both A2, C1 and A2, C2 was predicted to occur with a partial exciton mechanism, very short transfer times (about 10 fs), and high degree of coherence. In A1, C4 (having the largest energy gap), EET was found to occur with a hottransfer mechanism. More or less hottransfer dynamics appeared to be retained by all three systems at R=1.0 nm. Fully incoherent EET appeared to become operative only at distances larger than 2.0 nm.File  Dimensione  Formato  

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