Ultrafast Photoinduced Charge Separation in Naphthalene Diimide Based Multichromophoric Systems in Liquid Solutions and in a Lipid Membrane

Abstract

The photophysical properties of multichromophoric systems consisting of eight red or blue naphthalene diimides (NDIs) covalently attached to a p-octiphenyl scaffold, as well as a blue bichromophoric system with a biphenyl scaffold, have been investigated in detail using femtosecond time-resolved spectroscopy. The blue octachromophoric systems have been recently shown to self-assemble as supramolecular tetramers in lipid bilayer membranes and to enable generation of a transmembrane proton gradient upon photoexcitation (Bhosale, S.; Sisson, A. L.; Talukdar, P.; Fürstenberg, A.; Banerji, N.; Vauthey, E.; Bollot, G.; Mareda, J.; Röger, C.; Würthner, F.; Sakai, N.; Matile, S. Science 2006, 313, 84). A strong reduction of the fluorescence quantum yield was observed when going from the single NDI units to the multichromophoric systems in methanol, the effect being even stronger in a vesicular lipid membrane. Fluorescence up-conversion measurements reveal ultrafast self-quenching in the multichromophoric systems, whereas the formation of the NDI radical anion, evidenced by transient absorption measurements, points to the occurrence of photoinduced charge separation. The location of the positive charge could not be established unambiguously from the transient absorption measurements, but energetic considerations indicate that charge separation should occur between two NDI units in the blue systems, whereas both an NDI unit and the p-octiphenyl scaffold could act as electron donor in the red system. The lifetime of the charge-separated state was found to increase from 22 to 45 ps by going from the bi-to the octachromophoric blue systems in methanol, while a 400 ps decay component was observed in the lipid membrane. This lifetime lengthening is explained in terms of charge migration that is most efficient when the octachromophoric systems are assembled as supramolecular tetramers in the lipid membrane. Furthermore, the average charge-separated state lifetime of the red system in methanol is even larger and amounts to 750 ps. This effect cannot be simply explained in terms of Marcus inverted regime as the driving force for charge recombination in the red system is only slightly larger than in the blue one. A better spatial separation of the charges in the red system stemming from the localization of the hole on the p-octiphenyl scaffold could additionally contribute to the slowing down of charge recombination.

Excited-State Dynamics of Hybrid Multichromophoric Systems: Toward an Excitation Wavelength Control of the Charge Separation Pathways

“…The spectrum above 450 nm is very similar to that of R1 •-generated upon photoinduced CS between R1 in the lowest singlet excited state and an electron donor. 41 On the other hand, the negative TA band below 450 nm coincides very well with the S 0 f OPE-LES absorption band and can be assigned to the bleach of this transition. These TA spectra present very small changes with time, except for the intensity that decays almost entirely to zero after about 1.5 ns.…”

“…This positive amplitude is also probably associated with the NDI-LES, which has previously been shown to absorb between 400 and 500 nm. 41 The spectra associated with the longest time constants, τ 3 and τ 4 , are very similar and are essentially equal to the TA spectrum of the OPE/NDI-CSS. Therefore, these two time constants can be attributed to the decay of the OPE/NDI-CSS population back to the ground state upon CR (pathway 9 in Figure 5).…”

“…14,16 An original feature of these multichromophoric molecules is that they combine the functions of the antenna and of the reaction center. 41 Indeed, the large number of NDIs, eight in the individual molecule and 32 in the self-assembled transmembrane complex, ensures a high absorption cross section. Contrary to the natural photosynthetic apparatus, excitation is followed not by energy hopping toward the reaction center but rather by an ultrafast symmetry-breaking CS between two NDIs.…”

“…With such red NDI units, a CR component of about 1 ns was measured with the octachromophoric system (R8). 41 This slowing down cannot entirely be accounted for by the increase of driving force for CR of about 0.3 eV, associated with the different redox properties of blue and red NDIs. Instead, the occurrence of photoinduced CS between a red NDI and the POP scaffold, which is only energetically feasible with R8, could explain a longer-lived CSS as this process would lead to a better spatial confinement of the charges.…”

Ultrafast Excited-State Dynamics of Kynurenine, a UV Filter of the Human Eye

“…[17][18][19][20][21] The deactivation via hydrogen-bond-assisted proton transfer has been reported for methyl salicylate, 22 anthraquinones, 23,24 and some other aromatic molecules. [25][26][27][28][29][30] A fast deactivation of the photoexcited state was also found with o-aminoacetophenone (AAP) and its derivatives, whose chemical structures are similar to that of KN. 31,32 It has been shown that internal conversion (IC) is the main decay channel of the S 1 state, and that its rate constant is highly sensitive to the solvent: in polar aprotic solvents, the fluorescence lifetime of AAP exceeds 1 ns, while in polar protic and nonpolar solvents it decreases by one and two orders of magnitude.…”