Single-photon excited fluorescence chromophores

Two-photon excited fluorescence detection at the single-molecule level has been demonstrated for chromophores in cryogenic solids [M], room-temperature surfaces [61], membranes [62] and liquids [63, M and 65]- Although multiphoton excited fluorescence has been embraced with great enthusiasm as a technique for both ordinary confocal microscopy and single-molecule detection, it is not a panacea in particular, photochemical degradation in multiphoton excitation may be more severe than with ordinary linear excitation, probably due to absorption of more than the desired number of photons from the intense laser pulse (e.g. triplet excited state absorption) [61],... 

The vast majority of single-molecule optical experiments employ one-photon excited spontaneous fluorescence as the spectroscopic observable because of its relative simplicity and inlierently high sensitivity. Many molecules fluoresce with quantum yields near unity, and spontaneous fluorescence lifetimes for chromophores with large oscillator strengths are a few nanoseconds, implying that with a sufficiently intense excitation source a single... 

While the above-described instrumentation is an excellent choice for long-lived excited states such as the Ru " and Os polypyridyl complexes, organic-based chromophores and fluorescence labels frequently used by supramolecular chemists require higher time resolution. Commercially available time-correlated single photon counting (TCSPC) instruments can readily access... 

As shown in Figure 3, when the chromophores comprise an asymmetric D/A pair of mixed components in the molecule (102 and 104), their absorption maximum, >.max (of single photon absorption) is slightly red-shifted (ca. 5 nm) relative to that of their symmetric D/D pair counterpart (101 and 103). This is associated with a partial charge-transfer (CT) in the excited state of the asymmetric molecule. These oligomers are highly fluorescent (Of of 101 0.47,... 

In this method the fluorescence decay of the donor after pulse excitation is measured. Therefore the lifetime of the photon emitted upon fluorescence can be measured. This lifetime is the intrinsic property of the donor chromophore and its value is independent of variations in concentration. This fact is a major advantage of time-resolved methods over steady-state methods. If the decay is a single exponential, the measurement of the decay time in the presence and absence of the acceptor is straightforward. That means that, in circumstances where the decay is single exponential and the donor lifetime changes in the presence of the acceptor, then transfer of energy has taken place. The efficiency of this energy transfer can be calculated by ... 

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