20
J. Phys. Chem. A 2006, 110, 20-27
Correlations of Structure and Rates of Energy Transfer for Through-Bond Energy-Transfer Cassettes T. G. Kim,† J. C. Castro,‡ A. Loudet,‡ J. G.-S. Jiao,‡ R. M. Hochstrasser,† K. Burgess,‡ and M. R. Topp*,† Department of Chemistry, UniVersity of PennsylVania, Philadelphia, Pennsylania 19104, and Department of Chemistry, Texas A & M UniVersity, P. O. Box 30012, College Station, Texas 77842 ReceiVed: June 22, 2005; In Final Form: October 31, 2005
Fluorescent DNA-labeling cassettes are designed to have a common absorbing chromophore matched to a single exciting laser wavelength, but up to four different emitters. Experiments reported here have examined the energy-transfer rates and fluorescence polarization characteristics for two different types of cassette, involving three distinct relative orientations of the donor and acceptor transition moments and the axis of the rigid linker. Energy-transfer times range from 1 r S1 transient absorption of an anthracene donor species in compounds 1-9, followed by Sn>1 f S0 fluorescence of the anthracene moiety. The present work reports measurements of most of these samples via fluorescence gating methods (see Table 1), which allow verification of the earlier dynamical results and new comparisons of donor and acceptor fluorescence anisotropy. The apparatus incorporated an amplified Ti:sapphire laser delivering femtosecond-domain pulses near 810 nm at a repetition rate near 1 kHz at a power of >200 mW. Most commonly, second harmonic pulses near 405 nm were used for excitation, and detection was carried out near the maximum emission wavelength of both the donor and acceptor species. For anthracene-based cassettes, 405-nm excitation was sufficient to measure both the population relaxation lifetime and the transient anisotropy. This same wavelength was also sufficient to measure the dynamics of donor relaxation in the fluorescein-rosamine cases (compounds 10-18), a quantity not found to vary significantly with internal energy. However, anisotropy measurements of compounds 10-18 required excitation near the fluorescein absorption maximum at 490 nm to select an optimally polarized absorption transition. Those experiments used a custom-built noncollinear optical parametric amplifier (NOPA),17 which was tuned in our experiments over the range 450-530 nm. Ultrafast fluorescence gating is a now widely established experimental technique and needs little elaboration.18 Our apparatus (See Figure 7) employed linearly polarized optical pulses, either from the Ti:sapphire second harmonic (near 405 nm) or from the NOPA (450-530 nm), to irradiate the sample contained in a fused silica flow cell of optical path length of ∼1 mm. A matched pair of off-axis parabolic mirrors collected the emitted fluorescence and refocused it into a 0.5-mm path
compound no.
acceptor emission maximum (nm)
energy-transfer time (ps)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
515 520 545 660 690 615 650 570 590 535 580 605 620 580 575 600 600 590
0.4 0.49 0.55 0.33 0.47
