Anomalous Fluorescence Anisotropy

In this section, we discuss a manifestation of electrOTiic coherence in fluorescence spectroscopy fluorescence anisotropy exceeding the classical maximum of 0.4. Such anisotropy has been seen at short times after excitation with femtosecond laser pulses that cover a sufficiently broad band of wavelengths to excite multiple optical transitions coherently. 

To examine the effect of coherence on fluorescence anisotropy, consider a system with a ground state (state 1) and two electronically excited states (2 and 3), and assume as before that the interaction matrix elements 7/i2, T/is and H23 are zero in the absence of radiation. Suppose that an ensemble of systems in the ground state is excited with a weak pulse of light that is much shorter than the lifetime of the excited state (Ti). In the impulsive limit, the populatimi of excited state 2 generated by the pulse is 

The initial value of created by the excitation flash can be estimated similarly  

Here the square root can be taken arbitrarily to be positive. Equations (10.76a and 10.76b), which are a generalization of an expression suggested by Rahman et al. [42], arbitrarily assign (0) a purely real value. In general, (0) is complex and depends on the convolution of Eq. (BIO. 1.8) with the electric field in the excitation pulse, E v, t). The main point is that a short excitation pulse can create coherence between states 2 and 3 if it overlaps the absorption bands for excitation to both states. Such overlap is a common feature of measurements made with femtosecond laser pulses, which inherently have large spectral widths. 

Following the excitation pulse, p t) will execute damped oscillations with a period of How does the transient coherence between states 2 and 3 affect the 

---

Shi X, Basran J, Seward FIE, Childs W, Bagshaw CR, Boxer SG (2007) Anomalous negative fluorescence anisotropy in yellow fluorescent protein (YFP 10C) quantitative analysis of FRET in YFP dimers. Biochemistry 46 14403-14417... 

A series of aminostilbenes (lA-C, 2A-C) have been synthesized to test the effect of substitution of the amino group upon the photophysics and photochemistry of stilbenes [45]. This study indicated that the photophysics properties of trans-2-aminostilbene, lA, and trans-3-aminostilbene, IB, were similar. as-2-Aminostilbene, 2A, and ds-3-aminostilbene, 2B, showed similar fluorescent lifetimes. Anomalous behavior of emission anisotropy for short-living derivatives of stilbenes was described [46]. The absorption, fluorescence and polarization excitation, and emission spectra... 

---