CHEM 524

CHEM 524 -- Course Outline (Part 15)

XI. Molecular Luminescence Spectra (Chapter 15)

A. Two kinds of measurements

1. Luminescence -- spectra as function of l_lum, excited at fixed l_ex±Dl

2. Excitation -- spectra as function of l_ex, detect at fixed l_lum ±Dl

[equivalent to absorption under conditions, const. Quantum yield]

B. Instrumentation -- typically can control both ,

1. PE Model 650 -- source Xe, monochromator -- fast, PMT detect

minimal optics, shutters (for E_dark), slits (resolution, excit., lumin. Dl)

Stabilization (source instability) - eg arc wander, fluct. at slit - normalize

Options: thermal (broad, slow) or quantum detector (spectral, fast)

Horizontal slit, allows larger excited volume imaged onto lum. Mono.

2. Alternate models -- SLM - modular design, double mono excite (low scatt)

stabilize with dye fluorescence (after l_ex±Dl)

C. Measurements

1. Excitation -- increase sensitivity over absorption or

selectivity for specific component (that has luminescence) or

part of molecule (eg. Trp in proteins)

2. Fluorescence

-- normally see since fast relax vibrations to lowest excited state,

--if excite S₁ in absorption, then A and F overlap, hot bands - more overlap

--if excite S_n (n>1) then have a gap, DE > (E_Sn - E_S1)

3. Phosphorescence -- -- intersystem cross to triplet (DS 0)slow timing can discriminate process., temperature dependent (compete with radiationless decay or vibrational relax to ground state from S₁)

sensitive to heavy atoms (promote ISC - spin-orbit couple)

4. Time dependence -- lifetime: f = f^o exp(-t/t) where t -lifetime, (t)_1/2 = 0.69 t

phosphorescence -- measure decay directly f(t), t_P = (k_P + k_nr')^-1~10^-4-10⁴ sec

fluorescence -- deconvolve exciting pulse, E_F'(t) = Ú E_ex(t')E_F(t-t')dt'

t_P = (k_F + k_nr)^-1~10^-5-10^-8 sec

Equipment -- laser excite/fast PMT and boxcar amp/transient digitize

Phase resolved method -- modulate source

-- pulse correlation scheme

Quenching--external conversion--loss of excitation to another species

k_ec = k_q[Q] Stern-Volmer relation: f_FQ^o ^-1 = (f_F^o)^-1 + Kq[Q]/ (f_F^o)

K_Q = k_q/ (k_F + k_IC + k_ISC) -- use dipole acceptor for S₁ and triplet for T₁

5. Polarization anisotropy polarization ratio:

6. Linearity E_L ~ c for A < 0.005

7. Saturation -- approach equilibrium with excited state

8. Detection Limit -- 0.1-10 pM -- strength of fluorescence