29.06.2013 Views

View/Open - ARAN - National University of Ireland, Galway

View/Open - ARAN - National University of Ireland, Galway

View/Open - ARAN - National University of Ireland, Galway

SHOW MORE
SHOW LESS

You also want an ePaper? Increase the reach of your titles

YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.

Introduction<br />

where υ is the rate <strong>of</strong> each <strong>of</strong> the four processes listed, S is the ground state, S ∗<br />

is the excited singlet state, T ∗ the excited triplet state, and hνex and hνex are the<br />

energies <strong>of</strong> the incident and fluorescent photons respectively. If the absorbance <strong>of</strong><br />

the sample is low and the incident light intensity is relatively high, the steady-state<br />

approximation for the change in concentration <strong>of</strong> S ∗ can be given as:<br />

d[S ∗ ]<br />

dt = Iabs − kf[S ∗ ] − kic[S ∗ ] − kisc[S ∗ ] = Iabs − (kf + kic + kisc)[S ∗ ] = 0<br />

it follows that:<br />

Iabs = (kf + kic + kisc)[S ∗ ]<br />

Bringing in the quantum yield term from Equation 1.2 yields:<br />

φ =<br />

kf<br />

kf + kic + kisc<br />

(1.3)<br />

In steady state experiments the sample is illuminated with a constant beam <strong>of</strong><br />

light. Because <strong>of</strong> the short (typically nanosecond) timescale <strong>of</strong> fluorescence, a steady<br />

state is reached almost immediately. If the fluorophore is exposed to a pulse <strong>of</strong> light<br />

that generates the excited state, the intensity decays exponentially according to:<br />

[S ∗ ]t = [S ∗ ]0e −t/τ<br />

where the average time that the excited state exists is given by the fluorescence<br />

lifetime, τ.<br />

The observed fluorescence lifetime(τ) is comprised <strong>of</strong> the non-radiative (kic+kisc)<br />

and the radiative (kf) parts and is given by:<br />

τ =<br />

1<br />

kf + kic + kisc<br />

18<br />

= φ<br />

kf<br />

(1.4)

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!