Fluorophores electron energy levels

Fig. 6.1. Jablonski diagram, representing electron energy levels of fluorophores and transitions after photon excitation. S = electronic state, different lines within each state represent different vibrational levels. Blue arrows represent absorption events, green arrows depict internal conversion or heat dissipation, and orange arrows indicate fluorescence emission. Intersystem crossing into triplet states has been omitted for simplicity (see also Chaps. 1 and 12).

The fluorescence process is a three-stage process involving excitation of a fluorophore from ground state Sq to any one of the excited states Si, S2, or S (Fig. 5) by the absorption of a photon depending on the wavelength of the radiation. At each of these electronic energy levels, the fluorophores can exist in a number of vibrational... 

Figure 4.1 Simplified Jablonski diagram showing the electronic energy levels of a fluorophore, illustrating excitation (Ex), fluorescence (FI), and phosphorescence (Phos). Singlet states are labeled triplet states and virbrational energy levels Kx- Solid vertical lines illustrate radiative transitions in the direction of trhe arrows, broken lines are non-radiative transition dashed lines are inter-system crossing (ISC) and internal conversion (IC), and dotted lines are vibrational relaxations (VR).

Fluorescence is a process that occurs after excitation of a molecule with light. It involves transitions of the outermost electrons between different electronic states of the molecule, resulting in emission of a photon of lower energy than the previously absorbed photon. This is represented in the Jablonski diagram (see Fig. 6.1). As every molecule has different energy levels, the fluorescent properties vary from one fluorophore to the other. The main characteristics of a fluorescent dye are absorption and emission wavelengths, extinction... 

In a quantum dot, which is also often called an artificial atom, the excitons are confined in all three spatial dimensions. In a bulk semiconductor, an electron-hole pair is bound within the Bohr exciton radius, which is characteristic for each type of semiconductor. A quantum dot is smaller than the Bohr exciton radius, which causes the appearance of discrete energy levels. The bandgap, AE, between the valance and conduction band of the semiconductor is a function of the nanocrystaTs size and shape. Q-dots feature slightly lower luminescence quantum yields than traditional organic fluorophors but... 

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