Singlet-Triplet States

In an electronic transition, the electronic configuration changes, i.e., one or more electron(s) in the molecule are rearranged in available higher energy orbitals. Depending on the spin orientations of the excited electron in the new orbital, the states are designated as singlet (S) or triplet (T). In the following we discuss about the singlet and triplet electronic states.

Upon light absorption, electronic transitions result from the interaction between electrons and the electric field component of the light. The magnetic contribution to the absorption is negligible compared to the electric contribution because of the speed of electron rotation on itself is very weak compared to the light velocity [Total energy, F = (eE) + (evH /c) , where e, c, v, E, and H are, respectively, the electron charge, light velocity, speed of rotation of the electron on itself, and electric and magnetic components of the light wave]. Thus, a displaced electron preserves the same spin orientation during absorption excitation. Therefore, if the ground state of a molecule is a singlet state then the transition of the electron occurs to an excited singlet state. In other words, singlet to triplet transitions are forbidden, unless it is assisted by some other mechanism (such as spin-orbit couplings). Similarly, the electronic transition from T → S is formally forbidden (from the selection rules point of view). The absorption of light can excite the molecules from lowest energy singlet state S0 to higher energy singlet state S1 and even to S2 if the excitation energy is sufficient.

Fluorescence and Phosphorescence The Franck-Condon principle states that the heavier nuclei do not change their positions during the fast electronic excitation. This results in an initial geometry of the excited state which is usually not the energy minimum geometry. According to the Franck-Condon principle, the molecule will most probably be excited to one of the higher vibrational states of the excited electronic state. The molecule in an excited vibrational energy level loses energy rapidly (in 10-12 s or less) and moves to a lower (and finally to the lowest) vibrational energy level in the same excited electronic state. Once the molecule is in the lowest vibrational level of the excited electronic state, S1, the molecule can return to one of the vibrational states of the ground singlet state, S0 through an emission of photon radiation, called fluorescence, or via other relaxation path ways. In fluorescence, the acquired (absorbed) electronic energy is lost via the emission of a photon while transition occurs from lowest vibrational level of first excited singlet (i.e., v=0 of S1) to one of the vibrational states of the ground singlet state, S0. This radiative process takes about 10-9 second. Most compounds decay by non-radiative processes (such as heat) and are therefore not fluorescent.

If by some mechanisms, intersystem crossing (say, spin forbidden transition S1 → T1) occurs, then a radiative transition T1 to S0, called phosphorescence, may occur. In intersystem crossing, a molecule in the ground vibrational energy level of an excited electronic state passes into a high vibrational energy level of a lower energy electronic state with a different spin state (say, S1 to T1). Intersystem crossing requires a mechanism for converting the paired electron spins (↑↓) of the singlet state to unpaired electron spins (↑↑) of the triplet state. Spin-orbit coupling and vibronic coupling favor intersystem crossing. In spin-orbit coupling, the magnetic field arising from an electron's orbital motion around the nucleus interacts with the spin magnetic moment of the electron which helps in spin reversal or flipping of electron orientation (from ↑↓ to ↑↑). The strength of the orbitally generated magnetic field increases as the nuclear charge increases. The efficiency of this coupling varies with the fourth power of the atomic number. After intersystem crossing, the molecule as usual undergoes vibrational relaxation and moves down the vibrational energy levels of the T1 state by loss of energy in collisions with surrounding molecules. Then the molecule may transit from T1 to S0 by the emission of a photon, called phosphorescence. The forbidden transition of the molecule from T1 to S0 to occur requires a similar mechanism that permitted singlet to triplet intersystem crossing. Typical mean time for phosphorescence (1 millisecond to 10 second) is longer than that for fluorescence.