Triplet ground state fluorescence

Raman spectra of S2 in its triplet ground state have been recorded both in sulfur vapor and after matrix isolation using various noble gases. The stretching mode was observed at 715 cm in the gas phase [46], and at 716 cm in an argon matrix [71]. From UV absorption and fluorescence spectra of sulfur vapor the harmonic fundamental mode of the S2 ground state was derived as t e = 726 cm . The value corrected for anharmonicity is 720 cm [26, 27]. Earlier reports on the infrared absorption spectrum of 2 in matrix isolated sulfur vapor [72] are in error the observed bands at 660, 668 and 680 cm are due to S4 [17] and other species [73]. 

In addition to heat emission, radiative decay processes may also occur, in which light is emitted due to a transition from the lowest excited singlet or triplet state to the ground state (fluorescence or phosphorescence). In order to effect rapid and efficient conversion of optical energy (the laser) to heat, dyes which exhibit low fluorescence and in which excitation primarily involves the singlet states are the most suitable for heat-mode recording.196... 

Figure 10.2 Graphical representation of Metal-Enhanced Fluorescence (Top), Metal-Enhanced Phosphorescence (Middle), and for the generation of Singlet Oxygen (Bottom). F - Fluorophore, RB - Rose Bengal, P - Phosphorescence and MEP - Metal-Enhanced Phosphorescence, 302 — triplet ground state oxygen. 102 -singlet oxygen. Adopted from ref [25].

Emission of a photon from an electronically excited state is referred to as luminescence. Fluorescence and phosphorescence can be differentiated depending on whether the transition is between states of equal or different multiplicity and hence spin-allowed or spin-forbidden. (Cf. Section 5.1.1.) Thus, for molecules with singlet ground states fluorescence constitutes a pathway for deactivating excited singlet states whereas phosphorescence is observed in the deactivation of triplet states. 

A change in spin orientation is needed for a triplet state to return to the singlet ground state. Fluorescence is the emission that results from the return to the lower orbital of the paired electron. Such transitions are quantum mechanically allowed .. . . Phosphorescence is the emission that results from transition between states of different multiplicity, generally a triplet state returns to a singlet ground state. 

Once the excited molecule reaches the S state it can decay by emitting fluorescence or it can undergo a fiirtlier radiationless transition to a triplet state. A radiationless transition between states of different multiplicity is called intersystem crossing. This is a spin-forbidden process. It is not as fast as internal conversion and often has a rate comparable to the radiative rate, so some S molecules fluoresce and otliers produce triplet states. There may also be fiirther internal conversion from to the ground state, though it is not easy to detemiine the extent to which that occurs. Photochemical reactions or energy transfer may also occur from S. ... 

Similar to the fullerene ground state the singlet and triplet excited state properties of the carbon network are best discussed with respect to the tliree-dimensional symmetry. SurjDrisingly, the singlet excited state gives rise to a low emission fluorescence quantum yield of 1.0 x 10 [143]. Despite the highly constrained carbon network,... 

Fluorescence and phosphorescence are both forms of luminescence [3]. If the emission of radiation has decayed within 10 s after the exciting radiation is cut off it is known as fluorescence [4], if the decay phase lasts longer (because the electrons return to the ground state from a forbidden triplet state (Fig. 5), then the phenomenon is known as phosphorescence. A distinction is also made between... 

Fig. 5 Schematic representation of the electronic transitions during luminescence phenomena [5]. — A absorbed energy, F fluorescence emission, P phosphorescence, S ground state. S excited singlet state, T forbidden triplet transition.

Emission of light due to an allowed electronic transition between excited and ground states having the same spin multiplicity, usually singlet. Lifetimes for such transitions are typically around 10 s. Originally it was believed that the onset of fluorescence was instantaneous (within 10 to lO-" s) with the onset of radiation but the discovery of delayed fluorescence (16), which arises from thermal excitation from the lowest triplet state to the first excited singlet state and has a lifetime comparable to that for phosphorescence, makes this an invalid criterion. Specialized terms such as photoluminescence, cathodoluminescence, anodoluminescence, radioluminescence, and Xray fluorescence sometimes are used to indicate the type of exciting radiation. 

Fig. 14 Schematic representation of the electronic transitions of photochemically excited substances Sq = ground state, Sj = first excited singlet state, T = forbidden triplet transition, N = ground state of a newly formed compound, A = absorption, F = fluorescence, P = phosphorescence.

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