{"id":60,"date":"2023-02-23T01:17:35","date_gmt":"2023-02-23T06:17:35","guid":{"rendered":"https:\/\/pressbooks.bccampus.ca\/instanchem1\/?post_type=chapter&p=60"},"modified":"2025-01-25T15:22:09","modified_gmt":"2025-01-25T20:22:09","slug":"molecular-fluorescence","status":"publish","type":"chapter","link":"https:\/\/pressbooks.bccampus.ca\/instanchem1\/chapter\/molecular-fluorescence\/","title":{"raw":"Molecular Fluorescence","rendered":"Molecular Fluorescence"},"content":{"raw":" \r\n

A molecule is usually found in its ground electronic state (S0<\/sub>) and its ground vibrational state (v<\/em>0<\/sub>), written in combination as S0<\/sub>v<\/em>0<\/sub>. When a molecule gains energy, it transitions to an excited electronic state (Sn<\/em><\/sub>) and often an excited vibrational state (vm<\/sub><\/em>), written in combination as Sn<\/sub>vm<\/sub><\/em>. Molecules remain in their excited state for only a short period of time, relaxing back to their ground state by various mechanisms. One of these mechanisms is luminescence<\/em>, which is the emission of a photon.<\/p>\r\n

Fluorescence<\/em> and phosphorescence<\/em> are two specific mechanisms of photoluminescence<\/em>. All of these terms refer to the emission of light from a molecule that reached its excited state through the absorption of a photon.<\/p>\r\n\r\n

Excited-State Processes<\/h2>\r\n

Consider a molecule in an excited vibrational state of its first electronic excited state, an S1<\/sub>vm\u00a0<\/sub><\/em>state.\u00a0<\/em>The excess vibrational energy is quickly transferred to the surrounding environment as heat through collisions during bond vibrations. This process is called vibrational relaxation<\/em> and returns the molecule to the ground vibrational state of the excited electronic state,\u00a0S1<\/sub>v<\/em>0<\/sub>.\u00a0From here, the molecule may follow one of the following pathways:<\/p>\r\n

Fluorescence:\u00a0<\/strong>A photon is emitted as the molecule transitions back to the ground electronic state, S0<\/sub>vm<\/sub><\/em>. When m<\/em> \u2260 0, this photon emission is followed by vibrational relaxation to S0<\/sub>v<\/em>0<\/sub>. Because fluorescence is usually preceded and followed by vibrational relaxation, most fluorescence is at a longer wavelength than absorption.<\/p>\r\n

Internal conversion:\u00a0<\/strong>The molecule transitions to S0<\/sub>vm<\/sub><\/em> without emission of a photon, where vm<\/sub><\/em> is a highly-excited vibrational state of the ground electronic state (S0<\/sub>). Rapid vibrational relaxation to S0<\/sub>v<\/em>0<\/sub> follows internal conversion and all of the original excitation energy is lost as heat. Internal conversion competes with fluorescence at S1<\/sub>v<\/em>0<\/sub> but dominates at Sn<\/sub>v<\/em>0<\/sub> for n<\/em> \u2265 2, leading to\u00a0Kasha's Rule\u00a0<\/em>that fluorescence occurs only from S1<\/sub>v0<\/sub>.<\/p>\r\n

Intersystem crossing and phosphorescence:\u00a0<\/strong>Intersystem crossing converts the excited state to a triplet state (T1<\/sub>vm<\/sub><\/em>) by flipping the spin of the excited-state electron. After rapid vibrational relaxation to T1<\/sub>v<\/em>0<\/sub>, the molecule may phosphoresce, emitting a photon with a concurrent spin flip and transition to S0<\/sub>vm<\/sub><\/em>, or may internally convert with a concurrent spin flip to reach S0<\/sub>vm<\/sub><\/em>. Because the T1<\/sub> state is lower in energy than the S1<\/sub> state, phosphorescence occurs at longer wavelengths than fluorescence.<\/p>\r\nMolecules can also be excited to S2<\/sub>v<\/em>m<\/sub> by photon absorption. In this case, vibrational relaxation occurs to S2<\/sub>v<\/em>0<\/sub>, followed by internal conversion to S1<\/sub>v<\/em>m<\/sub>, and vibrational relaxation to S1<\/sub>v<\/em>0<\/sub>.\r\n\r\n \r\n\r\n\"Simple\r\n

Characteristics of Fluorescence<\/h2>\r\n

The quantum yield<\/em> (\u03a6) of fluorescence for a molecule is the probability that fluorescence occurs in preference to internal conversion or intersystem crossing. Non-fluorescent molecules have \u03a6\u00a0=\u00a00, whereas molecules with \u03a6\u00a0=\u00a01 relax only<\/em> via fluorescence. Molecules with non-zero quantum yields (i.e.<\/em> 0 < \u03a6\u00a0\u2264\u00a01) are often called fluorophores<\/em>.<\/p>\r\n

The fluorescence emission spectrum of a molecule is generally the mirror image of its absorption spectrum. The reason is that an absorptive transition from S0<\/sub>v<\/em>0<\/sub> to S1<\/sub>v<\/em>n<\/sub> and the converse fluorescence transition from S1<\/sub>v<\/em>0<\/sub> to S0<\/sub>v<\/em>n\u00a0<\/sub>have approximately equal probability. The aforementioned wavelength difference between the peaks of the absorption and fluorescence spectra is called the Stokes shift<\/em>.<\/p>\r\n

Fluorescence emission for most molecules occurs within a period of hundreds of picoseconds to tens of nanoseconds after excitation. A characteristic timescale for fluorescence emission from a given molecule is its\u00a0fluorescence lifetime<\/em>. Phosphorescence emission tends to occur within a period of microseconds to minutes. This difference in timescale arises from the low probability of intersystem crossing occurring in parallel with photon emission. The low probability also means that phosphorescence is often only observed at low temperatures or in solid matrices, whereas fluorescence is commonly observed at room temperature (and above) and in solution.<\/span><\/p>\r\n \r\n\r\n

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Connections<\/h3>\r\n