Photochemistry

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In order to absorb light in the region from 200 to 800 nm, the molecule must contain either pi bonds or atoms with non-bonding orbitals. A non-bonding orbital is a lone pair on, say, oxygen, nitrogen, or a halogen.

Fluorescence rarely results from absorption of UV of wavelength shorter than 250 nm because radiation at this wavelength has sufficient energy to deactivate the electron in the excited state by predissociation or dissociation.

Some principles of fluorescence and phosphorescence:

Phosphorescence quenching: oxygen can quench phosphorescence by interacting with the triplet state of the phosphorescent molecule, leading to non-radiative decay.
Thermal quenching: Higher temperatures can increase molecular vibrations and collisions with oxygen, which can quench fluorescence and phosphorescence by non-radiative decay. Therefore, phosphorescence is stronger at colder temperatures. Note that nitrogen is a much weaker quencher.
Non-radiative decay pathways are processes in which an excited molecule returns to its ground state without emitting a photon.
Quenching occurs when an excited molecule transfers its energy to another molecule (quencher) through collisional interactions, resulting in non-radiative decay. An example is oxygen molecules quenching the triplet state of a phosphorescent molecule, leading to energy dissipation as heat. Oxygen quenching affects both the fluorescence or phosphorescence intensity and lifetime.

The quantum yield of fluorescence or phosphorescence is the ratio of the number of photons emitted to the number of photons absorbed. Non-radiative decay pathways reduce the quantum yield because they provide alternative ways for the excited state to lose energy without emitting light.

The quenching process is often described by the Stern-Volmer equation: I₀/I = 1 + K_SV[Q] where I₀ is the fluorescence intensity in the absence of oxygen, I is the fluorescence intensity in the presence of oxygen, K_SV is the Stern-Volmer quenching constant, and [Q] is the concentration of the quencher (oxygen). K_SV depends on the specific fluorophore and its environment, including factors such as solvent, temperature, and the presence of other solutes.

Fluorescence but not phosphorescence:

-Vitamin B12, fluoresces yellow.
-Cumulenes, when crystallized, fluoresces orange.
-Circulenes, fluoresces blue to bluish-green.
-Matlaline, the fluorescent compound in the wood of the Eysenhardtia polystachya tree.
-Hyloin-L1, fluoresces blue, this was found in the polka-dot tree frog (Hypsiboas punctatus) in South America, was unintentionally discovered to be the 1st fluorescent amphibian in 2017.
-Perovskites.
-Gold nanoparticles (weakly), at wavelengths where there is plasmon excitation (520 nm).
-Ag₂ and Ag₃ clusters.
-Gold nanoclusters: Au₂₁(adamantanethiolate)₁₅ and Au₃₈S₂(adamantanethiolate)₂₀ (900 nm).

Pumpkin fluorescence: fluorescent extract from pumpkin seeds, crush them with nail varnish remover or ethanol gives an extract that glows red-orange in UV light, due to the chlorophyll precursor protochlorophyllide.

Organometallic:

The chelated lanthanide fluorophores are excited in the near UV and emit at long visible wavelengths. Their excited state lifetimes are long for fluorophores, up to ~1 millisecond, but not as long as the lifetimes of phosphorescent compounds such as certain mineral salts. The long lifetime allows time gating between excitation and detection of fluorescence emission, which greatly reduces background fluorescence from organic fluorophores, which have nanosecond lifetimes.

Gems:

-Diamonds: about 30% of diamonds exhibit some degree of fluorescence, with emitting blue light the most common, followed by yellow and green, but rarely red or purple. This is due to the variations in the atomic structure, such as the amount of nitrogen atoms present.
-Barite: it has been known for a long time that some specimens are fluorescent under UV exposure and emit white, yellow, green, or orange light.
-Calcite: can emit various colors, including orange, pink, blue, and green, depending on the impurities present in the crystal structure.
-Fluorite (CaF₂): named after fluorescence in 1852, fluorite is a well-known fluorescent gemstone. Most common emitting blue, but can also emit purple or green.
-Willemite: a zinc silicate mineral that fluoresces bright green under UV light (mineral discovered 1829 in Belgium).
-Sodalite (Na₈(Al₆Si₆O₂₄)Cl₂): most sodalite gemstones fluoresce orange under UV light. 1st discovered in 1811 in greenland, was not used as an ornamental stone until 1891 when discovered again in Ontario, Canada.

Note: a variety of fluorite, called chlorophane (known as cobra stone), can do fluorescence, triboluminescence, thermophosphorescence, and triboluminescence. It is reddish-purple in color, but can emit green when heated, or under UV light.

Other:

-Aflatoxins are famous for their intense fluorescense. Raw peanuts are especially vulnerable in supporting growth of yellow feared mold Aspergillus flavus, producing these toxins. Many natural oils from vegetable sources show intense fluorescence under UV, such as olive oil glowing brilliant red under UV.

Phosphorescence but not fluorescence:

Phosphoresce is rare compared to fluorescence and especially so in liquid solutions at room temperature and in the presence of oxygen.

-Benzophenone (has almost 100% intersystem crossing, forming triplets), emitting blue.
-Erythrosine.
-Acetone, for UV excitation.
-Diacetyl and possibly also fluorescence, depending on the conditions: solution or gas phase and oxygen, and quenching or not.
-Large, rigid compounds.
-Some common drugs that have phosphorescence properties include Aspirin, benzoic acid, morphine, and dopamine.

Inorganic.

Europium-doped strontium silicate-aluminate oxide, emitting blue.

Gems:

-The Hope diamond is blue and famously phosphoresces a deep red color.
-Hackmanite: this variety of sodalite, after being exposed to UV light, can glow a vibrant pink or orange. Hackmanite exhibits tenebrescence, a rare form of reversible photochromism where the mineral changes color when exposed to UV light and slowly returns to its original color (white or gray) in darkness.
-Wollastonite (CaSiO₃): certain types can phosphoresce a blue or violet glow after exposure to UV light, and sometimes yellow-orange.

Phosphorescence does not tend to be longer than 10 seconds (half-life), but there are longers ones in the world of minerals (geology).

Both fluorescence and phosphorescence.

Most compounds that phosphoresce, do both.

-Quinoidal thioamide.
-Opal: Opals are famous for their play-of-color, but some opals also fluoresce and phosphoresce, adding to their beauty.
-Scheelite (CaWO₄): exhibits both fluorescence and phosphorescence, appearing blue (or green with molybdenum impurities) under UV light.

Summary explanation for fluorescence and phosphorescence:

An excited state has initially 5 possible fates:

(1) internal conversion to a state of the same spin,
(2) intersystem crossing to a state of different spin (e.g. singlet to triplet),
(3) emission to a state of the same or different spin multiplicity, i.e. fluorescence, phosphorescence,
(4) chemical reaction such as bond breaking, isomerization, electron or proton transfer, or other photochemistry. Not all molecules exhibit chemical reaction (there may not be enough energy in an excited state for this to happen).
(5) quenching by other species in solution. This could be energy or electron transfer or 'heavy atom' effect which is the same as quenching by paramagnetic species such as dissolved oxygen. Clearly this pathway cannot occur with isolated molecules in the gas phase.

The probability of any 1 process occurring depends on the nature of the excited state and its closeness in energy to other states (and the nature of any quencher) but any process always occur in competition with all the others, it's just a matter of the yield of each process.

The longest range of absorption from emission:

Longer-range Stokes shifts (difference between excitation and emission peak wavelengths) of 100-150 nm occur for some organic compounds with UV excitation. For organic fluorophores with longer wavelength excitation, smaller Stokes shifts are usual. However, it is not necessary to use the peak excitation and emission wavelengths. At the cost of reduced fluorescence intensity, excitation at a shorter wavelength than the peak and emission measurement at a longer wavelength than the peak can be used in combination with a suitable optical filter to prevent crosstalk. Furthermore, the lanthanide chelates can provide very large Stokes shifts.

Chemiluminescence:

Chemiluminescence is light emitted from a chemical reaction, so it is not from absorbing light, so it is a separate category.

The 1st chemiluminescence discovered was 2,4,5-triphenylimidazole (lophine), in Germany, in 1877, to emit light when mixed with potassium hydroxide in aqueous ethanol in the presence of air.

In 1888, Eilhard Wiedemann was the 1st to classify different classes of phosphors according to the type of excitation, and is credited for introducing the terms luminescence, photoluminescence, electroluminescence, thermoluminescence, crystalloluminescence, triboluminescence and chemiluminescence

Chemiluminescent compounds:

-Luminol: blue, when mixed with an appropriate oxidizing agent. Discovered by Albrecht von Herzeele in 1902, its chemiluminescence was discovered in 1928.
-Lucigenin: bluish-green fluorescence, usually lasting 15 minutes. (lO,lO'-dimethyl-9,9'-biacridinium dinitrate), was examined in the 1930s. The confusion is the light is derived from energy transfer to fluorescent byproducts.
-Fluorescein: yellow-green (or sodium fluorescein).
-Rhodamine: red.

Examples: luminol (100 mg, a large spatulaful) is dissolved in NaOH (0.1 M, 200 mL), and the solution is added to 200 mL of chlorine bleach (about 5% sodium hypochlorite). Produces blue light.

In biological systems.

Aequorin is a protein that emits light when it binds with calcium ions. It was discovered in a species of bioluminescent jellyfish (Aequorea victoria) in the 1960s.

Persistent luminescence:

Persistent luminescence is the basis for glow-in-the-dark toys, and is commonly mistaken for phosphorescence. Classic examples include:

-Zinc sulfide (ZnS, discovered 1866).
-Strontium Aluminate (SrAl₂O₄).

Thermochromism.

Thermochromism is where a chemical changes color when the temperature changes. Probably the most classic example is vanadium(IV) oxide, VO₂.

VO₂: at room temperature, is brownish-yellow. Though if doped with tungsten, is yellow-green.
Above 67 C, is a darker gray to dark blue.

PbI₂ (toxic): at room temperature, is yellow.
At hotter temperatures, turns orange, then red at 200 C. Due to changes in energy levels in the solid state.

HgI₂: at room temperature, is red.
Above 127 C, is yellow (until 259 C). Due to how crystal lattice affects band gap. Reversible.

InSeI: at room temperature, is yellow.
Above 200 C, is orange.
At 77 K (-196 C), is pale yellow.

PbO (allotrope) can be red or yellow. The red form is tetragonal (called litharge), while the yellow form is orthorhombic (called massicot). However, they can be changed back and forth, from controlling temperature, at 486 C.

HgO is orange, and turns red when heated to 350 C. Both forms have the same structure of near-linear units of zigzag chains of 108 degrees (angle). The difference in form is due to molecule sizes. Note that because the 2 have the same geometry, this is not quite an example of allotropes.

S₄N₄: orange at room temperature.
At -30 C, is yellow, and deep red above 100 C.

Ag₂[HgI₄] (silver tetraiodomercurate): is yellow, turns orange at 50 C. Due to intermetallic charge transfer between Hg and Ag. Reversible.

Cu₂[HgI₄] (cuprous mercury iodide): scarlet, becomes maroon at 70 (or 67) C. Due to intermetallic charge transfer between Cu and Hg.

(Et₂NH₂)₂[CuCl₄] (bis(diethylammonium) tetrachlorocuprate(II)) light blue, turns yellow at 52 C, due to shift from square planar to a deformed tetrahedral.

(Et₂NH₂)₂[NiCl₄] (bis(diethylammonium) tetrachloronickelate(II)) lime (green due to humidity), turns blue at 110 C, but melting point 118 C. The yellow melts into a blue, and cooling goes back to yellow, where crystals reform. But yellow is the metastable state, the true form is red.

Most thermochromic compounds do revert back to their color if reverted back to room temperature. It is more common, when a compound is heated, to shift towards the IR direction (and to UV direction upon cooling) than the other way around. But there are compounds that can do the other way around. Colors are still more complicated, as often a material can absorb 2 or more wavelengths, and the relative intensities can vary with temperature.

However, some thermochromic compounds do not revert back to their original color, and some takes weeks to revert back. For irreversible examples, copper imidazolium nanoparticle networks is green, and turns to yellow at 180 C, in which the copper complex changes from planar to tetragonal. Diethylammonium copper tetrachloride is green, turns yellow at 52 C (square planar to tetrahedral).

Bis(dimethylammonium) tetrachloronickelate(II), [(CH₃)₂NH₂]₂NiCl₄ is raspberry red, becomes blue around 110 C, but upon cooling, becomes light yellow, taking 2 to 3 weeks to turn back to red.

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Some compounds are not thermochromic, but are dependent on geometry, is CoCl₂ in 3 M HCl solution. CoCl₂ is blue, can change color when wet (purple if dihyrate, pink if hexahydrate). When put in cold 5 M HCl, is purplish-blue (tetrahedral). Drip with water, until it turns pink (octahedral), it is now around 3 M HCl. You can heat it to turn dark blue (> 50 C), and pink back at room temperature. Like almost-all equilibrium constants, the equilibrium constant is temperature-dependent.

[Co(H₂O)₆]²⁺ + 2Cl^- <-> [Co(H₂O)₂Cl₂] + 4H₂O

The Br equivalent to this, is where anhydrous CoBr₂ is green, and adding HBr(aq) shifts to purple (with Br) or pink (without Br), as well as blue or violet is excess Br.

A formula for the origin of thermochromism is:

E_total = E_ZPR + E_LE + E_e-p, where

ZPR = zero-phonon renormalization, LE = lattice expansion, and e-p = electron-phonon coupling.

From SC-xray diffraction, the chains separate upon heating (van der Waals gaps open) while the tubule structure is retained.

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1. Can a carbon bonded to 4 benzenes, do fluorescence or phosphorescence?

For visible light, no, as the 4 benzenes would act as a nanorotor. This compound is called tetraphenylmethane. With too much rotational freedom, leads to non-radiative decay pathways. So when the molecule absorbs light, instead of that energy being re-emitted as fluorescence or phosphorescence, it is dissipated as heat. However, it does absorb UV-C, and fluoresce UV-A.

2. What about a cyclobutane, with each corner bonded to a benzene ring?

Sort of, to the extent you have to make the compounds more rigid, to avoid vibrational relaxation.

3. Can tetracene, pentacene, and hexacene do fluorescence and phosphorescence?

Only fluorescence, but they can do phosphorescence with a triplet sensitizer (explained in the quantum physics page), which can be done in the solid state. But they do not emit the same color, as extending aromaticity, extends the HOMO-LUMO gap.

From anthracene to hexacene, they fluoresce violet, green, orange, and red. For anthracene, the absorption are in the UV, and emission are in the violet. They are all powders, at white, orange, black, and blue-green. However, as with most compounds, have multiple wavelengths for absorbance. Pentacene absorbs very strongly in the UV-B region as well.

Phenanthrene, which is very similar to anthracene, fluoresces a near-UV color.

Pyrene, which is another type of 4-fused benzenes, fluoresces a violet to blue color when exposed to UV light, however, it can also exhibit excimer fluorescence. When pyrene molecules are close together, it can form an excimer (an excited dimer), which emits blue-green fluorescence. Pyrene was the 1st compound discovered for the excimer behavior, in 1954.

Rubrene, which is another derivative of tetracene, fluoresces different colors depending on circumstances. It is itself an orange-red powder. But it can also be in different crystal polymorphs: monoclinic, triclinic, and orthorhombic. It fluoresces green in benzene solution, yellow as amorphous form, and orange (600 nm) as orthorhombic crystal form. A 560 yellow-green emission is hard to observe in normal orthorhombic crystals when it is also emitting 600 nm (orange). In another study, where 500-nm rubrene nanoparticles are mixed with 50-nm silver nanoparticles, were modulated from yellow-green to red luminescence with an increasing emission angle, which shows optical properties of the rubrene layer change when viewing the angle. Another source showed luminescence of pristine rubrene nanoparticles were yellow-orange, but as the hydrothermal temperature increased to 180 C, now changed to blue.

4. What can I add to compounds to enhance their fluorescence or phophorescence?

Adding -OH for fluorescence and adding =S for phosphorescence (but reduces fluorescence), as well as phosphorus for both.

In general, for fluorescence, are adding electron-donating groups and electron-withdrawing groups, and for phosphorescence, adding the heavy-atom effect, such as platinum, ruthenium, and the halogens.

5. If a compound can do both fluorescence and phosphorescence, are they at different colors?

Yes, as singlet emission is always a lower wavelength than triplet emission.

6. It looks like photoluminescent compounds only emit spectral colors. So pink, brown, and white, are not spectral colors.

Compounds that luminesce those colors used dyes.

7. Is it true that it's rare for something to absorb UV light, and fluoresce red or violet?

Yes, those are farther range to fluoresce red, or close-range to fluoresce violet. It would be unlikely for something to absorb UV-A light and emit violet, but more common for something to absorb UV-B light, to emit violet, but UV-B lamps are more dangerous to work with. Compounds that fluoresce red under UV light tend to be complex, such as tris(bipyridine)ruthenium(II) chloride ([Ru(2,2'-bipyridine)₃]²⁺.

8. If 2 compounds are both fluorescent, can I combine them, and now they fluoresce 2 colors independently of each other?