Case Study 6.38: Macromolecular chemistry – photoinitiated polymerization of methacrylate
The extent of dimethylaminoethyl methacrylate (579) photopolymerization (Scheme 6.280) was found to be dependent on the type of photoinitiator.1503 The type I photoinitiator bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide (580), producing radicals by homolytic fragmentation (Scheme 6.281), was found to be more effective in promoting polymerization than the type II photoinitiator consisting of a mixture of 1-hydroxycyclohexyl phenyl ketone (581) and benzophenone. The ketone 581 apparently undergoes a-cleavage (Section 6.3.3) to initiate radical polymerization,1504 while electron followed by proton transfer (see Scheme 6.100 in Section 6.3.1), involving benzophenone or 581 and an amino group of the monomer (579) or polymer (Scheme 6.282), then promotes radical branching and cross-linking processes among macromolecules.
N
N
CH2
CH2
polymerization
O
O
O
O
O
O
O
O
N
N
579
n
Scheme 6.280
MeO
MeO
O
O
hν
O
O
MeO
P
OMe
MeO
+ P
OMe
R
O
R
O
OMe
OMe
580, R = 2,4,4-trimethylpentyl
Scheme 6.281
O OH
O
hν
HO
+
581
O
OH
hν
+ N
H
R
N
R
monomer or polymer
Scheme 6.282
Photosensitizers, Photoinitiators and Photocatalysts
437
Experimental details.1503 A mixture of the monomer 579 (1–3 mg) and a photoinitiator ( 3 wt%) in a sample (open-air) cell was irradiated with a high-pressure mercury discharge lamp (200 W) (Figure 3.11). In order to prevent evaporation of the monomer, the cell was covered with a thin poly(ethylene terephthalate) film. The extent of polymerization was evaluated by differential photocalorimetry.
Application of photoinitiators is one of the most efficient methods for achieving fast
polymerization or controlled chemical modification of polymers. The use of photoinitiators is common in many valuable industrial applications,174,1134,1505 such as
photolithography and UV curing (Special Topic 6.27).
Special Topic 6.27: Photolithography and UV curing
Photolithography (or optical lithography) is a photoimaging process (see also Special
Topic 6.32) designed to form microscale and nanoscale patterns (in so-called microand nanofabrication, respectively) on a solid surface.1505,1506 Together with particle beam
lithography, photolithography is the most common lithographic technique utilized in the microelectronics industry. The process employs a photoresist, an organic material that, upon irradiation through a mask, either degrades to become more soluble (positive
image generation) or cross-links to become insoluble (negative image generation) under specified conditions for development (Figure 6.27).1507,1508 The irradiated photoresist
layer prior to the development is called a latent image. In subsequent chemical treatment (etching), the upper layer of the substrate in the areas that are not protected by photoresist is removed, followed by etching the unprotected layers and stripping of the photoresist layer. The current highest reproducible resolution of a feature on the surface that can be achieved by photolithography is about 90 nm.1506
In positive-tone imaging, a high-sensitivity photoresist is usually composed of a photoinitiator and a polymer, the degradation of which is typically based on chemical
hν
mask
exposure
photoresist
silicon substrate
latent image
positive-tone
negative-tone
technique
technique
Figure 6.27 Photolithography
438
Chemistry of Excited Molecules
amplification, that is, a chain reaction initiated by photoproduction of a catalyst.1508 For example, a mixture of poly[4-(tert-butoxycarbonyloxy)styrene] (t-BOC resist) (582), a polymer with acid-labile tert-butyl carbonate groups, and triphenylsulfonium triflate (583), a photoinitiator that generates a strong acid (CF3SO3H) upon photolysis1509 (see also the photolysis of hypervalent iodine compounds in Scheme 6.242), creates a latent image upon irradiation at 250 nm through a mask. The exposed areas contain an acid, which catalyses the decomposition of the t-BOC groups to form poly(4-hydroxystyrene) (584), CO2 and isobutene with concurrent recovery of a proton upon heating to 100 C (Scheme 6.283). Thanks to the relatively low temperatures used, the unexposed surface of the resist remains unaffected, whereas that irradiated (containing phenol moieties) can be washed away with a solution of a base. This photoresist system can also be used for negative-tone imaging, when a developer – a less polar organic solvent, such as anisole – dissolves the unmodified polymer but not the poly(4-hydroxystyrene).
S
hν
CF3SO3H + side-products
CF3SO3
283
n
n
n +
H
+
100 oC
+ CO2 + H
O O t-Bu
O
OH
OH
O
O
282
284
Scheme 6.283
Novolak – diazonaphthoquinone positive-tone resists, the most important imaging system of semiconductor production today1510,1511 – is an archetypal example of the
industrial applications of photochemistry. Novolak is a phenol–formaldehyde polymer (Bakelite) that dissolves in aqueous hydroxide, but the addition of a small amount of the diazonaphthoquinone 585 dramatically decreases the solubility. When irradiated, 585 undergoes the photo-Wolff rearrangement (see also Scheme 6.171), leading to ring contraction and subsequently to carboxylic acid formation (Scheme 6.284). Such a photochemically altered site is readily soluble and can be removed with a basic developer solution.
Another industrial application, UV curing, is a technologically advanced process by which monomers/oligomers/polymers, usually in the presence of photoinitiators, undergo polymerization and cross-linking upon UV irradiation. The process is utilized
Photosensitizers, Photoinitiators and Photocatalysts
439
O
O
O
COOH
N2 hν
H2O
- N2
585
Scheme 6.284
to cure (harden, dry) inks, paints,1512 coatings,1513 adhesives,1513 and dental glues,1514 and also resists.174 This method has also recently been introduced in coating and printing processes for CD and DVD manufacture.1515
The most common dental polymers, used for prosthetic purposes and restorative dentistry (filling material), are polymethacrylates.1514 The polymerization process performed directly in the dental cavity has to meet strict demands; the reaction must be fast at a temperature below 50 C and it must avoid the formation of a toxic product. These requirements can be fulfilled by UV curing. For example, a mixture of camphorquinone (586), a chromophore (photoinitiator) with an absorption maximum at 468 nm and an amine 587 as a co-initiator (see also Scheme 6.100), initiates a radical polymerization reaction of the acrylate monomer 588 upon photolysis using a conventional blue lamp or laser (Scheme 6.285).
O
O
O O
O O
588
O
R
470 nm
O
R
+ H C
N CH
+ H C
N CH
3
3
2
3
O
OH
588
586
587
polymethacrylate
Scheme 6.285
In contrast, UV curing of poly(vinyl cinnamate) produces a cross-linked polymer (Scheme 6.286) via [2 þ 2] photocycloaddition (Section 6.1.5) in the absence of an photoinitiator. This reaction can be used in negative-tone photolithography because the complex photoproduct is poorly soluble.1516 UV curing can also be used to produce the high-definition images by UV nanoimprint lithography, in which a topographic pattern from a rigid mould (such as a silicon matrix) is transferred into a low-viscosity
monomer that is polymerized by UV irradiation to form solid structures on surfaces.1506,1517
440
Chemistry of Excited Molecules
O
O
O
hν
O
+
O O
O
O
= polymeric chain
Scheme 6.286
6.8.2Transition Metal Photocatalysts
Fe3+
Fe2+
D
hν
D
hν
TiO2
H2O
HO
+ H
A
A
Recommended review articles.170,1248,1518–1526
Homogeneous Transition Metal Photocatalysis
Photoreactions that involve transition metal ions, complexes or compounds can usually be classified as (photo)redox (simultaneous oxidation and reduction) processes. A representative non-photoassisted catalytic system is Fenton s reagent that produces HO. radicals on reaction of ferrous ions (Fe2 þ ) and hydrogen peroxide (Scheme 6.287a). Its photochemical counterpart is the photo-Fenton process,1527 in which ferric (Fe3 þ ) complexes in aqueous solutions (absorbing over 300 nm) are reduced to ferrous ions (Scheme 6.287b).
(a) Fe2+ + H2O2 Fe3+ + HO + HO
hν
(b) Fe3+ + H2O Fe2+ + HO + H
Scheme 6.287
Special Topic 6.28: Environmental remediation
Homogeneous transition metal photocatalysis reactions generating HO. radicals, often referred to as advanced oxidation processes (AOP),1518 are powerful methods to remediate (i.e. to remove or to destroy) organic pollutants from aqueous solutions.1519
Photosensitizers, Photoinitiators and Photocatalysts
441
Nonselective and efficient consecutive oxidation reactions ultimately lead to nontoxic mineralization products, such as CO2 and H2O.1519,1520 For example, an improved
version of the photo-Fenton system, utilizing ferrioxalate ion, very efficiently oxidizes organic compounds present in the aqueous solution.1528 This process affords a reactive C2O4. intermediate, which generates a superoxide radical anion (O2. ) from dissolved oxygen or directly attacks relatively inert molecules such as CCl4 (Scheme 6.288).1529
hν
[FeIII(C O ) ]3-
[FeII(C O ) ]2-
+
C O
2
4 3
2
4 2
2
4
C2O4
+
O2
2 CO2
+ O2
C2O4
+
CCl4
2 CO2
+ CCl3
+
Cl
Scheme 6.288
Transition metal catalysts can also be used in photochemical organic synthesis. For example, the photo-Bergman reaction (cycloaromatization; see also Scheme 6.62) of (Z)-hex- 3-en-1,5-diyne [(Z)-589], which is obtained by photostationary E–Zisomerization (Section 6.1.1) from (E)-589, occurs via transition metal-catalysed cycloaromatization reaction followed by photochemical dissociation of the arene ligand from the complex 590 (Scheme 6.289).1530 The product, 1,2-di(n-propyl)benzene (591), is obtained in 91% chemical yield.
Pr
Pr
hν
Pr
Pr
(E)-589
(Z)-589
Pr
Fe
Pr
590
hν
Pr
Pr
Pr
Pr
591
MeCN
Fe
NCMe
MeCN
Scheme 6.289
442
Chemistry of Excited Molecules
In another example, iron pentacarbonyl photochemically decarbonylates (see Section 6.3.9) to form the p-allyl complex 592 with an allyl alcohol 593, which then undergoes various dark reactions to give 594 (Scheme 6.290).1531
hν
R
[Fe(CO)5] (CO) Fe
3
H
OH
592
R
R
HO
(CO)3Fe
593
H OH
H3C
R
H3C
R
OH
(CO) Fe
3
OH
H3C R
O
594
Scheme 6.290
Heterogeneous Transition Metal Photocatalysis
The most common photocatalytic processes, in terms of both mechanistic analysis and practical use, involve insoluble semiconductor metal oxides or sulfides, which upon irradiation undergo dual interfacial electron transfer between the excited semiconductor surface and adsorbed donor (D) and/or acceptor (A) molecules (Scheme 6.291). Titanium dioxide (TiO2) is a particularly popular photocatalyst due to its good redox properties (see also Special Topic 6.29), high stability, low toxicity and low price.
TiO2 + D + A
hν
+ D + A
TiO2
Scheme 6.291
Special Topic 6.29: Excitons and redox reactions on a semiconductor
Irradiation of a semiconductor with photons having an energy exceeding its band-gap (the energy difference between the valence and conduction bands, usually considered
to be lower than 3 eV in TiO2) results in electron transfer from the valence to the conduction band.170,1248,1521–1524The generated negative charge (electron, e ) in the
conduction band associated with TiIII centres of TiO2, and positive charge (a quasiparticle, electron hole, h þ ) in the valence band associated with TiIV centres of TiO2, represent a bound excited state of an electron called an exciton that is formed within
Photosensitizers, Photoinitiators and Photocatalysts
443
Figure 6.28 Semiconductor photochemistry
femtoseconds.1532 The exciton energy is slightly lower than that of the unbound electrons and holes because it gains from the binding interaction of the electron with its hole. The charges recombine (in 10–100 ns) to generate heat (radiationless pathway) or photon emission or they can migrate to the semiconductor surface and be trapped by ambient molecules via electron transfer processes (Figure 6.28). The charge transfer probability for electrons and holes is related to the redox potentials of the adsorbed species and the positions of the band edges. For interfacial charge transfer, oxidation of a donor is faster (on the order of 100 ns) than reduction of an acceptor (on the order of milliseconds).
Both donor and acceptor molecules are indispensable for the accomplishment of the photoredox reaction and the electrochemical potentials of the donor (D/D. þ ) and acceptor (A/A. ) couples should lie within the semiconductor band gap. Oxidation reactions, photocatalysed by TiO2, are usually performed in the presence of easily reducible molecular oxygen as an electron acceptor, thereby generating a superoxide radical ion (O2. ) and
subsequently hydroxyl radicals. The resulting holes on the semiconductor surface can oxidize many compounds (Scheme 6.292), including alcohols, hydroxyl anion and even water.1522,1523
Photocatalytic reductions are less common because the reducing ability of an electron in the conduction band is considerably lower than the oxidizing ability of a positively charged valence band. Furthermore, reduction of most organic compounds usually cannot compete kinetically with that of oxygen.170 Photoreductions in aqueous solutions are often accompanied by the production of molecular hydrogen.
Semiconductors, such as TiO2, can be prepared in the laboratory by annealing (pyrolysis) of various TiIV compounds, such as titanium isopropoxide, Ti[OCH(CH3)2]4.170 The resulting semiconductor properties, such as surface area, morphology, particle size and
444
Chemistry of Excited Molecules
O2
H3O
H2O + HO2
O2
_
H
HO2
H2O2
hν
+
donor
donor
(e.g., HO
HO )
Scheme 6.292
surface character, then strongly influence its photophysical and photochemical behaviour. Semiconductors having high surface area can adsorb large amounts of organic molecules. Titanium dioxide in the anatase form is the most photoactive and most used in applications, such as water or air purification and water disinfection.1523 In photochemical experiments, the semiconductor can be irradiated in the form of powder suspended in a stirred solvent or deposited on a solid support, such as a zeolite. The irradiation wavelength below 400 nm is sufficient to photoactivate neat TiO2 (it is a white solid).
Titanium dioxide can also be modified by metal(0) deposition, which may significantly enhance its photoreaction efficiency. For example, Pt0 or Au0 deposited on TiO2 increases the charge separation lifetime. Further, the absorption properties of TiO2 can be improved by organic dye sensitization (see also Section 6.8.1). A sensitizer (S) then absorbs visible light to form an excited state (S ), which transfers an electron to the conducting band of the semiconductor (Scheme 6.293). The radical cation thereby generated (S. þ ) is reduced by an auxiliary electron donor (D) and, as a result, the sensitizer is regenerated. An example
of such a sensitizer is tris(2,20-bipyridine)ruthenium(II) (see also Section 6.4.4), which can be used in solar (photovoltaic) cells (Special Topic 6.31).1524,1533
Scheme 6.293
Photosensitizers, Photoinitiators and Photocatalysts
445
TiO2-based photocatalysts are undoubtedly the most commonly used, but some other heterogeneous catalysts also display promising photoactivity. Zinc and cadmium sulfides and iron, zinc and vanadium oxides are examples.170 Metal chalcogenides, cadmium
sulfide (CdS) and zinc sulfide (ZnS), have smaller band gaps, hence they can be irradiated at longer wavelengths (visible light).1534,1535 Compared with TiO2, which has a strong
photooxidizing power, metal chalcogenides have strong reductive properties. The electrons in the conduction band of CdS have sufficient negative potential to reduce water to form molecular hydrogen (see also Special Topic 6.26).1534 Unfortunately, metal chalcogenides are known to suffer photocorrosion, a process in which charge carriers in the illuminated semiconductor are not transferred to molecular donors or acceptors but oxidize or reduce the semiconductor itself, leading to its dissolution (i.e. releasing the metal ions to the solution). Some metal oxides, such as haematite (a-Fe2O3), absorb in the visible region. They generally display considerably lower photocatalytic activity than
TiO2, although ZnO was shown to produce hydrogen peroxide more efficiently than
TiO2.1536
Special Topic 6.30: Quantum dots
The absorption properties of semiconductors are dependent on the size of the
particles. Small semiconductor particles (usually 2–10 nm in diameter) are called quantum dots,1537–1539when they confine the migration of a small finite number of
conduction band electrons and valence band holes (i.e. excitons) due to electrostatic potentials and semiconductor surface/interface properties. Discrete absorption (but also emission) spectra are thus directly related to band-gap energies, which are larger in smaller particles (the colour of colloid solutions is changed with the size of the particles). For example, Figure 6.29 compares the optical properties of CdS sols formed in water; this mixture is yellow–green and the average particle size is 3 nm,
0.8
absorbance
0.6
0.4
0.2
0.0
300
400
500
600
wavelength / nm
Figure 6.29 CdS in water at 298 K (—) and in propan-2-ol at 213 K (- - -). Adapted from ref. 1534, copyright 1984. Reproduced with permission from IUPAC
Fe
Fe
