Lamp irradiation, excited state

DIBF has the absorbance spectrum shown in Figure 4. Coating mixtures containing this photoinitiator can be cured with an intense tungsten halogen lamp, such as a projector bulb. When DIBF is irradiated it forms an excited state which has a very low absorbance, i.e., it photobleaches (1). This allows light to penetrate deeply into the coating. 

Laser flash photolysis experiments48,51 are based on the formation of an excited state by a laser pulse. Time resolutions as short as picoseconds have been achieved, but with respect to studies on the dynamics of supramolecular systems most studies used systems with nanosecond resolution. Laser irradiation is orthogonal to the monitoring beam used to measure the absorption of the sample before and after the laser pulse, leading to measurements of absorbance differences (AA) vs. time. Most laser flash photolysis systems are suitable to measure lifetimes up to hundreds of microseconds. Longer lifetimes are in general not accessible because of instabilities in the lamp of the monitoring beam and the fact that the detection system has been optimized for nanosecond experiments. 

Furans are able to undergo photocycloaddition of the [W2S+ 2S] and the [W4S+ 4S] type to suitable substrates. With benzene (80JCS(P1)2174) five 1 1 products are obtained. The relative proportions of these products are highly variable and depend on the relative concentration of the reactants, the irradiation time, the light intensity and the temperature of the solution. For the shortest irradiation time with a low-pressure mercury lamp at 15 °C, the relative proportions are 1 1 10 40 2. The major product is the 2,5 l, 4 -adduct (301) and the next most prolific is the 2,3 l, 2 -adduct (302). Adduct (301) is unreactive to dienophiles but gives adduct (302) by Cope reaction at 60-70 °C. This reaction can also be achieved by irradiation of a cyclohexane solution of (301). Adduct (302) reacts readily with dienophiles in ethereal solution to form Diels-Alder adducts. The minor adducts possess structures (303), (304) and (305). The reaction is thought to involve the first excited triplet of benzene or an excited state complex. A [ .4s+ .4g] photoadduct (306) is formed... 

Irradiation of 2,2-dimethyl chromene through Pyrex using a 550-W Hanovia lamp initiates a retro 4 + 2 reaction to form the extended quinone methide 4, which reacts with methanol to form a pair of methyl ethers (Scheme 6A).18 Flash photolysis of coniferyl alcohol 5 generates the quinone methide 6 (Scheme 6B) by elimination of hydroxide ion from the excited-state reaction intermediate.19 The kinetics for the thermal reactions of 6 in water were characterized,20 but not the reaction products. These were assumed to be the starting alcohol 5 from 1,8-addition of water to 6 and the benzylic alcohol from 1,6-addition of water (Scheme 6). A second quinone methide has been proposed to form as a central intermediate in the biosynthesis of several neolignans,21a and chemical synthesis of neolignans has been achieved... 

Applying the same idea to the activation of an aliphatic C-H bond would be a major goal, but is not as simple as one may hope. Direct irradiation of the precursor can not be contemplated because simple aliphatic derivatives do not absorb at convenient wavelengths (i.e. those at which efficiently emitting lamps are available), but only in the extreme UV. At any rate, even when the way to reach the excited state is found, more than one chemical (fragmentation) path is usually available, making the process less clean that it may be desired. However, the use of photocatalysts may help, particularly for reactions via aliphatic carbon-centered radicals. 

The unit cell of a crystal of VO(acac)2 (acac = acetylacetonato, 511702 ) (Figure 5-26) expanded significantly when the single crystal at low temperature was irradiated with the xenon lamp. The VO(acac)2 complex was proposed to have a short-lived excited species due to a d-d transition under irradiation with visible light, [43] but the lifetime is too short to observe the emission. Because the anisotropy in expansion of the unit cell on photoirradiation clearly differed from the thermal one, it is possible to observe the excited structure of the VO(acac)2 complex in the equilibrium state even if the lifetime of the excited state is small. 

Another well-known example, viz. the lamp phosphor Cas(POa)jF Sb , Mn . Ultraviolet radiation is not ab.sorbed by Mn, but only by Sb. Under ultraviolet irradiation, the emission consists partly of blue Sb emission, and partly of yellow Mn emission. Since the Mn- ion was not excited directly, the excitation eneigy was transferred fruin Sb to Mn (see Fig. 1.4). The luminescence processes can be written as follows, where hr> indicates radiation with fiequency v and the asterisk an excited state ... 

Mollenauer and co-workers used the MCD effect in the absorption band of an F center in order to detect, for the first time, changes in the ground state spin polarization induced by ENDOR in the relaxed excited state. A He-Ne laser served for optical pumping as well as for monitoring the magnetic circular dichroism, while several preceding experiments still had used conventional lamps. The laser beam, which passed a quarter wave plate with a stress modulation frequency of SO kHz, was irradiated onto a KI crystal that contained the F centers under study. The transmitted laser light was monitored with a photomultiplier and processed with lock-in technique. 

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