Quantum yield azobenzenes

The simplest azo compound, azobenzene, exists as a mixture (Scheme 4-18) of a stable trans (4.19) and an unstable cis (4.20) form [38,39]. Formation of the cis isomer is induced by exposure to light, the quantum yield of the process depending upon the wavelength of the light employed [40]. The proportion of cis isomer can be appreciable in an equilibrium mixture. Thus a concentration of 24% of this unstable form builds up within a few hours when an acetic acid solution of azobenzene is exposed to sunlight in shallow white trays. Reversion to the trans form occurs readily on heating and is catalysed by a variety of substances that can function as electron donors or acceptors [41]-... 

Despite great interest in azobenzene photophysics, the basic photoisomerization mechanism remains disputed [173] in contrast to the expectations of Kasha s rule, the isomerization quantum yield decreases rather than increases with increasing photon energy. In Fig. 22, the two possible isomerization channels, proceeding via either a planar pathway (inversion) or a nonplanar, twisted pathway (torsion) are shown. Previous studies determined that isomerization in the first excited state S1 state proceeds along the inversion coordinate [171]. The second excited state is generally thought to be... 

Aromatic azo compounds show a clear wavelength dependence of cis-trans-isomerisation. The results of different groups on the photostationary state of azo benzene are however quite contradictory 24,28,47,48), Cfs-azobenzenes revert easily to the more stable fraws-isomers. This process has been shown to have an activation energy of about 20 kcal/mole. The quantum yields for cis-trans-iso-merisation differ starting from cis- or trans-iizo benzene 5°). 

In the 275-340 nm wavelength range, the concentrated solution of E-azobenzene absorbs the incident radiation completely (100% absorption). Thus the E Z isomerization, with a quantum yield of [Pg.147]

At 254 nm, the Z-isomer of azobenzene absorbs more strongly than the E-isomer (Fig. 1). Thus, for actinometric measurements of the 254 nm mercury line, the azobenzene actinometers are first preirradiated at 313 nm until a photostationary state (containing mostly Z-isomer) is obtained. The preirradiated actinometers are then exposed to the actinic radiation at 254nm and the Z—>E isomerization, with a quantum yield of 0 = 0.31, is monitored at 358 nm. The light intensity (at 254 nm) is obtained using Eq. (14), where W=2.3x 10 Einsteincm (15). 

The azobenzene actinometer has been calibrated for intensity measurements with pulsed nitrogen, excimer, and Nd YAG lasers. Comparative actinometric measurements of the radiation intensity of a weak nitrogen laser (337.1 nm, 5 ns, and <5mJ/pulse) utilizing ferrioxalate (vide supra) and a dilute (10" M) azobenzene solution resulted in an isomerization quantum yield identical to that obtained using the 334-nm mercury line (14). Thus, we conclude that a concentrated (6.4x 10" M) azobenzene solution can be used in the same way as described for actinometric measurements of the mercury line at 334 nm (15) with an calibration factor of W=3.6x 10 Einsteincm [see Eq. (14)]. 

In a similar way, anthracene triplet (4>,gj3=0.71, z =6A,700Mr cmr ) and the naphthalene triplet (4>jg = 0.75, e j = 24,500 M" cm" ) in cyclohexane solution have been introduced as transient chemical actinometers for the third-harmonic (355 run) and fourth-harmonic (266 nm) output of Nd YAG lasers, respectively (44). In summary, transient chemical actinometers are ideal for accurately measuring the energy of single laser pulses, provided the quantum yields and extinction coefficients of the transients are well known (45 7). Thus, the well-established benzophenone actinometer (42-44) has been used as a reliable reference to calibrate the azobenzene actinometer (see section "Laser Intensity Measurements with the Azobenzene Actinometer" Doherty S, Hubig SM, unpublished results) and the Aberchrome 540 actinometer (48,49) for intensity measurements with pulsed Nd YAG and/or XeCl excimer lasers. However, such actinometer can only be used when a complete set of laser flash photolysis equipment including a kinetic spectrometer is available. 

Gauglitz G. Azobenzene as a convenient actinometer for the determination of quantum yields of photoreactions. J Photochem 1976 541-547. 

Isomerization can be induced by light in both directions or by heat in the Z — E direction. The reverse thermal reaction is not observed at normal temperatures. Any one of the elementary reactions can be missing. Z-azobenzene in solution has a thermal Z E activation enthalpy AH 96 kJ moT and a half life time of 2 to 3 days at room temperature. Thus, the thermal reaction is irrelevant for the photoisomerization at usual irradiation intensities (for comparison Z-stilbene has Eg 180 kJ moT is liquid, and is kinetically stable). On the other hand, one of the photoreactions may not be active (e.g., when an irradiation wavelength is selected where one form does not absorb or when the quantum yield is too small). Inspection of Figure I.IB shows that E- and Z-azobenzene have virtually no spectral region without overlapping absorption. 

For the evaluation of Equation 1.2, the irradiation intensity Iq is needed. This is determined by actinometers, which may be either physical in nature or chemical reaction systems with known quantum yields. Because the values of for azobenzene are well documented, azobenzene... 

Azobenzene-type azoaromatics generally do not emit with reasonable quantum yields after excitation either to the (n,JC ) or to the states. 

Not every photon absorbed by azo compounds induces isomerization. Table 1.1 shows a series of quantum yields of azobenzene collected from different authors. The spread of the values reflects the experimental problems of a seemingly simple system. 

The quantum yields for azobenzene in solution are not concentration dependent. ... 

There are, however, azobenzenes that have wavelength-independent isomerization quantum yields and thus obey Kasha s rule. The structure of these molecules inhibits rotation. Rau and Liiddecke investigated azobenzeno-phane 9 and Rau the azobenzene capped crown ether 14, and these researchers found identical E,E —> E,Z, and E —> Z quantum yields respectively, regardless of which state was populated. The photoisomerization of azobenzenophanes and 13 could not be evaluated in the same way because the photoisomerization is intensity-dependent. A series of azobenzenes substituted in all ortho positions to the azo group has equal quantum yields for n —> n and k —> k excitation if the substituents are ethyl, isopropyl, tert.butyl, or phenyl. This provides clues for the elucidation of the isomerization mechanism (Section 1.6). 

FIGURE I.IO Temperature dependence of the quantum yields of azobenzene. (Data from reference 110.)... 

Z- E isomerization yield is nearly temperature-independent (Figure 1.10) or increases at low temperature, with only a small difference for excitation to the two lowest-excited states. So obviously, the E —> Z photoisomerization— after irradiation to the (n,7C ) state as well as the Z —> E isomerization— proceeds even at low temperature and in frozen solvents. In solid matrices, fast and slowly isomerizing molecules are observed on it —> it excitation. The fast process has a quantum yield of < = 0.14 that is temperature independent down to 4 K. With strong lasers, photoisomerization in the E —> Z direction have been exploited, even at 4 K in hole burning experiments. Thus, azobenzene photoisomerization cannot be frozen out. 

Bortolus and Monti have determined the quantum yields of azobenzene isomerization in different solvents.They found an increase of E Z isomerization but a decrease of Z E isomerization when solvents with high dielectric constant are used. This phenomenon is independent of the irradiation wavelength. Table 1.2 shows the special feature of wavelength-dependent quantum yields of azobenzene. 

RRLE 1.2 Solvent Dependence of Isomerization Quantum Yields of Azobenzene ... 

Gegiou et found only a very slight viscosity effect, both in the n-Ti and in the jt-jc absorption bands on the isomerization quantum yield. They used glycerol as a viscous solvent, but the result may also be transferred to polymer matrices. In solid matrices, several photoisomerization modes are observed (see the preceding section on the influence of temperature), A com parison between azobenzene isomerization in liquid methylmethacrylate and the slow mode in poly (methylmethacrylate) showed that the difference in quantum yields on Si (0.17) and S2 excitation (0.03) is retained in the solid matrix. The fast process is not observed in n —> n excitation. These data are important in relation to the use of the azobenzene isomerization method for the determination of the free volume in a polymer. 

Confinement of azobenzene in defined structures changes the quantum yields. In the cavity of fi-cyclodextrine, the < )e z are reduced and become nearly wavelength-independent, whereas the practically unaffected... 

There is one report of a concentration dependence of the 313 nm photo-stationary state of azobenzene and 4-methoxyazobenzene in cyclohexane— not in benzene or CCI2F-CCIF2—in the literature.A bimolecular excimer intermediate was postulated. Further work is needed to elucidate whether the absorption coefficients or the quantum yields are concentration-dependent, for instance by ground or excited-state association (cf. Equation 1.3). 

The quantum yield at 436 nm is significantly higher than it is at shorter wavelength irradiation (Table 1.3) which is like in azobenzene-type compounds. Indeed, in hydrocarbon solution, the absorption spectrum shows the azobenzene-type feature of a long wavelength n —> band (Figure 1.11). 

Pseudo-stilbenes may emit fluorescence that is, contrary to true stilbenes, generally weak at room temperature and often weak even at low temperatures. Protonated azobenzene-type molecules and many protonated azo dye molecules emit strong fluorescence in sulfuric acid at 77 K with quantum yields of about 0.1. Inclusion of azobenzene in the channels of AIPO4-5 crystals provides complexation of the n-electrons and space confinement. This leads to emission by protonated azobenzene at room temperature. For their cyclopalladated azobenzenes, Ghedini et al. " report quantum yields of ca. 1T0 and lifetimes of ca. 1 ns. In contrast, donor/acceptor pseudo-stilbenes, if emitting at low temperatures or when adsorbed to surfaces, are weak emitters. In textile chemistry, it has long been known that azo dyes adsorbed to fibers may show fluorescence. ... 

The crucial finding was that in the case of azobenzene, the isomerization quantum yields for excitation of the higher state are about one-half... 

Besides solving the quantum yield enigma, this concept also rationalizes some other results. If rotation is inhibited by, say, structural design as, for instance, in azobenzenophanes or constraint from outside as, for instance, in restricted spaces as in iS-cyclodextrin " or zeolites or in solid matrices or low temperature down to 4 K, then the internal conversion from the (7i,7t ) to the (n,7t ) state provides a virtually barrierless path of isomerization. The fact that the stilbenophane analogue of Tamaoki s azobenzeno-phane shows isomerization does not invalidate this reasoning—the azobenzenes choose the easiest isomerization path. 

In a very new report, Fujino et al. challenge the two-isomerization-mechanism concept on the basis of their time-resolved and time-integrated femtosecond fluorescence measurements of B-azobenzene following excitation of the (7t,7t ) State. They use the extremely weak fluorescence (cf. Figure 1.8) as an indicator for the population of the emitting state. From the ratios of their measured fluorescence lifetimes (S2 0.11 ps Sp 0.5 ps) and the radiative lifetimes deduced from the (absorption-spectra-based) oscillator strengths, they determine the fluorescence quantum yields 2.5310 for the emission and 7.5410" for the Si—>So emission. By comparison... 

The photoisomerization mechanism in aminoazobenzene- and pseudo-stilbene-type compounds has attracted far less attention than the mechanism for azobenzenes. In pseudo-stilbenes, the (n,7t ) state is buried under the intense it —> Jt band and cannot be populated selectively. No state-specific quantum yields are available because the yields are independent of the exciting wavelength.There is only a very narrow experimental basis for a discussion of these two mechanisms. However, this may change when pseudo-stilbenes are subjected to ultrashort-time experiments. 

The photoisomerization of all types of azobenzenes is a very fast reaction on either the singlet or triplet excited-state surfaces according to the preparation of the excited state, with nearly no intersystem crossing. Bottleneck states have lifetimes on the order of 10 ps. The molecules either isomerize or return to their respective ground states with high efficiency. So photoisomerization is the predominant reactive channel, and the azobenKnes are photochemically stable. Only aminoazobenzene-type molecules and pseudo-stilbenes have small quantum yields of photodegradation. 

Gauglitz, G. (1975). Azobenzene as a Convenient Actinometer for the Determination of Quantum Yields of Photoreactions. Photochem. 5, 41- 6. 

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