Azobenzene isomerization quantum yields

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... 

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)]. 

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). 

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. 

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

In recent years, optical dichroism and birefringence based on photo-induced trans-cis-trans isomerization of azobenzene groups has been observed with preoriented liquid-crystalline polymers [31-35] at temperatures above the glass transition temperature, and also with various amorphous polymers at temperatures well below the glass transition temperature. In the case of a polyimide (see Chart 5.7), a quasi-permanent orientation can be induced [36-38]. Here, the azobenzene groups are rather rigidly attached to the backbone and photoisomerization occurs at room temperature, i.e. 325 °C below the glass transition temperature, Tg = 350°C. This behavior is in accordance with the fact that the isomerization quantum yields of azobenzene compounds are very similar in solution and in polymer matrices 0 trans cis) 0,1 and 0(cis trans) 0.5. 

Photoirradiation of EE- lC at 365 nm afforded almost quantitative (> 95%) isomerization to the ZZ isomer, as shown by H NMR and UV-vis spectra. The values determined for the EZ photo-isomerization quantum yield (0.095) and rate constant for ZE dark isomerization (1.4 x 10 s ) are typical of the azobenzene unit and are virtually unaffected by the presence of ring 2. Thus, both the end groups of IH —either alone or surrounded by 2—can be effectively switched between the E and Z isomers. Cycling between EE and ZZ states could be performed with no sign of degradation. The H NMR spectra also showed that ZZ-IH" remains complexed by 2 (Fig. 8.1a). 

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). 

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. 

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. 

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. 

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 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. 

For all Azo-PURs, the quantum yields of the forth, i.e., trans—>cis, are small compared to those of the back, i.e., cis—>trans, isomerization—a feature that shows that the azo-chromophore is often in the trans form during trans<->cis cycling. For PUR-1, trans isomerizes to cis about 4 times for every 1000 photons absorbed, and once in the cis, it isomerizes back to the trans for about 2 absorbed photons. In addition, the rate of cis—>trans thermal isomerization is quite high 0.45 s Q 1 shows that upon isomerization, the azo-chromophore rotates in a manner that maximizes molecular nonpolar orientation during isomerization in other words, it maximizes the second-order Legendre polynomial, i.e., the second moment, of the distribution of the isomeric reorientation. Q 1 also shows that the chromophore retains full memory of its orientation before isomerization and does not shake indiscriminately before it relaxes otherwise, it would be Q 0. The fact that the azo-chromophore moves, i.e., rotates, and retains full orientational memory after isomerization dictates that it reorients only by a well-defined, discrete angle upon isomerization. Next, I discuss photo-orientation processes in chromophores that isomerize by cyclization, a process that differs from the isomeric shape change of azobenzene derivatives. 

Polarized light absorption orients both isomers of photisomerizahle chromo-phores, and quantified photo-orientation both reveals the symmetrical nature of the isomers photochemical transitions and shows how chromophores move upon isomerization. Photo-orientation theory has matured by merging optics and photochemistry, and it now provides analytical means for powerful characterization of photo-orientation by photoisomerization. In azobenzenes, it was found that the photochemical quantum yields and the rate of the cis—>trans thermal isomerization strongly influence photo-... 

It is clear now that irradiation of phenyl azide at room temperature gives dehydroazepine. At high concentration of azide, the dehydroazepine polymerizes rapidly in competition with its slow isomerization to triplet phenyl nitrene. The major product formed from photolysis of phenyl azide under conditions where its quantum yield for disappearance is claimed to be greater than unity is poly-1,2-azepine [48], not azobenzene. Of course, the polymer does not elute from an HPLC, and analysis of reaction mixtures by chromatography will show only two components. 

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