Page 57 - Photoreactive Organic Thin Films

HERMANN RAU

187
conducted by Lednev et al. They find a transient absorption in the region
red to the n —> n* absorption band with two lifetimes: fi ~ 1 ps and T 2 =
11-15 ps. The ground state recovers (in part) with a single time constant of
T 3 = 13 ps. This indicates that the S 2 state does not deactivate directly to the
ground state, but rather that a "bottleneck" state S* with 13 ps lifetime is
passed. The authors find their results to be compatible with rotation in the S 2
188
state. An extended study by Lednev et al. also included n ~» n* excitation.
This creates transient absorptions near 400 nm (strong) and 550 nm (weak),
Both bands decay with a single time constant of T = 2.5 ps when the excita-
tion is near the origin of the absorption band. On jc —» ji* excitation, the
improved time resolution of the equipment allowed a modification of the
earlier results: An immediate transient at 475 nm decays very fast (t >0.2 ps)
and creates the band at 400 nm with the same time constant. This 400 nm
band in turn decays biexponentially with the times of ca. 0.9 and ca. 15 ps.
189
In a third paper, Lednev et al. investigate the molecule 14 where rotation is
inhibited. They find, regardless of excitation wavelength, a transient absorp-
tion with a decay time of ca. 2.6 ps and the same time constant of the
(partial) recovery of the ground state. From the persistent ground-state
absorption change, they estimate <|>E-»Z = 0.26 for S 2 excitation, in agreement
34
with the steady-state result. This is in accord with Figure 1.15. Nagele et
190 1
al. give an idea about the relative slopes of the (n,Jt*) inversion curve at
the Z~ and E-geometries by the time constants of the disappearance of the
first transient absorption in the 450 to 550 nm region: 170 fs for the Z- and
320 fs for the E-isomer. After the intramolecular processes, the vibrationa!
cooling of the ground state by energy transfer to the solvent occurs in about
191 192
20 ps, in agreement with the results of Terazima et al.
Picosecond time-resolved Raman studies give information about the early
time dynamics of the isomerization process. For n —» n* excitation, the N=N
bond of E-azobenzene shrinks by 3 pm, and the C-N bond is elongated by
78 193
1 pm within 5 to 30 fs, ' which is in accord with an inversion mechanism.
79
Fujino and Tahara see the properties of the (n,ft*) and ground state after
n —» it* excitation. In the picosecond realm, the N=N vibrations in the
1
1
ground (1440 crcf ) and the (n,^*) states (1428 cm" ) are nearly the same,
indicating the retention of the N=N double bond, which suggests the reten-
tion of a planar geometry. The authors infer internal S 2 - S t conversion from
their findings.
61
In a very new report, Fujino et al. challenge the two~isomerization~
mechanism concept on the basis of their tirne-resolved and time-integrated
femtosecond fluorescence measurements of E-azobenzene following excita-
l
tion of the (n,n*) 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 (S 2: 0.11 ps; S 2: 0.5 ps) and the
radiative lifetimes deduced from the (absorption-spectra-based) oscillator
s
strengths, they determine the fluorescence quantum yields: 2.53 10" for
7
the S2-T>S 0 emission and 7.54" 10~ for the $i~»S 0 emission. By comparison
with the integrated S 2—>S 0 and St-^So fluorescence intensities, they derive an
efficiency of unity (0 = 1.07± 0.15) for the S 2 (ft,?t*) -> Sj (n,7t.*) internal
conversion. On this basis, they exclude any isomerization in the (TC,JC*) state.