Page 73 - Photoreactive Organic Thin Films

5 2 TAKAYOSHI KOBAYASHi AND TAKASHI SAITO

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nism. There is still an argument about the mechanism. In the second part of
this paper, the reaction mechanism in relation to the real-time dynamics of
molecular vibration in the excited state of fra«s-4-(dimethylamino)azoben~
zene (DMAAB, methyl yellow) in solution is studied by pump-probe spectro-
1 2
scopy, using a sub-5fs visible pulsed laser recently developed by our group. '

2.2 EXPERIMENTAL SECTION

1PA2N was purchased from Aldrich and used without further purification.
Experiments were performed with 1PA2N in methylcyclohexane, glycerin, or
mixed solvents of methylcyclohexane and cyclohexanol. Output pulses from
an Nd glass laser with 6-ps width and 40-mJ energy were used as a basic
pulsed light source. The 532-nm with 6-ps width was used for both the pump
and the generation of a picosecond broadband continuum for the interroga-
tion of the transient spectral change. All data of picosecond spectroscopy of
1PA2N were obtained at room temperature.
DMAAB (guaranteed reagent grade, Tokyo Kasei) and dimethyl sulfoxide
(DMSO; special reagent grade of Japan Industry Standard, Kanto Chemicals)
used as solute and solvent, respectively, were utilized without further purifica-
tion. Output pulses from the pulse-front-matched noncollinear optical
1 2
parametric amplifier (NOPA) ' with 4.7-fs width and 5~uJ energy at 1-kHz
repetition rate were used as both pump and probe pulses. The spectrum of
the pulses covers from 520 to 730nm with a nearly constant phase. Laser
pulse energies of the pump and probe pulses are about 20nJ and 2nj, respec-
tively. The pump beam was mechanically chopped at SOOHz in synchroniza-
tion with the laser pulse at 1kHz. The probe pulse intensity was measured
with a Si photodiode attached to a rnonochromater with 4-nm resolution.
Transmitted probe intensity change induced by the pump was detected with a
lock-in amplifier. All measurements of sub-5-fs spectroscopy of DMAAB were
performed at room temperature.

2.3 RESULTS AND DISCUSSION
2.3.1 IPA2N
2.3.1.1 Absorption Spectrum
The absorption spectrum of l-phenylazo-2-naphthol in various solvents
and the spectra of the photoinduced colored species at low temperature
generated by the exposure of this compound to light at various wavelengths
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was studied in detail. For any o-hydroxyazo compound, e.g., l-phenylazo-2-
naphthol, the combined results of tautomerization and rotation around single
and double bonds lead to the eight configurations ATI, AT2, AC1, AC2, HT1,
10 15
HT2, HC1, and HC2, ' where H and A represent the hydrazone and azo
forms, respectively, and T and C denote the trans and cis configurations,
respectively.
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The study by Gabor et al. showed that the absorption spectrum of
1PA2N is a superposition of A, absorbing in the region 350-440 mm, and of