878 J. Phys. Chem. A, Vol. 105, No. 5, 2001
Marcinek et al.
TABLE 1: Spectroscopic Characterization of the Transient Species Detected
low-temp
matrices (77 K)
solution
1H
2H
3H
1H
2H
transient species
λmax [nm]
λ
max [nm]
λ
max [nm]
λ
max [nm]
λmax [nm]
a
375, 565b
335, 375, 640b
355, 480, 520
380, 570c
385, 565c
270, 410, 480
radical cations
radicals
365, 570
∼450
a
430, 455, 485b
b,c
260, 390, 440d
d
a
-Chlorobutane glass. b MCH/n-BuCl glass. LFP in acetonitrile. d Pulse radiolysis in aqueous solution.
c
2
nm in this matrix, is already observed after initial thermal
relaxation of the matrix (see spectrum b in Figure 2B). The
solubility of 2 in this solution is rather poor, and most of the
Low-Temperature Radiolysis. The steady-state measure-
ments of 2-chlorobutane glassy samples containing 1H (saturated
solution) were performed in a temperature controlled nitrogen-
cooled cryostat (Oxford Instruments). The optical spectra were
measured with a Cary 5 (Varian) spectrophotometer. Irradiation
of the samples (a few pulses of 600 Gy each) was made with
the ELU-6 linear electron accelerator. A detailed description
+
ionic products precipitated from the solution. Thus, the absorp-
tion growth at 505 nm on prolonged annealing was limited.
+
However, formation of the monomer 2 was clearly followed
by its dimerization, which was identified on the basis of a
characteristic difference in the spectra of dimer dication (two
2
7,28
of the pulse radiolysis system is given elsewhere.
2
4,25
+
close bands at 490 and 505 nm) and monomer cation.
Pulse Radiolysis. Reduction of 1 was achieved by reaction
with 2-propanol ketyl radicals generated in aqueous solutions
of 2-propanol (1 M) saturated with N2O. This method enables
a quantitative conversion of the major products of water
Superposition of the absorptions due to the radical and closed-
shell cation are also responsible for the strong, broad band
between 400 and 500 nm seen in the spectrum shown in Figure
•
•
2A and in the spectra obtained in the laser flash photolysis
radiolysis (i.e., eaq (2.6), OH (2.7), H (0.6) - numbers in
parentheses are the G values, i.e., yields of radicals per 100 eV
experiments (see Figure 3).
2
9
of energy absorbed) to a reducing agent, namely 2-propanol
ketyl radical, over a wide pH range.
3. Conclusions
Transient radical cations and radicals observed during se-
•
-
e +N O f OH + OH +N
2
(1)
(2)
(3)
aq
2
quential oxidation of 1H and 2H, embedded in cryogenic
matrices, have been stabilized and spectroscopically character-
ized. Spectroscopic features of the identified species are
summarized in Table 1.
•
•
OH + (CH ) CHOH f (CH ) C OH +H O
3 2
3 2
2
•
•
H + (CH ) CHOH f (CH ) C OH +H
Conformational relaxation of the radical cations in rigid
matrices, induced by thermal softening of the matrix, led to
flattening of the intermediate radical cations. This was associated
with a blue shift of their long-wavelength electronic transitions.
It is interesting to note that flattening of the acridans during
oxidation begins immediately upon ionization. The removal of
one electron produces the radical cations which further depro-
tonate to give the radicals. Oxidation of the radicals, in turn,
results in formation of the adequate closed shell cations.
For all of the compounds studied a quantitative oxidation can
be achieved upon photolysis in aerated polar solvents. Laser
flash photolysis experiments revealed participation of the radical
cations in these processes.
3 2
3 2
2
R-Hydroxy alkyl radicals formed in reactions 2 and 3 with
the 85% yield (remaining 15% are unreactive â-hydroxy
radicals) are strongly reducing species that react with many
compounds via one-electron transfer. The pulse radiolysis
measurements in solution were made on system equipped with
the ELU-6 linear accelerator (17ns pulses delivering a dose of
2
7
1
0-50 Gy).
Laser Flash Photolysis. The laser flash photolysis system
employed a Lambda Physik EMG 101 excimer laser utilizing
XeCl for 308 nm emitting line with a pulse energy of 150 mJ
and a pulse width of ca. 20 ns. The concentrations of the 1-cm
thick samples were kept to have absorbance of 0.3-0.5 at 308
nm. An EG&G Princeton Applied Research model 1460 optical
multichannel analyzer (OMA) recorded transient absorption
spectra. The laser flash photolysis apparatus is described in more
4
. Experimental Section
Compounds. 3,6-Diamino-10-methylacridan (1H) was pre-
pared by sodium borohydride reduction of 3,6-diamino-10-
methylacridinium chloride, which was isolated from acriflavine
neutral (Aldrich) following a published procedure.26 The reduc-
tion product was purified by crystallization from CHCl3 to give
light-yellow needles (30%, mp. 127 °C, dec).
30
detail elsewhere.
Acknowledgment. This work was supported by the State
Committee for Scientific Research (grant no. 3/T09A/037/17),
the M. Skłodowska-Curie Joint Fund (grant no. MEN/NSF-97-
303), and the Lodz City Research Council. J.G. thanks Founda-
tion on behalf of Polish Science for support from the program
“Subsidies for Scholars”. We thank also Professor Thomas Bally
and Dr. Paweł Bednarek for calculation of the dihedral angles.
3
,6-Bis(dimethylamino)-10-methylacridan (2H) was prepared
by sodium borohydride reduction of 3,6-bis(dimethylamino)-
0-methylacridinium methyl sulfate, which was in turn obtained
by reaction of acridine orange (Aldrich) with dimethyl sulfate
Aldrich) in toluene solution. The reduction product was
crystallized from CHCl3 to give orange needles (45%, mp. 103
C, dec).
0-Methylacridan (3H) was prepared by sodium borohydride
1
(
References and Notes
°
(
1) Adamus, J.; G e¸ bicki, J.; Ciebiada, J.; Korczak, E.; Denys, A. J.
1
Med. Chem. 1998, 41, 2932.
reduction of 10-methylacridinium chloride according to a known
procedure and recrystallized three times from 96% EtOH. 10-
Methylacridinium chloride was prepared from acridine (Fluka,
purum) via 10-methylacridinium methyl sulfate.11
(2) Dean, A. C. R. In Acridines; Acheson, R. M., Ed.; Wiley:
Interscience: New York, 1973; pp. 789-813.
(
3) Nakamura, H. Mem. Konan UniVersity Sci. Ser. 1995, 42, 93.
4) Adamus, J.; G e¸ bicki, J.; Ciebiada, I.; Korczak, E.; Denys, A. Acta
(
Polon. Pharm.-Drug Research 1999, 56, 65.