Transcript Two-photon absorption in tetraazachlorin

Two-Photon Absorption in Tetraazachlorin and its Benzo- and 2,3-naphto-fused
Derivatives: Effective symmetry of p-conjugation pathway
Mikhail Drobizheva, Nikolay S. Makarova, Aleksander Rebanea, Elena A. Makarovab, Evgeny A. Luk’yanetsb
aPhysics
Department, Montana State University-Bozeman, USA
bOrganic Intermediates and Dyes Institute, Moscow, Russia
1. In stable tautomers of H2TAC and H2TBTAC, the aromatic pathway is much
more similar to that of tetraazaporphyrin (and Pc), than to that of chlorin.
2. According to quantum chemical calculations of p-electron ring currents [2], in
tetraazaporphyrins and phthalocyanines the main current pathway does not
encounter the outer parts of two pyrrolenine rings, see figure:
800
10
300
8
6
2(i0/-1)
2
200
100
10
4
5
2
0
0
900
1000
1100
15000
laser, nm
20000
H2TAC and H2TBTAC:
Linear absorption, arb. u.
15
N
NH
N
Transition frequency, cm
H2TBTAC
1600
200
1200
-1
800
10
6
100
4
H2TBTAC and H2TNTAC
500
N
N
50
2
3000
H2TBTAC
0
900
1000
1100
15000
laser, nm
20000
25000
0
T1 T2
T2 T1
2
P = 6.4 mW/cm , T = 77K
0.6
0.4
0.4
0.0
700
800
600
, nm
0.4
T2
T1/2 = 82K
0.0
70
80
90
100
110
Temperature, K
120
T1 optical density
Absorption
0.6
0.2
800
Dark stability of T2 at 77K
0.40
T1
0.8
700
, nm
Thermal stability of photoproduct (T2)
8
30
6
20
4
10
2
2(i0/-1)
1000
1050
1100
0
10000
0.35
0.30
0.25
0.20
0
100
200
H2Pc
300
Sf
Gf
200
h
Ag->Ag
mfi
Si
h
100
mi0
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1000
laser, nm
1100
0
800
S0
900
laser, nm
1000
0
15000
20000
25000
Transition frequency, cm
1. Free-base chlorin analogues of tetraazaporphyrin, phthalocyanine,
and naphthalocyanine demonstrate photo-tautomerization
temperatures, intermediate between those of chlorin and
corresponding porphyrazine molecule.
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2. 2PA spectra of H2TAC and H2TBTAC resemble those of symmetric
tetraazaporphyrins and phthalocyanines. They show narrow and
strong 2PA peak near 920-940 nm, which is not reproduced in 1PA.
Peak 2PA cross sections are much larger in H2TAC and H2TBTAC, than
Main Results of 2PA Spectroscopy and Open Issues
0.2
600
50
laser, nm
0.2
0.0
100
0
950
0s
20 s
40 s
60 s
120 s
180 s
240 s
0.6
0s
20 s
40 s
60 s
120 s
240 s
Absorption
Absorption
0.8
150
N
Conclusions:
2
P = 12 mW/cm , T = 77K
40
Linear absorption, arb. u.
Photochemical switching between two tautomers in PVB at 77K
10
200
2, GM
Photo-Tautomerization and
Temperature Stability of T2 of H2TBTAC
N
H
N
Two-photon absorption spectrum of H2TBTAC (left) reminds that of symmetrical
metal-free phthalocyanine [3], H2Pc, right: Both show rather narrow peak at 920940 nm. For H2Pc it was assigned to Ag -> Ag transition [3]. An order of magnitude
increase of peak cross section in H2TBTAC can be explained by better resonance
conditions for 2PA and other factors. About the same enhancement is observed for
H2TAC, compared to H2TAP [4].
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250
2
Are these molecules closer to chlorin or tetraazaporphyrin
(and Pc, Nc) in terms of optical and photochemical
properties?
H2TNTAC
N
N
-1
2PA laser wavelength, nm
1200
C
N
N
H
0
30000
Transition frequency , cm
20001600
50
N
-
"Ag->Ag "
1000
0
N
HN
2000
2, GM
2
150
1000
N
NH
4. In H2TNTAC, the symmetry is lost either due to larger conjugation system,
or because of different tautomer (with reduced bond in one pyrrolenine ring)
is more stable. Therefore, in this case, one- and two-photon absorption have
similar selection rules, and, since one photon absorption is almost absent in
the spectral region 480 – 540 nm, 2PA is weak.
8
1500
HN
N
N
3. Therefore, the p-conjugation pathway an electronic wavefunction remain
centro-symmetric, like in symmetric tetraazaporphyrins and phthalocyanines.
2PA laser wavelength, nm
2000
N
H2TNTAC:
N
0
30000
25000
N
N
2, GM
H2TAC
1200
Linear absorption, arb. u.
H2TAC
1600
20
2(i0/-1)
Chemical Structures
Possible Explanation
2PA laser wavelength, nm
2, GM
Two-photon absorption (2PA) of non-symmetrical
metal-free tetrapyrroles can find use in ultrahigh capacity
(Terabit) re-writable 3D memory and other applications,
such as deeper-penetrating photodynamic therapy and
optical power limiting. In this work we study 2PA spectra
and cross sections of a family of substituted free-base
tetraazachlorin (H2TAC), tribenzo-tetraazachlorin
(H2TBTAC), and trinaphto-tetraazachlorin (H2TNTAC).
Because of non-symmetrical structure, all three molecules
can exist in the form of two unequivalent tautomers with
completely different absorption spectra. We sow, that
both tautomers are metastable at temperatures < 100 K
and can be interconverted by irradiation with light
resonant with the corresponding absorption peak. At room
temperature one of the form completely dominates (more
stable). Free-base tetraazachlorin and tribenzotetraazachlorin with non-symmetrical structure
demonstrate very narrow and strong 2PA-transition (~
1000 GM for the second), which is not present in onephoton absorption spectrum and reminiscent to that of
symmetrical phthalocyanines. We explain this fact by the
conservation of quasi-symmetric p-electron conjugation
pathway in the stable tautomer form of tetraazachlorin
and tribenzo-tetraazachlorin.
Two-Photon Absorption Spectra
(room temperature, in CH2Cl2)
2, GM
Abstract
300
Time, min
T1/2 = 82K is close to an average between chlorin
(50 K) [1] and Pc (120K) [1] values!
1. Strong and narrow 2PA peak is observed in H2TAC and
H2TBTAC spectra in the region between Q- and B-bands, near
900-950 nm.
2. Despite the absence of centrosymmetry, there is no onephoton counterpart of this peak.
Question (1): Why?
3. 2PA peak cross section of H2TBTAC is 5 times larger that of
H2TAC. This can be explained by more extensive conjugation
system in the former.
4. H2TNTAC does not show any distinct peak in the same region,
but rather increasing 2PA with photon frequency.
Question (2): Why?
5. Average 2PA cross section of H2TNTAC in this region is similar
to that of H2TAC, regardless much larger conjugation system
of the former.
Question (3): Why?
in H2TAP and H2Pc, respectively, which can be due to resonance enhancement
and stronger excited-state transition.
3. We explain the dissimilarity of one- and two-photon absorption
spectra in H2TAC and H2TBTAC by an effective centro-symmetry of
electronic wavefunctions in them.
4. H2TNTAC does not show any distinct peak in the same region and has
lower 2PA cross sections. This can be due to a breaking of
centrosymmetry of p-conjugation in this molecule.
References
1.
I. Renge, H. Wolleb, H. Spahni, U.P. Wild, J. Phys. Chem., A, 101 (1997) 6202.
2.
Y.B. Vysotsky, V.A. Kuzmitsky, K.N. Solovyov, Theor. Chim. Acta, 59 (1981) 467.
3.
M. Drobizhev, N.S. Makarov, Y. Stepanenko, A. Rebane, J. Chem. Phys., 124 (2006) 224701.
4.
M. Drobizhev, A. Karotki, M. Kruk, N.Zh. Mamardashvili, A. Rebane, Chem. Phys. Lett., 361 (2002) 504.
Acknowledgments: This work was supported by AFOSR and MBRCT