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Excited states and laser properties of tricarbocyanine dyes
Volume
18, number
EXCITED STATES AND LASER PROPERTIES J.P. FOUASSIER,
August
OPTICS COMMUNICATIONS
3
OF TRICARBOCYANINE
1976
DYES
D.J. LOUGNOT and J. FAURE
Laboratoire de Photochimie Gh&arale, b’quipe de Recherches Associhe au C.N.R.S. No 386, lkole Supkieure de Chimie 3, rue Alfred Werner, 68093 Mulhouse Cedex, France Received
4 February
1976,
Revised
version
received
13 April 1976
Photoisomer absorption and emission were obtained in various tricarbocyanine dyes by using a laser photolysis technique. Triplet spectra were also recorded in the wavelength range of the laser emission. The results allow the discussion of the behaviour of these compounds as saturable absorbers.
1. Introduction The process of photoisomerisation has been extensively studied, for example, in azocompounds [I] , stilbenic [2] or butadiene [3] derivatives, carbocyanines [4-71, polymethine dyes [9-131, etc. In the last cases, the photochemical characteristics of these molecules are of particular interest to workers in the field of dye lasers. It was shown [14] that the laser efficiency in flashlamp pumped systems is increased by a centrally located partial stiffening of the polymethine chain which would reduce the isomerisation process. Experiments on quenching effects in flashlamp excited tricarbocyanine dye lasers [ 151 provide evidence for substantial variations of the laser efficiency which can be related to the particular transient absorption of the molecule (photoisomer, triplet, . ..). Moreover some considerations about dye laser modelocking led to the suggestion that it was due to the generation of a photoisomer with a long lived ground state [16]. The investigation of l-3-3-, l’-3’-3’ hexamethyl indotricarbocyanine iodide (HITC) by conventional flash photolysis and picosecond spectroscopy technique [13] allowed us to obtain some information about photoisomer and triplet generation and excited singlet state absorption. The laser efficiency of HITC and its saturable absorber behaviour may be discussed in terms of these results. In this paper, measurements of photoisomer and triplet generation and fluoroes-
AS
Fig. 1. Experimental
setup for laser photolysis
measurement.
cence emission in tricarbocyanine dyes are submitted and mode-locking results described in a previous paper [ 171 are discussed.
2. Measurements
of photoisomer
and triplet generation
2. I. Experimental The excitation source consists of an oscillatoramplifier laser head with ruby, capable of delivering 1 J in a train of 6 to 7 pulses 300 ps fwhm (fig. 1) and has been described previously [ 13 ] . For excited-state absorption measurements, we use a high-pressure pulsed xenon arc XBO 150 W connected to a detection system (monochromator M25 263
Volume
18, number
Table 1 Spectroscopic DMSO. Dye
3
OPTICS COMMUNICATIONS
characteristics
of the tricarbocyanines
Absorption maximum
Emission maximum
(nm)
(run)
748 700 780 830 810
in DTTC
Photoisomer absorption maximum (run)
HITC DOTC DTTC 2,2’DTC DDTTC
780 750 820 870 850
805 730 830 880 855
1976
DDTTC (1, 22’DTCra
Relative generation yield of photoisomerisation 1 1.2 0.7 0.1 0.3
Huet, photomultiplier RCA XPlOOS, oscilloscope Tektronix 7904 with probe) which allows investigations beyond 20 ps. As will be shown hereafter, the subject of this work requires the kinetic behaviour of the transient to be followed in the order of 1~s. The xenon arc was accordingly pulsed to obtain a high brightness over a few hundred microseconds which is essential to decrease the relative fluorescence signal of the sample. The light intensity is increased by a factor of 25 in the range of investigated wavelengths. 2.2. Results The generation of the photoisomer was investigated in various cyanine dyes: 3,3’dimethyltricarbocyanine iodide (DOTC), 3,3’diethylthiatricarbocyanine iodide (DTTC), 1 ,l’diethyl 2,2’thiacarbocyanine iodide (2,2’DTC), 3,3’diethyl2,2’(4,5,4’,5’) dibenzothiatricarbocyanine iodide (DDTTC). Table 1 shows the results obtained in dimethylsulfoxide solution (optical density = 1 at 694 nm): wavelengths of absorption and emission maxima of the normal form, wavelength of photoisomer absorption maxima and relative generation yield of photoisomerisation (the results obtained in HITC [ 131 are quoted for comparison). The transient spectra are shown in fig. 2. The ground-state relaxation of the photoisomer was observed to decay via mixed first order kinetics and the actual lifetimes were in any case in the order of 20 ps and 200 /.QG. These complex kinetics will be analysed in a forthcoming paper in connection with the nature of the solvents used and of the molecules. 264
August
Fig. 2. So-Sr (--), T-T (- - - - -) and photoisomer of five polymethines dyes: HITC, DOTC, ( a.***) absorption DTTC, DDTTC and 2,2’DTC. T-T spectra are plotted in arbitrary units. Only photoisomer spectra of DDTTC and 2,2’DTC were recorded, as referred to in the report.
This fact supports the assumption that two photoisomers with similar absorption spectra are present. No triplet generation was observed when studying directly solutions of cyanine in DMSO or methanol, except for DOTC where a very weak transient absorption was detected in the order of some hundred nanoseconds. Transient spectra nevertheless were obtained for HITC, DTTC, DOTC dyes in DMSO solution by adding 1 M CH,I to generate an external heavy atom effect which increases the intersystem crossing (fig. 2). Even under such conditions, the absorptions were weak in DDTTC and DTC and did not allow any spectrum to be recorded. In aerated solution, the triplet lifetime was about 300 ns in methanol and 900 ns in DMSO. By outgassing the solution with N, bubbling, we obtained lifetimes in the order of a few microseconds.
Table 2 Photoisomer
August 1976
OPTICS COMMUNICATIONS
Volume 18, number 3
fluorescence maxima.
Dye
Photoisomer fluorescence maximum (run)
HITC DOTC DTTC 2,2’DTC DDTTC
860 800 890 960 910
3. Measurements of photoisomer fluorescence We used the mode locked ruby laser with a maximum density of excitation power of 300 MWcmp2 on the sample. The cell aperture was smaller than the laser beam diameter and caused all the dye in the cell to be irradiated. Upon removal of the diffuse 694 nm light, the fluorescence emitted normally to the laser beam and collected by a lens is focused upon the slit of a Huet monochromator and directed to a RCA XP1005 photomultiplier. The time integrated fluorescence intensity was read on an oscilloscope. The linearity of the detection system was checked over the range of intensities employed. The incident laser beam intensity was monitored using a beam splitter with a biplanar photodiode ITL and an oscilloscope Tektronix 7904 to control the fluctuation from shot to shot. Neutral density futers, calibrated at 694 nm provided the required attenuation. In high level excitation, we obtained, under low self absorption conditions, an anomalous fluorescence in the range of long wavelengths. By subtracting the normal fluorescence spectrum obtained from a spectrophotofluorometer, we determined the photoisomer fluorescence spectrum as is shown by fig. 3a in the case of HITC. The small distortion in the 790 nm range which disappears when the excitation power is decreased was ascribed to the stimulated emission of HITC. The maximum emission wavelengths are shown in table 2. The difference of energy between the So-S1 absorptions of the normal and photoisomer forms are in the order of a few kcal mole-l. Upon decreasing the excitation energy over a range of about 1O4 fold, the log-log plot of the fluorescence intensity at the normal and photoisomer emis-
x 0.5
X=780nmx/
./
-OS - / /’
/J
.
.
.
l
l
X=860nm
1f
-as-
x
./ log ~cxcifatlon 5
6
mrenslryl
7
Fig. 3. a) Fluorescence spectra obtained under high (curve a) and low (curve b) level excitation. Curve c is attributed to the fluorescence spectrum of the photoisomer form. The dashed lines in curves b and c originate from a lack in precision related to problems of measurement of curve b in the long wavelengths range and from the presence of stimulated emission in the 800 nm range which is superimposed on the fluorescence signal. b) Fluorescence intensities at 780 nm and 860 nm versus the excitation intensity.
sion maxima versus the laser power density assumed a slope of one (fig. 3b). At higher excitation, the photoisomer fluorescence is drastically increased as compared to normal fluorescence, owing to the fact that a significant amount of photoisomer molecules are pumped by the 694 nm beam, so that the same molecule can emit several photons of fluorescence during the running time of the excitation train. It should be noted that the photoisomer fluorescence is more difficult to observe when the spectra are red-shifted. In that case, indeed, the absorption cross section of the photoisomer at 694 nm becomes smaller. Moreover, we showed recently [ 181 that the back process from the photoisomer excited singlet state to 265
Volume 18, number 3
OPTICS COMMUNICATIONS
August 1976
Table 3 Characteristics of mode-locking dyes .
___Laser dye
Saturable absorber
Mode-locked laser emission. (nm)
-
x 10-16
7
0,” x 10-16
0; x 10-16
7s
(cm2)
(ns)
(cm21
(cm21
(ns)
De
DOTC
HITC
795-805
4
_
1
7.5
1.2
HITC
2,2’DTC
870-880
1.2
1.2
1
5
-
HITC
DTTC
857-863
1.3
1.2
< 0.5
1.6
2
-
the normal form is negligible in HITC. This result in tricarbocyanine is different from the one obtained by Arthurs et al. [ 161 in the case of a dicarbocyanine (DODC) and illustrates the significant differences in the behaviour of excited states of polymethine dyes.
4. Discussion of the mode-locking
process
Results obtained by Hirth et al. [ 171 in the field of near infra-red dye laser mode-locked by polymethine molecules lead to the assumption that it would be necessary to have a species with a singlet state lifetime shorter than the normal form. This hypothesis was also considered by Arthurs et al. [ 161 in the case of the rhodamine 6G dye laser mode-locked by 3,3’diethyloxadicarbocyanine iodide. In the theory of mode-locking of quasi continuous lasers developed by New [ 191 , three conditions must be taken into account to get a good mode-locking:
where ua is the absorption cross section of the saturable absorber, CT,the emission cross section of the laser dye, re the cavity round trip time, r and rs the relaxation times of the amplifying medium and of the saturable absorber respectively. Table 3 shows the three experimental cases studied by Hirth [17] and the related mode-locking range. We also report the values of ua in the case when the saturable absorber is the normal form (cJ,“) or the photoisomer form (u,P).’ It follows that in each case the first of the three conditions u,/u, > 1, is fulfilled only for the photoisomer as a saturable absorber. Experimentally, the ,266
best mode-locking is obtained for me = 1.2 ns [ 17 1, which means that TV must be lower than 1.2 ns. This condition is difficult to fulfil when rs is relevant to the values relating to the normal form of the molecule. A better agreement can be obtained by taking the value rs of the lifetime of the first excited singlet state of the photoisomer which presumably is shorter.
5. Conclusion We obtained photoisomer- and triplet absorption in tricarbocyanine molecules which are good Baser dyes in the near infrared [ 151. Spectra were recorded and the relative generation yields were measured: From the results obtained, we clearly show that HITC, DTTC, 2,2’DTC cannot be effective as saturable absorbers for mode-locking dye lasers operating in the spectral range referred to by table 3. In fact, the physical conditions set forth fully by New [19] in his theory of mode-locking of quasi continuous lasers are fulfilled only under the assumption that the photoisomer created during the pumping pulse functions as a saturable absorber in the dye lasers mode-locked by these polymethine compounds; these conclusions support previous speculations and comments concerning the behaviour of di- and tri-carbocyanine dyes [ 16, 171. In a more general way, this work must be put back into the context of our studies on photoisomerisation mechanisms [13,18,20] thus affording a better insight into the primary process of the excited states of tricarbocyanine dyes and allowing significant differences with carbocyanine [4,5,7] and dicarbocyanine [ 1 l] to be shown. The diagram of whole energy levels and the evidence for the processes in the excited states, allow the characteristics of the laser
Volume 18, number 3 emission described tions
of flashlamp in connection
OPTICS COMMUNICATIONS pumped
polymethines
to be
with their transient
absorp-
[20].
References [l] D.L. Ross and J. Blanc, in: Photochromism III (Wiley, Interscience, New-York, 1971). [2] D.V. Bent and D. Schulte-Frohlinde, J. Phys. Chem. 78 (1974) 446. [3] C. Rulliitre, J.P. Morand and .I. Joussot-Dubien, Opt. Comm. 15 (1975) 263. [4] W. Cooper and A. Rome, J. of Phys. Chem. 71 (1974) 16. [5] W. West, S. Pearce and F. Brum. J. Phys. Chem. 71 (1967) 1316. [6] P.J. MC&tin, J. Chem. Phys. 42 (1965) 2980. [7] J.T. Knudtson and E.M. Eyring, J. Phys. Chem. 78 (1974) 2355. [8] D. Madge and M.W. Windsor, Chem. Phys. Lett. 27 (1975) 31. [9] G.E. Busch, K.S. Greve, G.L. Olson, R.P. Jones and
August 1976
P.M. Rentzepis, Chem. Phys. Lett. 33 (1975) 412; 33 (1975) 415. [lo] D.J. Bradley, M.H.R. Hutchinson and H. Koetser. Proc. Roy. Sot. A329 (1972) 105. 1111 E.G. Arthurs, D.J. Bradley and A. Roddie, Chem. Phys. Lett. 22 (1973) 230. 1121D.N. Dempster, T. Morrow, R. Rankin and G.F. Thomp son, J. Chem. Sot. Faraday II, 68 (1972) 1479. I131 J.P. Fouassier, D.J. Lougnot and J. Fauri, Chem. Phys. Lett. 35 (1975) 189. [I41 J.P. Webb, F.G. Webster and B.E. Plourbk, Eastman Org. Chem. Bull. 46 (1974) 1. 1151 A. Hirth, J. Faure and D.J. Lougnot, Opt. Commun. 7 (1973) 339. [I61 E.G. Arthurs, D.J. Bradley and A.G. Roddie, Opt. Commun. 8 (1973) 118. 1171 A. Hirth, J. Faurk and D.J. Lougnot, Opt. Commun. 8 (1973) 318. 1181 D.J. Lougnot, J.P. Fouassier and J. FaurB, C.R. Acad. Sci. (Paris) 282C (1976) 265. 1191 G.H.C. New, IEEE J. Quantum Electronics 10 (1974) 115. 1201 J.P. Fouassier, D.J. Lougnot and J. Faur&, to be published.
267
18, number
EXCITED STATES AND LASER PROPERTIES J.P. FOUASSIER,
August
OPTICS COMMUNICATIONS
3
OF TRICARBOCYANINE
1976
DYES
D.J. LOUGNOT and J. FAURE
Laboratoire de Photochimie Gh&arale, b’quipe de Recherches Associhe au C.N.R.S. No 386, lkole Supkieure de Chimie 3, rue Alfred Werner, 68093 Mulhouse Cedex, France Received
4 February
1976,
Revised
version
received
13 April 1976
Photoisomer absorption and emission were obtained in various tricarbocyanine dyes by using a laser photolysis technique. Triplet spectra were also recorded in the wavelength range of the laser emission. The results allow the discussion of the behaviour of these compounds as saturable absorbers.
1. Introduction The process of photoisomerisation has been extensively studied, for example, in azocompounds [I] , stilbenic [2] or butadiene [3] derivatives, carbocyanines [4-71, polymethine dyes [9-131, etc. In the last cases, the photochemical characteristics of these molecules are of particular interest to workers in the field of dye lasers. It was shown [14] that the laser efficiency in flashlamp pumped systems is increased by a centrally located partial stiffening of the polymethine chain which would reduce the isomerisation process. Experiments on quenching effects in flashlamp excited tricarbocyanine dye lasers [ 151 provide evidence for substantial variations of the laser efficiency which can be related to the particular transient absorption of the molecule (photoisomer, triplet, . ..). Moreover some considerations about dye laser modelocking led to the suggestion that it was due to the generation of a photoisomer with a long lived ground state [16]. The investigation of l-3-3-, l’-3’-3’ hexamethyl indotricarbocyanine iodide (HITC) by conventional flash photolysis and picosecond spectroscopy technique [13] allowed us to obtain some information about photoisomer and triplet generation and excited singlet state absorption. The laser efficiency of HITC and its saturable absorber behaviour may be discussed in terms of these results. In this paper, measurements of photoisomer and triplet generation and fluoroes-
AS
Fig. 1. Experimental
setup for laser photolysis
measurement.
cence emission in tricarbocyanine dyes are submitted and mode-locking results described in a previous paper [ 171 are discussed.
2. Measurements
of photoisomer
and triplet generation
2. I. Experimental The excitation source consists of an oscillatoramplifier laser head with ruby, capable of delivering 1 J in a train of 6 to 7 pulses 300 ps fwhm (fig. 1) and has been described previously [ 13 ] . For excited-state absorption measurements, we use a high-pressure pulsed xenon arc XBO 150 W connected to a detection system (monochromator M25 263
Volume
18, number
Table 1 Spectroscopic DMSO. Dye
3
OPTICS COMMUNICATIONS
characteristics
of the tricarbocyanines
Absorption maximum
Emission maximum
(nm)
(run)
748 700 780 830 810
in DTTC
Photoisomer absorption maximum (run)
HITC DOTC DTTC 2,2’DTC DDTTC
780 750 820 870 850
805 730 830 880 855
1976
DDTTC (1, 22’DTCra
Relative generation yield of photoisomerisation 1 1.2 0.7 0.1 0.3
Huet, photomultiplier RCA XPlOOS, oscilloscope Tektronix 7904 with probe) which allows investigations beyond 20 ps. As will be shown hereafter, the subject of this work requires the kinetic behaviour of the transient to be followed in the order of 1~s. The xenon arc was accordingly pulsed to obtain a high brightness over a few hundred microseconds which is essential to decrease the relative fluorescence signal of the sample. The light intensity is increased by a factor of 25 in the range of investigated wavelengths. 2.2. Results The generation of the photoisomer was investigated in various cyanine dyes: 3,3’dimethyltricarbocyanine iodide (DOTC), 3,3’diethylthiatricarbocyanine iodide (DTTC), 1 ,l’diethyl 2,2’thiacarbocyanine iodide (2,2’DTC), 3,3’diethyl2,2’(4,5,4’,5’) dibenzothiatricarbocyanine iodide (DDTTC). Table 1 shows the results obtained in dimethylsulfoxide solution (optical density = 1 at 694 nm): wavelengths of absorption and emission maxima of the normal form, wavelength of photoisomer absorption maxima and relative generation yield of photoisomerisation (the results obtained in HITC [ 131 are quoted for comparison). The transient spectra are shown in fig. 2. The ground-state relaxation of the photoisomer was observed to decay via mixed first order kinetics and the actual lifetimes were in any case in the order of 20 ps and 200 /.QG. These complex kinetics will be analysed in a forthcoming paper in connection with the nature of the solvents used and of the molecules. 264
August
Fig. 2. So-Sr (--), T-T (- - - - -) and photoisomer of five polymethines dyes: HITC, DOTC, ( a.***) absorption DTTC, DDTTC and 2,2’DTC. T-T spectra are plotted in arbitrary units. Only photoisomer spectra of DDTTC and 2,2’DTC were recorded, as referred to in the report.
This fact supports the assumption that two photoisomers with similar absorption spectra are present. No triplet generation was observed when studying directly solutions of cyanine in DMSO or methanol, except for DOTC where a very weak transient absorption was detected in the order of some hundred nanoseconds. Transient spectra nevertheless were obtained for HITC, DTTC, DOTC dyes in DMSO solution by adding 1 M CH,I to generate an external heavy atom effect which increases the intersystem crossing (fig. 2). Even under such conditions, the absorptions were weak in DDTTC and DTC and did not allow any spectrum to be recorded. In aerated solution, the triplet lifetime was about 300 ns in methanol and 900 ns in DMSO. By outgassing the solution with N, bubbling, we obtained lifetimes in the order of a few microseconds.
Table 2 Photoisomer
August 1976
OPTICS COMMUNICATIONS
Volume 18, number 3
fluorescence maxima.
Dye
Photoisomer fluorescence maximum (run)
HITC DOTC DTTC 2,2’DTC DDTTC
860 800 890 960 910
3. Measurements of photoisomer fluorescence We used the mode locked ruby laser with a maximum density of excitation power of 300 MWcmp2 on the sample. The cell aperture was smaller than the laser beam diameter and caused all the dye in the cell to be irradiated. Upon removal of the diffuse 694 nm light, the fluorescence emitted normally to the laser beam and collected by a lens is focused upon the slit of a Huet monochromator and directed to a RCA XP1005 photomultiplier. The time integrated fluorescence intensity was read on an oscilloscope. The linearity of the detection system was checked over the range of intensities employed. The incident laser beam intensity was monitored using a beam splitter with a biplanar photodiode ITL and an oscilloscope Tektronix 7904 to control the fluctuation from shot to shot. Neutral density futers, calibrated at 694 nm provided the required attenuation. In high level excitation, we obtained, under low self absorption conditions, an anomalous fluorescence in the range of long wavelengths. By subtracting the normal fluorescence spectrum obtained from a spectrophotofluorometer, we determined the photoisomer fluorescence spectrum as is shown by fig. 3a in the case of HITC. The small distortion in the 790 nm range which disappears when the excitation power is decreased was ascribed to the stimulated emission of HITC. The maximum emission wavelengths are shown in table 2. The difference of energy between the So-S1 absorptions of the normal and photoisomer forms are in the order of a few kcal mole-l. Upon decreasing the excitation energy over a range of about 1O4 fold, the log-log plot of the fluorescence intensity at the normal and photoisomer emis-
x 0.5
X=780nmx/
./
-OS - / /’
/J
.
.
.
l
l
X=860nm
1f
-as-
x
./ log ~cxcifatlon 5
6
mrenslryl
7
Fig. 3. a) Fluorescence spectra obtained under high (curve a) and low (curve b) level excitation. Curve c is attributed to the fluorescence spectrum of the photoisomer form. The dashed lines in curves b and c originate from a lack in precision related to problems of measurement of curve b in the long wavelengths range and from the presence of stimulated emission in the 800 nm range which is superimposed on the fluorescence signal. b) Fluorescence intensities at 780 nm and 860 nm versus the excitation intensity.
sion maxima versus the laser power density assumed a slope of one (fig. 3b). At higher excitation, the photoisomer fluorescence is drastically increased as compared to normal fluorescence, owing to the fact that a significant amount of photoisomer molecules are pumped by the 694 nm beam, so that the same molecule can emit several photons of fluorescence during the running time of the excitation train. It should be noted that the photoisomer fluorescence is more difficult to observe when the spectra are red-shifted. In that case, indeed, the absorption cross section of the photoisomer at 694 nm becomes smaller. Moreover, we showed recently [ 181 that the back process from the photoisomer excited singlet state to 265
Volume 18, number 3
OPTICS COMMUNICATIONS
August 1976
Table 3 Characteristics of mode-locking dyes .
___Laser dye
Saturable absorber
Mode-locked laser emission. (nm)
-
x 10-16
7
0,” x 10-16
0; x 10-16
7s
(cm2)
(ns)
(cm21
(cm21
(ns)
De
DOTC
HITC
795-805
4
_
1
7.5
1.2
HITC
2,2’DTC
870-880
1.2
1.2
1
5
-
HITC
DTTC
857-863
1.3
1.2
< 0.5
1.6
2
-
the normal form is negligible in HITC. This result in tricarbocyanine is different from the one obtained by Arthurs et al. [ 161 in the case of a dicarbocyanine (DODC) and illustrates the significant differences in the behaviour of excited states of polymethine dyes.
4. Discussion of the mode-locking
process
Results obtained by Hirth et al. [ 171 in the field of near infra-red dye laser mode-locked by polymethine molecules lead to the assumption that it would be necessary to have a species with a singlet state lifetime shorter than the normal form. This hypothesis was also considered by Arthurs et al. [ 161 in the case of the rhodamine 6G dye laser mode-locked by 3,3’diethyloxadicarbocyanine iodide. In the theory of mode-locking of quasi continuous lasers developed by New [ 191 , three conditions must be taken into account to get a good mode-locking:
where ua is the absorption cross section of the saturable absorber, CT,the emission cross section of the laser dye, re the cavity round trip time, r and rs the relaxation times of the amplifying medium and of the saturable absorber respectively. Table 3 shows the three experimental cases studied by Hirth [17] and the related mode-locking range. We also report the values of ua in the case when the saturable absorber is the normal form (cJ,“) or the photoisomer form (u,P).’ It follows that in each case the first of the three conditions u,/u, > 1, is fulfilled only for the photoisomer as a saturable absorber. Experimentally, the ,266
best mode-locking is obtained for me = 1.2 ns [ 17 1, which means that TV must be lower than 1.2 ns. This condition is difficult to fulfil when rs is relevant to the values relating to the normal form of the molecule. A better agreement can be obtained by taking the value rs of the lifetime of the first excited singlet state of the photoisomer which presumably is shorter.
5. Conclusion We obtained photoisomer- and triplet absorption in tricarbocyanine molecules which are good Baser dyes in the near infrared [ 151. Spectra were recorded and the relative generation yields were measured: From the results obtained, we clearly show that HITC, DTTC, 2,2’DTC cannot be effective as saturable absorbers for mode-locking dye lasers operating in the spectral range referred to by table 3. In fact, the physical conditions set forth fully by New [19] in his theory of mode-locking of quasi continuous lasers are fulfilled only under the assumption that the photoisomer created during the pumping pulse functions as a saturable absorber in the dye lasers mode-locked by these polymethine compounds; these conclusions support previous speculations and comments concerning the behaviour of di- and tri-carbocyanine dyes [ 16, 171. In a more general way, this work must be put back into the context of our studies on photoisomerisation mechanisms [13,18,20] thus affording a better insight into the primary process of the excited states of tricarbocyanine dyes and allowing significant differences with carbocyanine [4,5,7] and dicarbocyanine [ 1 l] to be shown. The diagram of whole energy levels and the evidence for the processes in the excited states, allow the characteristics of the laser
Volume 18, number 3 emission described tions
of flashlamp in connection
OPTICS COMMUNICATIONS pumped
polymethines
to be
with their transient
absorp-
[20].
References [l] D.L. Ross and J. Blanc, in: Photochromism III (Wiley, Interscience, New-York, 1971). [2] D.V. Bent and D. Schulte-Frohlinde, J. Phys. Chem. 78 (1974) 446. [3] C. Rulliitre, J.P. Morand and .I. Joussot-Dubien, Opt. Comm. 15 (1975) 263. [4] W. Cooper and A. Rome, J. of Phys. Chem. 71 (1974) 16. [5] W. West, S. Pearce and F. Brum. J. Phys. Chem. 71 (1967) 1316. [6] P.J. MC&tin, J. Chem. Phys. 42 (1965) 2980. [7] J.T. Knudtson and E.M. Eyring, J. Phys. Chem. 78 (1974) 2355. [8] D. Madge and M.W. Windsor, Chem. Phys. Lett. 27 (1975) 31. [9] G.E. Busch, K.S. Greve, G.L. Olson, R.P. Jones and
August 1976
P.M. Rentzepis, Chem. Phys. Lett. 33 (1975) 412; 33 (1975) 415. [lo] D.J. Bradley, M.H.R. Hutchinson and H. Koetser. Proc. Roy. Sot. A329 (1972) 105. 1111 E.G. Arthurs, D.J. Bradley and A. Roddie, Chem. Phys. Lett. 22 (1973) 230. 1121D.N. Dempster, T. Morrow, R. Rankin and G.F. Thomp son, J. Chem. Sot. Faraday II, 68 (1972) 1479. I131 J.P. Fouassier, D.J. Lougnot and J. Fauri, Chem. Phys. Lett. 35 (1975) 189. [I41 J.P. Webb, F.G. Webster and B.E. Plourbk, Eastman Org. Chem. Bull. 46 (1974) 1. 1151 A. Hirth, J. Faure and D.J. Lougnot, Opt. Commun. 7 (1973) 339. [I61 E.G. Arthurs, D.J. Bradley and A.G. Roddie, Opt. Commun. 8 (1973) 118. 1171 A. Hirth, J. Faurk and D.J. Lougnot, Opt. Commun. 8 (1973) 318. 1181 D.J. Lougnot, J.P. Fouassier and J. FaurB, C.R. Acad. Sci. (Paris) 282C (1976) 265. 1191 G.H.C. New, IEEE J. Quantum Electronics 10 (1974) 115. 1201 J.P. Fouassier, D.J. Lougnot and J. Faur&, to be published.
267