Absorption and emission spectroscopic characterization of some azo dyes and a diamino-maleonitrile dye

Chemical Physics 333 (2007) 49–56 www.elsevier.com/locate/chemphys

Absorption and emission spectroscopic characterization of some azo dyes and a diamino-maleonitrile dye T. Susdorf a, A.K. Bansal a, A. Penzkofer a

a,*

, Sheng-Li Guo b, Jing-Min Shi

c

Institut II – Experimentelle und Angewandte Physik, Universita¨t Regensburg, Universita¨tstrasse 31, D-93053 Regensburg, Germany b Department of Physics, Nanjing University of Information Science and Technology, Nanjing 210044, PR China c Department of Chemistry, Shandong Normal University, Jinan 250000, PR China Received 4 November 2005; accepted 3 January 2007 Available online 9 January 2007

Abstract A linear and nonlinear optical spectroscopic characterization is carried out on three azo dyes (Reactive orange 1, Reactive violet 8, and Acidproof purplish red), and on N-(p-hydroxybenzylidene)-diamino-maleonitrile. Fluorescence quantum distributions, fluorescence quantum yields, and fluorescence lifetimes are measured. The saturable absorption is studied by nonlinear transmission measurements with intense picosecond laser pulses. The ground-state absorption recovery is studied by picosecond time-resolved pump and probe measurements. Absolute ground-state absorption cross-sections, excited-state absorption cross-sections, and dye concentrations are extracted from saturable absorption studies. The azo dyes have fluorescence lifetimes and ground-state absorption recovery times of around 2 ps and their excited-state absorption cross-sections are small (measured at 527 nm) making them good mode-locking dyes for picosecond and femtosecond lasers. The investigated diamino-maleonitrile dye exhibits sub-picosecond fluorescence lifetime and slow ground-state absorption recovery (>1 ns).  2007 Elsevier B.V. All rights reserved. Keywords: Reactive violet 8; Reactive orange 1; Acidproof purplish red; Maleonitrile dye; Fluorescence up-conversion; Pump–probe spectroscopy; Absorption spectroscopy; Fluorescence spectroscopy; Saturable absorption; Ground-state absorption cross-section; Excited-state absorption crosssection; Fluorescence lifetime; Absorption recovery time

1. Introduction An absorption and emission spectroscopic characterisation of the azo-dyes [1] C.I. Reactive orange 1 (RO1), C.I. Reactive violet 8 (RV8), Acidproof purplish red (APPR), and of the maleonitrile dye N-(p-hydroxybenzylidene)diamino-maleonitrile (2-amino-[[(4-hydroxyphenylmethylene]amino])-2-butenedinitrile, abbreviated DAMND) [2] is undertaken. The investigated azo-dyes find application in textile and paper colouring where acid proof purplish red is a direct dye, while reactive orange 1 and reactive violet 8 are reactive dyes [1,3]. DAMND crystals are new potential nonlinear optical materials [2].

Here we carry out linear absorption and emission measurements, and nonlinear transmission measurements. Fluorescence lifetimes are determined by femtosecond laser pulse fluorescence up-conversion. The ground-state absorption-recovery after picosecond pulse excitation is determined by pump-and-probe transmission measurements. Absolute ground-state absorption cross-sections, excited-state absorption cross-sections, and dye concentrations are extracted from non-linear transmission (saturable absorption) measurements and numerical simulations. The results indicate that the dyes may be applied in fast optical gating by saturable absorption [4,5]. 2. Experimental

*

Corresponding author. Tel.: +49 941 943 2107; fax: +49 941 943 2754. E-mail address: [email protected] (A. Penzkofer). 0301-0104/$ - see front matter  2007 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2007.01.003

The structural formulae of the investigated dyes are displayed in Fig. 1. Reactive orange 1 (RO1) is available from

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T. Susdorf et al. / Chemical Physics 333 (2007) 49–56 H N

NaO3S

N

Cl

N NaO3S

N

N

N OH

Cl

SO3Na

RO1: Reactive Orange 1 Cl H3CO

N

N

OH

N HN

N N

NaO3S

Cl

NaO3S

RV8: Reactive Violet 8 O

O OH

OH HN

N

N

N

N

N H

O NaO3S

N H

N H

SO3Na

APPR: Acidproof Purplish Red NC

NH2

NC

N

C H

OH

DAMND: Diamino-Maleonitrile Derivative Fig. 1. Structural formulae of investigated dyes. RO1: C.I. Reactive orange 1. Sum formula: C19H9Cl2N6Na3O10S3. Molar mass: Md = 717.38 g mol1. RV8: C.I. Reactive violet 8. C20H12Cl2N6Na2O8S2. Md = 645.36 g mol1. APPR: Acidproof purplish red. C37H28N8Na2O11S2, Md = 870.78 g mol1. DAMND: N-(p-hydroxybenzylidene)-diamino-maleonitrile. C11H8N4O, Md = 212.21 g mol1.

Shanghai Dyestuffs Coroporation, Limited, China. Reactive violet 8 (RV8) is distributed by Yick-Vic Chemicals & Pharmaceuticals (H.K.) Ltd., Kowloon, Hong Kong. Acidproof purplish red (APPR) is manufactured by Dezhou Xinda Chemical Co., Ltd, China. The diamino-maleonitrile dye DAMND was self-synthesized (J.-M.S). The method of synthesis is described in [2]. Information on the azo dyes is found in the Colour Index [3,6]. RO1 is a water soluble monoazo dye with a reactive dichlorotriazinyl group [7]. RV8 is also a water soluble monoazo dye with a dichlorotriazinyl group [8]. APPR [9] is a diazo dye with good solubility in water and dimethylformamide (DMF). DAMND [2] absorbs in the near UV. It is water insoluble, but good soluble in methanol and DMF. Transmission measurements, T(k), have been carried out with a commercial spectrophotometer (Beckman ACTA M IV and Varian Cary 50). The dye transmission is related to

the dye absorption coefficient, a, and the dye absorption cross-section, ra, by T ðkÞ ¼ exp½aðkÞ‘ ¼ exp½ra ðkÞN d ‘ ¼ exp½ra ðkÞ

wd mN A ‘; M dV

ð1Þ

where Nd is the number density of dye molecules in the solution and ‘ is the sample length. The number density of the dye molecules in solution is given by Nd ¼

wd mN A ; M dV

ð2Þ

where wd = md/m is the dye mass content in the dyestuff, m is the in-weighted mass of dyestuff (dye plus impurities), md = NdMdV/NA is the mass of the dye, Md is the molar

T. Susdorf et al. / Chemical Physics 333 (2007) 49–56

mass of the dye, NA is the Avogadro constant, and V is the volume of the solution. The fluorescence spectra are measured with a self-assembled fluorimeter in front face collection arrangement [10,11]. A high-pressure mercury lamp together with appropriate interference filters is used as excitation source. The intrinsic fluorescence quantum distributions, EF(k), of the dyestuffs are determined under conditions of vertically polarised excitation and magic angle fluorescence detection (54.7). The intrinsic fluorescence quantum yield, /F, is calculatedR from the fluorescence quantum distribution ð/F ¼ EF ðkÞ dkÞ. Dyes of known fluorescence quantum yield with absorption and emission in similar wavelength ranges as the studied dyes are used as fluorescence standards. These applied reference dyes are quinine sulphate dihydrate in 1 N H2SO4 (/F,R(C) = 0.546/(1 + 14.5C) at 25 C, where C is the dye concentration in mol dm3[12]) for DAMND, rhodamine 6G in methanol (/F,R = 0.94 [13]) for RO1 and APPR, and rhodamine 101 in ethanol (/F,R = 1.0 [13]) for RV8. The degree of fluorescence polarisation, PF = (SF,k  SF,?)/(SF,k + SF,?), is determined by measuring the fluorescence signal polarized parallel (SF,k) and perpendicular (SF,?) to the excitation light. The fluorescence lifetimes were measured by excitation with femtosecond pulses (of a mode-locked Ti–sapphire laser system (Hurricane from Spectra-Physics, second-harmonic pulses at kSH = 400 nm, vertical polarisation, pulse duration DtL  110 ps) and fluorescence up-conversion in a BBO crystal [14–16] (sum-frequency mixing of fluorescence light with horizontal polarized fundamental laser pulses at kL = 800 nm). The experimental arrangement is shown in [17]. Saturable absorption studies were carried out by intensity dependent transmission measurements [18]. The dyes RO1, RV8, and APPR were excited with second harmonic picosecond pulses (duration 6 ps) of a mode-locked Ndphosphate glass laser [19] (wavelength 527 nm, duration 6 ps). The diamino-maleonitrile dye DAMND was excited with second harmonic pulses of a mode-locked ruby laser [20] (duration 35 ps, wavelength 347.15 nm). The energy transmissions were measured as a function of pump pulse peak intensity. The absolute ground-state absorption cross-section is extracted from the rise of transmission with excitation intensity by comparison with numerical simulations: At a fixed small-signal transmission, the higher the ground-state absorption cross-section the lower is the number density of absorbing dye molecules and the smaller is the intensity needed to bleach the absorption. The excited-state absorption cross-sections are extracted from the energy transmission at high excitation intensity (depletion of ground-state population). The ground-state absorption recovery of the dyes is studied by picosecond laser pump and probe transmission measurements. The samples are excited with intense pump pulses (duration 1.4 ps, wavelength 400 nm, pulse energy 200 lJ) and probed with attenuated time-delayed pulses (same duration, wavelength, and polarization direction)

51

which are counter-propagating through the samples. The experimental arrangement is shown in [17]. The purity of the dyestuffs was tested by thin-layer chromatography using pre-coated TLC plates Silica Gel 60 without fluorescence indicator (# 5724 from Merck) and a mixture of 40 vol-% methanol and 60 vol-% toluene. No impurity was resolved by visual inspection and by transmission detection along the traces. 3. Results The absorption coefficient spectra of the investigated dyes are displayed in Fig. 2. For RO1 in water the absorption peak is at 500 nm. RV8 in methanol has its absorption peak at 565 nm. A vibronic structure of the S0–S1-band is resolved. For APPR the absorption spectrum is shown in the solvent dimethylformamide (DMF). The absorption peak is at 530 nm. DAMND in methanol has its absorption peak at 375 nm. The fluorescence quantum distributions, EF(k), of the investigated dyestuffs are shown in Fig. 3. The applied dye transmissions are listed in the figure captions. The excitation wavelengths, kexc, are written to the distribution R curves. The fluorescence quantum yields, /F ¼ EF ðkÞ dk, are collected in Table 1. In all cases the fluorescence quantum yields of the samples are low. The degrees of fluorescence polarisation, PF, are also listed in Table 1. They are large indicating fluorescence lifetimes, sF, short

Fig. 2. Absorption coefficient spectra: (a) reactive orange 1. Solvent: water. Dyestuff mass density m/V = 8.85 · 104 g cm3; (b) reactive violet 8. Solvent methanol. m/V = 1.27 · 104 g cm3; (c) acidproof purplish red. Solvent DMF. m/V = 1.92 · 104 g cm3 and (d) DAMND. Solvent methanol. m/V = 1.325 · 104 g cm3.

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T. Susdorf et al. / Chemical Physics 333 (2007) 49–56

compared to molecular reorientation times, sor, (PF ! 0 for sor  sF, and PF ! 0.5 for sor  sF [21]). The results of the fluorescence lifetime measurements are displayed in Fig. 4. The up-converted fluorescence traces are normalized to their peaks values, i.e., Supc(t)/ Supc,max is presented. The fluorescence excitation wavelength was kSH = 400 nm. The dot-connected triangles show the system response function. It was determined by femtosecond white-light generation [22,23] in a 1 mm quartz glass cell filled with water at the position of the fluorescence cell and stronger focussing to the sample. It has a trapezoid shape with 0.4 ps decay time on the 80–20% level. The fluorescence traces fit well to a bi-exponential decay (two-component single-exponential decay) according to S F ðtÞ ¼ S F;0 ½x1 expðt=sF;1 Þ þ x2 expðt=sF;2 Þ;

Fig. 3. Fluorescence quantum distributions. Excitation wavelengths are given in the figures. Sample length ‘ = 2 mm. (a) RO1 in water. Transmission, T0 (470 nm) = 0.14; (b) RV8 in methanol. T0 (526 nm) = 0.2; (c) APPR in DMF. T0 (500 nm) = 0.17; and (d) DAMND in methanol. T0 (365 nm) = 0.24.

ð3Þ

where x1 and x2 = 1–x1 are the weight factors, and sF,1 and sF,2 are the time constants. The fractions and fluorescence lifetimes are listed in the figures. Obtained sF,1 values are additionally listed in Table 1. The bi-exponential decay may indicate the presence of two decay channels caused by the short-wavelength excitation. It cannot be excluded that it is due to some artefact (laser and reflection subpulses). The probe-pulse transmission versus delay time for DAMND in DMF is displayed in Fig. 5 (pump and probe laser wavelength 400 nm, pulse duration 1.4 ps, cell length 1 mm, counter-propagation of pump pulse and probe

Table 1 Spectroscopic parameters Dye

Reactive orange 1

Reactive violet 8

Acidproof purplish red

DAMND

Solvent

Water

Methanol

DMF

Methanol

kS1,max (nm) kF,max (nm) nA nF /F,ds /F,d PF sF,1 (ps) srad (ns) ku (nm) sa,rec (ps) g (mPa s) sor (ps) ra,L (cm2) rex,L (cm2) rex,L/ra,L T0,L Nd (cm3) m/V (g cm3) wd

480 578 1.337 1.333 (1.6 ± 0.2) · 104 4.7 · 105 0.42 ± 0.02 1.18 ± 0.1 24.9 ± 3 420 1.2 0.89 [36] 258 1 · 1017 1.74 · 1018 0.174 0.48 7.34 · 1017 8.85 · 104 0.988

565 640 1.332 1.3294 (6.9 ± 0.5) · 104 9.6 · 105 0.40 ± 0.02 1.8 ± 0.2 18.8 ± 2 440 1.8 0.547 [37] 143 6.6 · 1017 1.06 · 1017 0.16 0.45 1.21 · 1017 1.27 · 104 1.0

525 592 1.431 1.4265 (8 ± 2) · 104 8 · 104 0.42 ± 0.02 2.9 ± 0.2 3.61 ± 0.5 450 2.9 0.805 [36] 264 4.8 · 1017 1.59 · 1017 0.22 0.51 1.40 · 1017 1.92 · 104 0.984

380 440 1.350 1.336 (1.15 ± 0.1) · 103 2 · 104 0.30 ± 0.03 0.5 ± 0.1 2.5 ± 0.3 291 >1000 0.547 [37] 47 1 · 1016 1.08 · 1017 0.162 0.11 2.21 · 1017 1.325 · 104 0.588

Comments

Fig. 2 Fig. 3 R /F; ds ¼ EF ðkÞ dk /F,d = sF,1/srad Fig. 4 Eq. (7) Fig. 2 Figs. 4 and 5 at 25 C Eq. (5) Fig. 6 Fig. 6 Fig. 6 Fig. 6 Eq. (5) Eq. (6)

Abbreviations: kS1,max: wavelength position of maximum S0–S1 absorption. kF,max: wavelength position of fluorescence maximum. nA: average refractive index in S0–S1 absorption region. nF: average refractive index in S1–S0 emission region. /F,ds: intrinsic fluorescence quantum yield of dyestuff. /F,d estimated intrinsic fluorescence quantum yield of dye. PF: degree of fluorescence polarization. sF,1: fluorescence lifetime determined from fluorescence upconversion measurements. srad: radiative lifetime. ku: upper wavelength limit for S0–S1 absorption. sa,rec: ground-state absorption recovery time. g: dynamic viscosity. sor: molecular reorientation time. ra,L: ground-state absorption cross-section at pump laser wavelength kL = 527 nm for RO1, RV8, APPR, and at kL = 347.15 nm for DAMND. rex,L: excited-state absorption cross-section at kL. T0,L: small-signal transmission at kL (sample length ‘ = 1 mm). Nd: dye number density. m/V: dyestuff concentration in g/mL. wd: dye mass content.

T. Susdorf et al. / Chemical Physics 333 (2007) 49–56

53

through the sample of 1 mm thickness. The transmission rises within the pump–probe transit overlap region. Then the transmission reduces partly with a time constant of about 30 ps, probably due to molecular reorientation, and afterwards remains constant within the time period of observation (540 ps). For the other dyes a pump pulse bleaching at 400 nm was observed (RO1 in water: T0 = 0.291, Tpu = 0.425; RO8 in methanol: T0 = 0.111, Tpu = 0.158; RO8 in water: T0 = 0.288, Tpu = 0.373; APPR in DMF: T0 = 0.264, Tpu = 0.302; APPR in water: T0 = 0.245, Tpu = 0.317). In the time delayed probe pulse measurements no probe pulse bleaching could be resolved indicating a ground-state recovery roughly within the time resolution of the pump– probe arrangement of about 6 ps (a probe pulse signal decrease artefact was observed in the pump and probe overlap region by some nonlinear optical effect like pump pulse induced probe pulse defocusing). The saturable absorption behaviour of the dyes RO1 in water, RV8 in methanol, APPR in DMF, and DAMND in methanol is shown in Fig. 6. For RO1, RV8, and APPR Fig. 4. Temporal fluorescence behaviour determined by femtosecond laser flurescence up-conversion. Line-connected circles: normalized fluorescence up-conversion signals. Dot-connected triangles: system response function. Dash-dotted curves: bi-exponential fits (Eq. (3)). Dyes and solvents together with the fit parameters are given in the sub-figures.

Fig. 5. Probe pulse transmission as a function of time delay between pump pulse and probe pulse for DAMND in methanol.

pulse). The small-signal transmission, T0, of the sample is indicated by a horizontal bar. The pump pulse transmission, Tpu, (increased transmission due to saturable absorption by ground-state depletion) is written in the figure. The time resolution is about 6 ps due to the finite pump and probe pulse duration and due to counter-propagation

Fig. 6. Intensity dependent energy transmission. Sample length ‘ = 1 mm. Dot-connected filled circles are measured. Solid curves are calculated. (a) Reactive orange 1 in aqueous solution (Millipore water). Input parameters are: small-signal transmission T0,L = 0.48, ground-state absorption recovery time sa,rec = 1.2 ps, reorientation time sor = 258 ps. Fit parameters are: ra,L = 1.0 · 1017 cm2, rex,L/ra,L = 0.174; (b) reactive violet 8 in methanol. Input parameters are: T0,L = 0.45, sa,rec = 1.8 ps, sor = 292 ps. Fit parameters are: ra,L = 6.6 · 1017 cm2, rex,L/ra,L = 0.106; (c) acidproof purplish red in DMF. Input parameters are: T0 = 0.51, sa,rec = 2.9 ps, sor = 264 ps. Fit parameters are: ra,L = 4.8 · 1017 cm2, rex,L/ra,L = 0.022 and (d) DAMND in methanol. Input parameters are: T0 = 0.11, sa,rec = 1000 ps, sor = 47 ps. Fit parameters are: ra,L = 1 · 1016 cm2, rex,L/ra,L = 0.162.

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T. Susdorf et al. / Chemical Physics 333 (2007) 49–56

the intensity dependent energy transmission of laser pulses of duration DtL  6 ps and wavelength kL = 527 nm (second harmonic of mode-locked Nd-glass laser) was measured. For DAMND the nonlinear transmission measurements were carried out with second harmonic pulses of a mode-locked ruby laser (DtL  35 ps, kL = 347.15 nm). In all cases the energy transmission rises with excitation intensity (saturable absorber behaviour) showing that the excited-state absorption is less than the ground-state absorption at the excitation wavelength. Below, the experimental energy transmission data will be fitted by numerical simulations in order to extract the dye number densities, Nd, the ground-state absorption crosssections, ra,L, and the excited-state absorption cross-sections, rex,L, of the investigated dyes.

a

4. Discussion The absorption cross-sections, ra,L, and the excited-state absorption cross-sections, rex,L, at the laser wavelength kL of saturable absorption measurement are extracted from the intensity dependent transmission results in the following. The complete absorption cross-section spectra are obtained from the absorption coefficient spectra, a(k), by the relation ra;L ra ðkÞ ¼ aðkÞ; ð4Þ aL

b

where aL is the absorption coefficient at kL. The number density of dye molecules, Nd, is extracted from the relation between aL and the absorption cross-section, ra,L, i.e., N d ¼ aL =ra;L ¼  lnðT 0;L Þ=ðra;L ‘Þ;

ð5Þ

where T0,L is the initial small-signal transmission, and ‘ is the sample length. The experimental energy transmission curves displayed in Fig. 6 are fitted by numerical simulations. The energy level scheme applied for the saturable absorption simulations is depicted in Fig. 7a. The pump laser excites the chromophores from the S0 ground-state to a Franck–Condon level 2’ in the S1 band. From there the molecules relax to a thermalized level 2 with the Franck–Condon relaxation time constant, sFC (sFC = 0.5 ps [24] is used in the simulations). From the S1 band excited-state absorption occurs to a higher lying singlet band Sn (level 3). The higher excited chromophores relax quickly back to the first excited singlet state with a time constant sex (sex = 60 fs is used in simulations [25]). The S1 state back relaxation to the ground-state is taken into account by the ground-state absorption recovery time, sa,rec. The laser excitation with linear polarized light causes population anisotropy because of the electric dipole interaction (preferential excitation of molecules with transition dipole moment parallel to polarization direction of excitation light). The reorientation time, sor, of the transition dipole moments of absorption is due to molecular reorientation which is approximately given by the Stokes–Einstein relation [26,27]

P1

P2

Fig. 7. (a) Energy level diagram for saturable absorption simulations and (b) proposed energy level scheme for DAMND consisting of two conformations (isomers), P1 and P2.

sor ¼

gV h ; kB#

ð6Þ

where g is the dynamic viscosity, Vh is the hydrodynamic volume of the molecule, kB is the Boltzmann constant, and # is the temperature. The hydrodynamic volume, Vh, may be approximated by the molecular volume, Vd, which is given by Vd = Md/(NAqd), where Md is the molar mass, NA is the Avogadro constant, and qd is the mass density (assumed to be qd = 1 g cm3 in the estimates). The viscosities, g, and estimated reorientation times are included in Table 1. The differential equation system for the intensity dependent laser pulse transmission through the samples is given in [28] (Eqs. 6–16 there) and is not repeated here. The curves in Fig. 6 are calculated with the parameters in the figure captions. The small-signal transmission, T0,L, the absorption recovery time, sa,rec, and the re-orientation time, sor, are used as input parameters. The ground-state

T. Susdorf et al. / Chemical Physics 333 (2007) 49–56

absorption cross-section, ra,L, and the excited state absorption cross-section, rex,L, are fitted. The dye number density, Nd, is given by Eq. (5). The best fits of ra,L and rex,L together with the extracted Nd values are given in Table 1. The dye mass content, wd, is estimated by use of Eq. (2), i.e., wd ¼ ðN d M d V =N A mÞ:

ð7Þ

The obtained dye mass contents of the investigated dyestuffs are included in Table 1. They are near to 100% for the three investigated azo-dyes. The uncertainty of the determination is estimated to be about 10%. For DAMND we determine a mass content of about 60%. We speculate that the dyestuff is rather pure, but that it consists of two conformations (isomers), P1 and P2 of number densities NP1 and NP2 (N0 = NP1 + NP2, wd = NP1/N0), with different ground-state absorption cross-sections. Photo-excitation is thought to cause transfer of P1 to P2 with a fast time constant of photo-induced transfer, sPIT  sF  0.5 ps. The photo-induced excess population of P2 in the ground-state is thought to recover to the P1 ground-state by thermally induced ground-state transfer with time constant, sGST  sa,rec > 1 ns. An energy level scheme with proposed transitions is shown in Fig. 7b. In this model description the overall grounds-state absorption at the laser frequency is aL = Ndra,L = Ndra,P1 + (N0  Nd)ra,P2, and aex,L = Ndrex,L giving ra,L = ra,P1 + [(1  wd)/wd]ra,P2, and rex,L = ra,P2/wd, or ra,P2 = wdrex,L and ra,P1 = ra,L  (1  wd)rex,L. For our experimental parameters we get ra,P1  9.3 · 1017 cm2 and ra,P2  9.5 · 1018 cm2 at kL = 347.15 nm. A more detailed analysis of DAMND is beyond the scope of this paper. The dyes RO1, RV8, and APPR have absorption recovery times short compared to the excitation laser pulse duration (sa,rec  sF,1 < DtL  6 ps). For the applied excitation laser pulse duration they are fast saturable absorbers (saturable absorption depends on pump pulse intensity) [29]. For DAMND in methanol the absorption recovery is slow compared to the excitation laser pulse duration (sa,rec  DtL  35 ps). DAMND acts as a slow saturable absorber (saturable absorption depends on pump pulse energy density [30]). For all investigated dyes the excited-state absorption cross-section, rex,L, was found to be small compared to the ground-state absorption cross-section, ra,L (good saturable absorbers). The S1–S0 radiative lifetime of the dyes is given by the Strickler–Berg formula [31–33], which reads R Z 1 8pc0 n3F em EF ðkÞ dk aðkÞ ra;L dk ¼ ; ð8Þ R 3 srad nA aL k EF ðkÞ dk abs k em where nF is the average refractive index of the dye solution in the fluorescence region, nA is the average refractive index in the emitting absorption band (S0–S1 absorption band), and EF(k) is the fluorescence quantum distribution. The integrals extend over the fluorescence region (em) and over the S0–S1 absorption band (abs). The relation ra(k) = a(k)-

55

ra,L/aL (Eq. (4)) is included in Eq. (8). The obtained radiative lifetimes are listed in Table 1. The used upper borders of the S0–S1 absorption bands, ku, are indicated in Fig. 2. The fluorescence efficiency of the investigated samples is rather small. The determined fluorescence quantum yields, /F,ds, of the samples (dyestuffs) are upper limits for the dyes since small quantities of moderate to good fluorescing impurities contribute over-proportionally to the measured fluorescence signals. The real fluorescence quantum yields, /F,d, of the dyes are estimated from the dye fluorescence lifetimes, sF,1, and the radiative lifetimes, srad, of the dyes, i.e., /F,d = sF,1/srad. The estimated fluorescence quantum yields, /F,d, are included in Table 1. The short fluorescence lifetimes and absorption recovery times of the dyes RO1, RV8, and APPR indicate an efficient internal conversion from S1 to S0. For DAMND the fluorescence lifetime is very short (sF  0.5 ps) and the ground-state absorption recovery is long (sa,rec> 1 ns). Some photo-isomerisation or intra-molecular molecular charge transfer may take place. A two-conformation model is proposed in Fig. 7b with ground-state population fractions wd in conformation P1 and 1-wd in conformation P2. The fast absorption recovery times of 1–3 ps and the small excited-state absorption cross-sections (small rex,L/ ra,L values) make the dyes RO1, RV8, and APPR good fast saturable absorbers in the wavelength range around 530 nm. They may be applied for passive mode-locking of solid-state lasers and dye lasers for picosecond and femtosecond pulse generation [18,34,35] and for fast optical gating with intense picosecond laser pulses [4,5]. The dye DAMND acts as a fast optical switch from low transmission to high transmission. 5. Conclusions Three azo dyes and one diamino-maleonitrile dye have been characterized by optical absorption and emission spectroscopy. The ground-state absorption crosssections, excited-state absorption cross-sections, and dye concentrations were extracted from saturable absorption measurements in connection with fluorescence-lifetime (femtosecond laser fluorescence up-conversion) and ground-state absorption recovery time (picosecond pump– probe transmission) measurements. The azo dyes, reactive orange 1, reactive violet 8, and acidproof purplish red, turned out to be fast saturable absorbers, while the investigated diamino-maleonitrile dye DAMND acts as a slow saturable absorber. All the investigated dyes may find application in nonlinear optical photonics. Acknowledgements The authors thank A. Merkel for technical assistance, and Professor R. Deutzmann for helpful discussions.

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