Volume 121. number 4,5 CHEMICAL PHYSICS LETzlERS lS.November 1985

PICOSECOND TIME-RESOLVED ABSORPTION AND GAIN SPECTROSCOPY OF A. GIANT DIPOLE MOLECULE, 4-DIE ~~NO~-N~OS~LBE~ IN SOLUTION AND IN POLYMER FILM

Takayoshi KOBAYASHI, Hiroyuki OHTANI ’ and Kenji KUROKAWA

Deparxmettr of Physics. Facuisy of Science. Universify OJ Tokyo. Hongo 7-3-i, Bunh30. Tokyo J J3. .J~pon

Received 12 July 1985; in find form 7 Augusl1985

Picosecond absorpGm and gain spawa of ~dicrhylamin~-nilroslilbenc (DEANS) were measured wiih a mode-locked Nd:YAG laser. The largcs~ gain ever reponed (1.1 X103 cm-‘) was obtained by a DEANS (0.7 mol dm-“) doped polymer film. The S, -* S,, gain of DEANS (1.2x lo-’ mol dm -3) in benzene decreases with Ihe photon tlwt of the probe light from 6 x Ib” to 3 x 10” phoIons/cm’ s in khe X30-680 nm region.

1. Introduction

Stilbene derivatives with electronilonative group(s) attached to one benzene ring and electron-withdrawing group(s) to the other are interesting molecules with respect to their fluorescence with an anomalously large Stokes shift which is sensitive to solvent polarity [l-3]. Other interesting features are trans-cis photo- isomerization f4-81, participation of triplet state in this isomerization [9], the possibility of laser oscillation [ lo,1 11, and large non-linear optical properties [ 12- 14] expected from their giant perm~ent dipole moments especially in the excited singlet states [l 2,

1 I]. Lippert et al. observed large Stokes shifts in the emission spectra in polar solvents and attributed these shifts to their dipole moments in the lowest excited states [2]. Dipole moments increase in the process of equ~ibration in the lowest excited singlet states. The equilibration includes charge transfer in the molecule and reorientation of solvent molecules within excited slr@et lifetimes { 1,2]_ Such a large dipole moment induced by excitation will obviously result in a strong solvent-solute interaction and affect the isomerization [4-81. For example, Schulte-Frohlinde found that the trans-cis photoisomerization of ~ime~yl~~o-

’ Visiting research Ccllow from Ha mama~~ Pholonics K. K, Hamamalsu. Shizuoka 435, Japan.

4’-nitrostilbene did not take place in alcoholic solvents while it was efficient in non-polar solvents [4]. In our previous papers, we proposed the possibility of the application of molecular systems with giant dipole moments to optically bistable devices utilizing its large third-order non-linear ~sceptib~ity induced by the dipole moments [I 3,141. In order to pursue the possible application, we need fundamental data of linear and non-linear spectroscopy of the systems. In This paper we report on the picosecond time-resolved absorption and gain spectra of 4-diethyhunino4’- nitrost~bene (DEANS) in benzene solution and in polymer film.

2. Experimental

2.1. picosecond laret spectroscopy

A mode-locked Nd :YAG laser (Quantel, YG 472) was used for both excitation and monitoring light sources. The third harmonic (9 mJ, 30 ps, 300 MW) of the amplified output with a two-stage amplifier from the laser was used as an excitation light source. The diameter of the excited area on the sample cell was 2 mm. The 1064 nm pulse was focused into a 10 cm cell containing D20 to generate the picosecond cont~uum light used as a mo~to~g light. The con-

356 0 009-2614/85/S 03.30 0 EL&ier Science Publishers B-V. ~o~-Ho~~d physics publishing Division)

-Volume 121, number 43 CHEMICAL PHYSICS LETTERS 15 November 1985

tinuum light was detected with two combined systems of a polychromator and a multichannel photodiode array. All data.for the measurement of transient ab- sorption and gain spectra were averaged over 12-16 excitation-non-excitation data sets.

2.2. Samples

A synthesized polycrystalline sample of DEANS and a polycarbonate ftim doped with DEANS were kind gifts from Dr. Tetsuro Murayama, Mitsubishi Chemical Industries Ltd. Research Center. Crystalline DEANS was purified by repeated recrystallization from tetrahydrofuran and benzene solutions. Benzene (Wake, spectrograde) was used as a solvent without further purification. The concentrations of DEANS in benzene and in the film were 1.2 X 10m3 mol dmm3 (1.2 mM) and 0.7 mol dmS3, respectively_ Internal thickness of benzene solution sample was 2 mm while polymer thickness was 1 m.

3. Results and discussion

3.1. Picosecond time-resolved absorplion and gain spectra

The maxima of the ordinary absorption spectra of DEANS in the film and benzene solution are located at 450 and 440 nm, respectively. The longer wavelength for the polycarbonate film sample than that for the benzene solution is due to its higher polarity. The picosecond time-resolved absorption and gain spectra observed at delay times of -135,42, -10,41,90, and 490 ps are shown in fig. L The intensities of the

fluorescence emitted spontaneously from samples both in solution and in film were found to be much weaker than the picosecond continuum light and not to affect the absorption/gain spectrum. Since the excitation and probe.pulses had almost the same pulse width of 30 ps, the spectrum observed at -13.5 ps gave the noise level of the picosecond spectroscopy apparatus used in the present study. The noise level was f 0.014 in units of absorbance or gain for the spectral region measured (500-650 nm). In all the tune-resolved spectra except at -135 and 42 ps, the absorption (positive absorbance change) was observed in the spectral region shorter than about 590 nm. In the

b

I. J 500 550 600 650

Wavelenalhfnm)

Fig. I_ Picosecond time-resolved absorption and gain spectrum of DEANS (1.2 X 1r3 mol dmm3) in benzene at delay times of (a) -135, t-b) -42. (cl -10, (d) 41. (e) 90. and (f) 490 ps. Negative delay times indigte that the intensity maximum of the probe pulse arrives at the sample before the peak of the excitation pulse. A positive change is due to absorption while a negative one is due not to the ground state depletion but to gain, since there is no ground-state absorption in the spectral region where a negative change is observed.

region longer than 5 90 nm, gain (negative absorbance change) was observed. The peak wavelength of the new absorption was around 570 nm; that of the gain was not clear, but it seemed to be between 610 and 645

nm. In this experiment the probe light energy density was kept between 1 and 5 fl/cmz. The normalized spontaneous fluorescence spectrum,f@), of the sample was obtained by setting /rf@) dh equal to the fluo- rescence quantum yield. The gain spectrum So) was obtained by

S(X) -N(SF) h4f( ?1)/8ircrn 2 , (1) where 7 is the fluorescence lifetime, n is the refractive

index of the solvent, c is the light velocity in vacuum, and N(SF) is the concentration of DEANS in the lowest excited singlet state after relaxation (SR)_ This equation is obtained on the assumption that t k - ere rs

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Volume 121. number 4,5 CHEMICAL PHYSICS LETTERS 15 November 1985

a, i!!

0.1

2

B 0.c a -4

-0.1

- 0.1 5

I-

,*

I”

1’

I .

,_

lot 1 550 600 650 Wavelenglh (nm)

Fig. 2. The observed (curve a) nod corrected (curve b) S, -SF absorption. The correction was made by the use of the QI- culated gain spectrum (curve c) obtained by eq. (1) in the text and the normalized spontaneous emission spectrum f m.

no absorption due to S, (the nth excited singlet state) +-SF transition or T, (the nth excited triplet state) +T1 (the lowest excited triplet state) transition in the spectral region of the gain. The obtained gain spectrum, S(X), is shown in fig. 2_ With this calculated result and the assumption that there is no transient absorption in the 620-680 nm region we can obtain the corrected transient absorption in the overlap region of Sy + So (ground state) gain and transient absorp- tion. The result is shown in lig. 2.

The time dependence of the intensity of the tran- sient absorption at 570 nm and gain at 630 nm of DEANS in benzene is shown in fig. 3. The data are obtained after the correction of the intensity fluctua- tion of the exciting pulse at 355 nm. The rise time of the transient absorption, obtained from the data to be 44 f 3 ps, agrees that of the gain, 41 + 3 ps, within experimental error. The observed time constants cor- respond to the reorientation of the solvent molecules induced by the large increase in the dipole moment in the lowest excited state S, _ The permanent dipole moment in the excited state increases further with the change in the configuration of the solvent mole- cules. The relaxation from the Franck-Condon state in the lowest excited singlet state STC to the relaxed lowest excited singlet state (SF) is due to both the solvent reorientation and conformation reorganisation

3.58

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. I

.

0 0

02 - 0

-+

+-

0.0 - e, -MO 0 200 Loo

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0.L

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D-0

Fig. 3. The time dependence of the transient absorbance change, at 570 nm (AA) and gain at 630 nm (C) of DEANS (1.2 x 1r3 mol dmm3) in benzene solution.

. .

induced by the change in the electronic configuration of the intramolecular charge-transfer complex of DEANS.

The observed gain spectrum agreed well with that observed for DEANS in solution pimped with nano- second pulses from N2 laser f 151. The agreement is also satified in the wavelength region where the gain and absorption overlap each other. The gain spectrum 10 ns after the excitation agrees with that obtained after several tens to several hundreds of picoseconds. The lifetimes of the gain and the transient absorption are identical. Since both the rise times and lifetimes of the gain and the transient absorption are common, it is concluded that there is a common initial state of the transitions relevant to these two processes. There- fore, the transient absorption is concluded to be due to S, + SF transition.

3.2. Gain saturation with the increase in probe iight intensiiy

Fig. 4 shows the time-resolved gain and absorption spectra 90 ps after the excitation of DEANS in benzene by 355 nm picosecond pulse. When the probe light intensity was increased, the gain decreased. The gain was approximately inversely proportional to the probe light intensity. The lifetimes of the excited singlet state in 4-d~e~ylamino~‘-~trost~bene in benzene and chlorobenzene are 3.3 ns [3] and 2.2 ns [3], re-- spectively. Therefore, the initial state of both gain and absorption processes is the lowest excited sin

i? et

state after the solvent reorientational relaxation; S, _ There are four tentative reasons for the saturation of

Volume 121, number 4.5 CHEMICAL PHYSICS LETTERS 15 November 19 85

500 550 600 650 Wavelwqth(nm)

Fig. 4. Probe intensity dependence of the transient absorption and gain spectrum 90 ps after excitation. The relative in- tensity of the probe light is decreased from top to bottom. The probe photon fluxes are (a) 3 X 1023, (b) 1.5 x 1023, (c) 1 x 10’3. and (d) 6 X 1OJ2 photons/cm2 s.

the gain: (1) The leading edge of the probe pulse in-

duces isometiation of DEANS in solution. This de- crease in the gain is due to the appearance of the cis

form in the ground state. (2) The leading edge of the probe pulse induces the population in the higher ex- cited states S, by S, + SF transition_ The S, -+ S, (m > n) absorption between 590 and 680 nrn may reduce the gain in the spectral region. (3) The leading edge of the probe pulse initiates the process of trans.4 phantom state [ 161. Thus the phantom state may have absorption in the gain spectral region (590-680 nm). (4) The reduction in the SF population is more sensitive to the Sf + So gain than the S, + SF absorption_ This can cause gain saturation for the following reason: The relaxation from the Franck-Condon state (Sic) to the equilibrium configuration in So is much slower than the relaxation from the Sn states, which is popu- lated by the probe pulse via S,., + SF transition. Thus the population of Stc is higher than that of the S, states, and the SF + SEC gain is saturated by the same

probe pulse by which no saturation of the S, + SF absorption takes place.

The highest probe pulse energy of 6 nl, which cor- responds to 2 X lOlo photons in the 590-680 nm region, was focused on an area of 0.2 rnm2_ The photon flux is therefore about 3 X 1O23 photons/cm’ s. Since the gain cross section of ordinary molecules is on the order of 10-l’ cm’, the rate constant for the gain process is estimated to be lo6 s-l _ This value is much smaller than the SR decay rate, which is on the order of 10*-l 010 swl’ in ordinary medium-size molecules. Hence there is hardly any possibility of the reduction in SF population. Therefore, scheme (4) can be ruled out. The cis-DEANS absorption spectrum occurs in the wavelength region (< 530 nm) much shorter than the gain region, while the observed spectral change is in the wavelength region of 590-680 nm. Therefore, scheme (1) can also be ruled out. The lifetime of the higher excited state (S,) is usually be- tween a few picoseconds and several hundred femto- seconds. The lifetime of the phantom state should be shorter than the ground-state recovery time, which is a few nanoseconds in benzene solution [ 111. Since the absorption cross section in the 590-680 nm region is also of the order of 1 O-17-1 0-l 8 cm2, the highest population rate S, + S, or phantom state + S, is 106-lo5 s-l- There can be no efficient population in the S, state with a lifetime of the order of 10-12- lo-13 s &d/or a phantom state with a lifetime shorter than a few nanoseconds. Therefore, schemes (2) and (3) can also be eliminated.

Since none of the tentavive schemes seem plausible, the unusually low gain saturation level cannot be ex- plained at the present stage. We are undertaking studies of the dependence of the gain saturation on the ex- xitation and probe wavelengths and the solvent effect.

3.3. Absorption andgainspectraofDEAh5inpoIyn~er firm

The picosecond time-resolved absorption 2nd gain spectra of DEANS in the polycarbonate film with 1 pm thickness at several delay times were measured. An extremely hig;l gain (1.1 X IO3 cm-l) was ob- tained for highly concentrated DEANS film. ltoh et al. measured optical gains of some counarin dye- doped polymethylmethacrylate (PMMA) films. The gain of 0.24 mol dm-3 4-trifluoromethyl-6-methyl-7-

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Volume 121, number 4,s CHEMICAL PHYSICS LETTERS 15 November 1985

ethylcoumarin (CF3-coumarin) doped in PMMA film was 50 f 5 cm-1 [17]. Itoh et al. claimed that the value is the highest among various dyes reported. The optical gains of ordinary dyes (several to several ten mol m-3) in solution are of the order of 10 cm-l _ The extraordinarily large gain of DEANS in the poly- carbonate film is due to the high concentration of DEANS dissolved in polymer film and the high ex- citation density (10 mJ/cmZ). The gain of 0.7 mol dmm3 DEANS in polycarbonate film is more than twenty times as large as that of 0.24 mol dm-’ CFa- coumarin in PMMA film. The concentration of DEANS in polymer film was about 0.7 mol dmm3 while that in benzene was about 1.2 X 10m3 mol drnm3 _ The

wavelength of the transient absorption maximum was

around 570 nm in both the film and solution. On the other hand, the gain maximum of DEANS in benzene was located at 610-645 nm and-that in polycarbonate film was a wavelength longer than 650 nm_ The wave-

length of the equal intensity of absorption and gain in benzene was about 590 while that in the polymer film was about 615 nm. These experimental results can be explained as follows.

The gain spectrum corresponds to the Sr + So transition, while the transient absorption corresponds to the transition S, + SF _ The SR state has a very large dipole moment, while the d! pole moment of the S, state is considered to be much smaller for the following reason. There are not only charge-transfer states but also several locally excited configuration states near S,, . Therefore, there is possible configuration mixing among them. Hence the dipole moment of the S,., state is smaller than that of SF, which is con- sidered to be an almost purely charge-transfer state. The SF state with charge-transfer character is stabilized in the film which is more polar than benzene. There- fore, the gain spectrum in the film shifts to the longer wavelength region than in benzene.

Acknowledgement

This research was partly supported by the Ministry of Education, Science and Culture, Toray Science Foundation, Kajima Foundation, and Kurata Foundation.

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