Spectral hole burning: Electric field effect on resorufin, oxazine-4 and cresylviolet in polyvinylbutyral

Journal of Luminescence 39 (1988) 181—187 North-Holland, Amsterdam

181

SPECTRAL HOLE BURNING: ELECTRIC FIELD EFFECT ON RESORUFIN, OXAZINE-4 AND CRESYLVIOLET IN POLYVINYLBUTYRAL Alois RENN, Stephan E. BUCHER, Alfred J. MEIXNER, Erich C. MEISTER and Urs P. WILD Physical Chemistry Laboratory, Swiss Federal Institute of Technology, ETH Zentrum, CH-8092 Zurich, Switzerland Received 30 June 1987 Revised 17 November 1987 Accepted 30 November 1987

High resolution optical studies have been performed on the dyes resorufin, oxazine-4 and cresylviolet in polyvinylbutyral films. Spectral hole burning with holographic detection allows to probe shallow holes at low laser power. From the analysis of the spectral line shape profiles under the influence of an external electric field (Stark effect) the magnitude of the difference between the dipole moment in the ground and excited state I = I ~(S~) — ~(S0) the angle 0 between ~ and the transition dipole moment D, as well as matrix induced contributions were derived. Homogeneous linewidths of 330, 450 and 600 MHz were obtained at 1.7 K for the three dyes resorufin, oxazine-4 and cresylviolet. Time-correlated single photon counting was used for the measurement of the fluorescence lifetimes of the three dyes in polyvinylbutyral at 77 K.

I. Introduction The differences in the local environment of organic molecules in amorphous hosts lead to inhomogeneously broadened electronic absorption spectra. Spectral hole burning as a method to overcome this broadening has become a powerful tool in high-resolution spectroscopy of molecules in condensed phase [1]. The spectral resolution attainable is limited by the homogeneous line-5 width respect and is increased by aonfactor of iO~to i0 with to studies inhomogeneously broadened bands. Data on physical properties of guest molecules as well as on guest—host interaction have been obtained from hole-burning experiments. Studies of the homogeneous linewidth in glasses showed a temperature dependence following a power law T~with 0.59
excited state was derived [2,8—12].Astonishingly large matrix-induced dipole moments have been observed for centrosymmetric molecules such as perylene [10,11] and octaethylporphyrin [12]. In previous papers lineshape calculations for persistent spectral holes in an electric field were presented [11,12]. The lineshape depends on the magnitude I I of the difference of the electric dipole moment in the S1 and S0 states and on the orientation of Z~Lm witheffect respect to the transition dipole moment D. The of polarized light on burning and probing spectral holes was discussed. The linear Stark effect occurring in centrosynimetnc molecules was explained by matrix-induced dipole moments. Due to guest—host interaction the molecules are polarized by random electrostatic matrix fields. These matrix fields were assumed to be isotropic and to have a gaussian distribution in magnitude. Spherical and disc models of the tensor polarizibility were considered [10—12]. In this paper we present electric field effect data, which have been measured using the holographic detection method [13] on the ionic dyes resorufin, oxazine-4 and cresylviolet dissolved in polyvinylbutyral (PVB) films. The lineshape func-

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tions were analyzed in terms of the models described in ref. [12]. The experimental data are compared with results obtained from semiempincal CNDO calculations. The dyes resorufin and cresylviolet have been the subject of previous

Two different types of Stark cells as shown schematically in fig. 1 were used, allowing either a perpendicular (a) or parallel (b) orientation of the electric field (Es) with respect to the polarization (E0) of the light. The first type was prepared as

investigations [8,14,15]. The first Stark effect measurement by means of the hole-burning method in an amorphous host had been performed on resorufin and a value of 0.2 D for I I was estimated [8]. A rather large homogeneous linewidth of 5.1 GHz has been found for cresylviolet in a PVA3 matrix at 1.65 K [14]. In recent studies a T’ temperature dependence of the homogeneous unewidth for resorufin and cresylviolet in PMMA was reported in the temperature range from 0.3 to 6 K and the homogeneous linewidths are believed to approach the values determined by the excited state lifetimes at T 0 K [15]. Photon echo measurements of resorufin in an ethanol glass [16] showed even narrower linewidths suggesting that results obtained by spectral hole burning are affected by spectral diffusion processes taking place on short time scales. Probably this may also be the case for the linewidths reported in this paper and the values must be considered as upper limits for the homogeneous linewidths. In order to compare these linewidths obtained from hole-burning data with the lifetime-limited values, excited state lifetime measurements have been performed at 77 K on a fluorescence lifetime apparatus using synchronously pumped dye laser excitation and time-correlated single photon counting [17].

follows: the raw films were inserted between two glass plates with a conducting, optically transparent coating on the inner side. Good optical quality and uniform thickness of the films was obtained after annealing the samples at 100°C under pressure. The thickness of the films was adjusted to 120—300 tsm with an Stark accuracy a few ~sm by precision spacers. This cellofpreferentially used for molecules with Z~J~D is easy to prepare and allows to apply electric fields up to iO~V/cm. The second type of Stark cell (fig. ib) is mainly used for molecules with ~imjID and consists of a sample film of good optical quality inserted between two polymethylmetacrylate (PMMA) blocks. The electric field is applied by means of two copper electrodes clamped to the block perpendicular to the film. The maximum field strength obtained was 2.5 kV/cm being limited by the available high voltage supply. Cooling of the samples was achieved by direct contact with liquid helium in a homebuilt bath cryostat. Helium was pumped to a pressure of 12 mbar corresponding to a temperature of 1.7 K. A

—*

E P F

E

—~

F

2. Experimental

F

The dyes resorufin (CAS reg. no. 635-78-9), oxa.zine-4 perchlorate (CAS reg. no. 41830-81-3), and cresylviolet perchlorate (CAS reg. no. 4183080-2) were dissolved in a small amount of ethanol and added to a solution of polyvinylbutyral (PVB) in dichloromethane. Raw films of 150—300 ~tm thickness were obtained after evaporation of the solvent. The concentration of the dyes was chosen to give films with an optical absorbance of about 0.5—1 at the maximum of the S S 0 absorption band.

k

E~

~

k

a

E~ ~

b

Fig. 1. Schematic view of the Stark cells used for the expenments with the electric field perpendicular (a) and parallel (b) to the polarization of light. G: glass plate; C: optical transparent conducting layer; E: copper electrodes; F: doped poly-

~—

mer film; P: PMMA-blocks; E0, k: polarization and propagation directions of the light; E~:electric field.

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CR599-21 actively stabilized single-mode dye laser was used for burning and probing the holes. Rhodamine 6G and DCM were utilized as laser dyes. The laser frequency was monitored simultaneously by a Tropel 240 spectrum analyzer with a free spectral range of 1.5 GHz acting as a scan control and a Burleigh WA-b wavemeter for absolute wavelength calibration. The optical set-up used for holographic detection of hole spectra was described in detail in a previous paper [12]. For writing the holograms the laser beam was split into an object and a reference

observed at 90° with respect to excitation through a polarizer set at the magic angle. The apparatus response function of the detection system, including a high-resolution 0.75 m Czerny Turner monochromator (Spex 1400) and a Hamamatsu R928 photomultiplier, is 160 ps [18]. Data acquisition was controlled by a DEC LSI-11/73 minicomputer and data deconvolution was processed in Fourier space [19]. The samples were cooled to 77 K in an Oxford DN 704 liquid nitrogen cryostat. In order to get an understanding of the polarization of the S~ S0 transition and of the magni-

beam, which were brought to interference on the sample at an angle of 15°.2). TheFor spot diameter at burning holes the sample was 2 mm (1/e the beams had an intensity of 1—5 p~Weach, and the burning times used in our experiments were 10—60 s. The interference pattern of the two beams (fringe spacing about 2 ~tm) was stored by the hole-burning process as a spectrally narrow spatial grating. For detection of the holes, the object beam was blocked and the diffracted light intensity was measured by a photomultiplier, followed by single-photon counting electronics. In order to prevent bleaching of the grating during read-out the laser power was attenuated by a factor of 50—100. The external electric field was set by a programmable high-voltage amplifier which provided a voltage range of ±1000 V. The laser frequency, the electric field setting and the data acquisition were controlled by a DEC LSI-11/73 computer. During hole burning the laser frequency

tude of the dipole moment difference, semiempirical calculations CNDO methodanhave been performed forusing the the three dyes using extended version of the program of Baumann [20]. Interatomic distances from X-ray structure data were used. All singly excited configurations within 10 eV excitation energy were considered in the configuration interaction procedure. In order to facilitate the calculations for oxazine-4 the methyl groups and the ethyl groups were substituted by hydrogen atoms.

and the electric field strength were kept constant. For probing the holes the laser was tuned over a frequency range of 30 0Hz, centered at the burning position. The diffracted light intensity was measured as a function of the frequency at different values of the electric field strength. Maximum hologram efficiencies were in the order of i0~. The fluorescence lifetime measurements were carried out on a time-correlated single-photon counting system described in detail by Canomca et a!. [17]. The excitation source consists of a CR-590 Rhodaniine 60 dye laser synchronously pumped by a mode-locked CR Innova 90-5 argon ion laser. The pulses had a width of 8 ps at a repetition rate of 75.6 MHz. The emission was

~—

3. Results In fig. 2 the low-temperature absorption spectra of the dyes resorufin, oxazine-4 and cresylviolet in PVB recorded at 4 K are given in the wavelength range of 520—640 nm. The absorption bands show the strong inhomogeneous broadening of the S~ S 0 electronic transitions which is typical for these dyes. Most of the spectral features which relate to the homogeneous bandwidth of the guest molecules are hidden in these unstructured bands. The hole-burning experiments have been performed near 595 nm for resorufin, 623 nm for oxazine-4 and 615 nm for cresylviolet. The anionic dye resorufin and the cationic dye oxazine-4 have C2~symmetry. The twofold z-axis passes through the nitrogen and oxygen atoms in the central ring. All atoms contributing to the IT-system lie in the x—z plane. The ir-orbitals are classified as 2 or b2 and the resulting irtr * configurations and states transform according to A1 or B1. The Irsr* transitions are polarized either parallel to the long axis (x; B1) or the short axis ~—

184

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/ Spectral hole burning the holographic method areelectric shownfield for different electric detection field strengths. The

~/\~

-

-

Cl)

_______________________________ -

___________________ ~

was applied perpendicularly to the polarization of the incoming light. A broadening linear with the electric field was observed. The dip in the center of the signals is a clear indication of an angle of 90° between ~i and the transition moment D. A Lorentzian lineshape was fitted to the signal obtamed at zero electrical fieldstrength giving the position of the line center and the holewidth (1.2

-

~O~HC

H

0Hz). These two parameters and the angle of 90° were kept constant when theoretical functions as described in ref. [12] were fitted to the electric field dependent experimental curves. From the curve fits a value of I I 0.66 D was derived for the molecular dipole moment difference and a value of I I 0.28 D for the broadening parameter of the matrix-induced dipole moments assuming a Gaussian distribution for the magnitude of the matrix fields and a disc-like tensor polarizability of the guest molecules. In the case of resorufin the signal shapes are very similar to those measured for oxazine-4; a value of I L~m I 0.52 D was obtained and a broadening parameter of I ~Iuind I 0.13 D was derived for a fixed angle 0=90°.

-

u_i

-

=

C—) -

=

U)

-

~O~NH,e

=

=

________________________ I

520

I

I

560

I

600

I

640

WAVELENGTH [NM]

_____________________________ I I

Fig. 2. Absorption spectra of the dyes resorufin, oxazine-4 and cresylviolet and in PVB matrices at 4.2 K showing the inhomogeneously broadened S~~— S~transitions.

‘

oxazine

E

5=0

--4 0

A1). Due to symmetry the direction of the permanent electric dipole moment in any electronic state is restricted to the z-direction. Thus also the difference of the dipole moments of two electronic states must be oriented along the z-axis. For cresylviolet the symmetry is lowered by the additional aromatic ring and from symmetry considerations it is only required that the dipole moments and the transition moments lie in the molecular plane. For the S~ S~transition dipole moment D an angle of 7° and for L’~qL~ an angle of 36° with respect to the x-axis was predicted. In fig. 3 hole spectra obtained with an oxazine-4 sample at a temperature of 1.7 K and recorded by

Es42kV/cm

(z;

-

-

I..

-~ -~

__________

__________

I

—10

0.0 frequency

[GHzJ

10

Fig. 3. Hole spectra of oxazine-4 in PVB obtained with holographic detection at 1.7 K. The hole was burned at zero electric field strength. A slightly power-broadened holewidth of 1.2 GHz was found for the signal at E~ = 0 V. Applying electric fields 4.2 kV/cm and 8.3 ky/cm during read-out leads to a broadening of the signals and a characteristic dip arises mdicatinganangleof 8=90° between L115m and D.

A. Renn et aL I

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‘ce~y1’v’ioIet

/ Spectra! hole burning

I

I

-

E~J.. E0

• a

0,0

‘ce~1v’io’1et E~ II E0

hO’

frequency [GHz]

185

E30

‘~1~’r

•

-

~

frequency [GHz}

b

Fig. 4. Hole spectra of cresylviolet in PVB obtained for the different experimental setups with the polarization of the light perpendicular (a) and parallel (b) to the electric field. The synunetric broadening in (a) and the dip in (b) indicate a relatively small angle between ~l,i and D.

In fig. 4(a) hole spectra obtained with a cresylviolet sample are shown for field strengths of 0, 1.7 and 3.4 kY/cm. In comparison with the results shown for oxazine-4 the signals are only broadened by the electric field, indicating an orientation of the dipole moment difference almost parallel to the transition moment. Additional information is provided by data obtained from an experimental setup with the electric field parallel to the polarization direction of the exciting laser radiation (fig. 4(b)). The experimental hole shapes exhibit a dip in the center as predicted by the model calculations [12] for molecules with a small angle between ~i and D. The partial filling of the dip can be explained either by contributions of matrix induced dipole moments or by an angle in the order of 45° between z~ and D. A reliable determination of the molecular parameters can be

achieved when the data from both geometrical arrangements are considered at the same time in a curve fit. The best fits were obtained for an angle 0 28°, a molecular dipole moment difference I I 2.1 D and a broadening parameter I I 0.9 D for the distribution of matrix-induced dipole moments. The estimated errors are 5% for I L~mI and 10% for I I. The values for I I and for I ~ILifld I obtained from the lineshape analyses are summarized in table 1 and compared with results obtained from the semiempirical CNDO calculations. The lifetimes were measured using doped films with very low optical absorbance (<0.06). Resorufin and cresylviolet were excited at the maximum of the S1 S0 absorption bands (585 and 618 nm), oxazine~.4at the vibromc band at 598 nm. Fluorescence emission was observed from =

=

=

-

Table 1 Summary of results for differences of dipole moments of resorufin, oxazmne-4 and cresylviolet in PVB matrices obtained from hole-burning experiments at 1.7 K and from semiempirmcal CNDO-calculations Molecule

Resorufin Oxazine-4 Cresylvmolet

I

1(D)

I

8 (deg.)

Exp.

CNDO

Exp.

CNDO

0.42 0.2 0.66 2.1

2.88 (ref. [81) 2.54 1.75

90

90

0.13

90 28

90 29

0.28 0.9

1(D)

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Spectral hole burning

S~ Nat. Homog. lifetime linewidth linew. (ns) (MHz) (MHz)

Ref.

transfer is now mainly oriented from this ring to the pyridinic N-atom. The transition moment D of the S S transition is calculated to lie almost in the direction of the long molecular axis. The change in the dipole moment z1~z points from

4.8

33

Oxazine-4 5.0 Cresylviolet 4.0

31 36

this work [8](PMMA) a) [15](PMMA) this work this work

the central nng towards the additional nng forimng an angle of 29° with respect to D. From the analysis of the experimental hneshapes obtained with the two different geometical setups a value of 28° has been obtained being in good agreement with the calculated value. The magnitude of ~Lm

Table 2 Comparison of excited singlet state lifetimes (at 77 K), the corresponding lifetime-limited linewidths and homogeneous hnewidths obtained from hole-burning spectroscopy at 1.7 K

_____________________________________________ Molecule

Resorufin

330 3000 390 450 600

.

b)

~

a)

___________________________________________ a)

.

~—

.

Calculated from ref. [15] for 1.7 K.

~ From ref. [8] using the hole width of 0.2 cm~’at 1.8 K. ~ From ref. [14] using the hole width of 0.34 cm~ at 1.65 K.

vibronic satellite lines shifted by about 575 cm1 to the red from the excitation wavelength. The results of the lifetime measurements at 77 K are given in table 2. The corresponding lifetime-limited linewidths are derived and compared with the ]inewidths obtained from measurements on very shallow holes at 1.7 K.

4. Discussion As derived from the semiempirical calculations the main characteristics of the Si S 0 transition in the three dyes consist of a charge transfer of about 0.3 electron charges towards the pyridiic nitrogen atom. The anionic and cationic dyes resorufin and oxazine-4 show a very similar change in the charge density distribution. The resulting changes in dipole moment are oriented along the z-axis and are almost identical, 2.88 and 2.54 D respectively, considerably larger than the experimental values. As expected, the magnitude of the charge transfer upon S~ S0 excitation is overestimated by the semiempirical calculations. In resorufin and in oxazine-4 the lowest absorption band is predicted to be x-polarized. Thus the angles between the transition dipole moment and are expected to be 90° which is in agreement with our experimental results, The additional ring in cresylviolet acts as an electron donating group and the direction of charge

.

.

is quite well predicted by the calculations. In order to get a good fit of the model functions to the experimental lineshapes also matrixinduced dipole moments had to be included. For the three dyes 0.13, 0.28 and 0.9 D were obtained. These values are in the order of the change of dipole moments I I which are 0.42, 0.66 and 2.1 D for resorufin, oxazine-4 and cresylviolet, reflecting a considerable polarization of these molecules due to guest—host interaction. If we assume that similar matrix fields are interacting with the different guests the different values for the matrix-induced contributions for the three molecules are due to different polarizibilities of the molecules. These results are supported by the observation of rather large crystal-field-induced dipole moments [21]. The linewidths observed in our experiments are

~—

~—

considerably smaller than most of the previous results reported on nonphotochemical hole-burning systems [7,8]. In recent measurements on resorufin and cresylviolet in PMMA matrices very narrow homogeneous linewidths have been obtamed [15]. However, it has been shown that, at least for the glassy hosts ethanol [16] and glycerol [22], hole-burning measurements are affected by spectral diffusion processes and homogeneous linewidths obtained from photon echo data are smaller by a factor of about 5. Therefore, the results derived from our hole-burning measurements must be regarded as upper limits for the homogeneous linewidths. The linewidths, 330 MHz for resorufin, 450 MHz for oxazine-4 and 600 MHz for cresylviolet, show an increase with increasing magnitude of the dipole moment differences. This behaviour confirms assumptions that dipolar coupling to the host [22,23] contrib-

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utes to dynamic line broadening mechanisms such as optical dephasing and spectral diffusion in amorphous solids. The excited state lifetimes for resorufin reported here show good agreement with values reported for different solvents [22,24]. In conclusion we want to point out that Starkeffect data provide valuable information on molecular properties as well as on guest—host interaction. We believe that the experimental results of dipole moment differences reported here are also of considerable interest to test quantum chemical calculations. The investigated molecular systems which show large Stark shifts and give narrow holewidths are interesting candidates for frequency- and field-selective holographic storage devices [25,26].

Acknowledgements We would like to thank Dr. H. Baumann and J Keller for valuable help in the semiempirical calculations. Technical assistance by M. Lüönd is gratefully acknowledged. This work was supported by the Swiss National Science Foundation.

References [1] RI. Personov, in: Spectroscopy and Excitation Dynamics of condensed molecular Systems, Vol. 4, eds. V.M. Agranovich and R.M. Hochstrasser (North-Holland, Amsterdam, 1983). [2] F.A. Burkhalter, G.W. Suter, U.P. Wild, V.D. Samoilenko, N.Y. Rasumova and R.I. Personov, Chem. Phys. Lett. ~ (1983) 483. [3] H.P.H. Thijssen, R. van den Berg and S. Völker, Chem. Phys. Lett. 97 (1982) 295.

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[4] A. Gorokhovskii, V. Korrovits, V. Palm and M. Trummel, Chem. Phys. Lett. 125, (1986), 355. [51W. Breinl, J. Friedrich and D. Haarer, J. Chem. Phys. 81 (1984) 3915. [61B.L. Fearey and G.J. Small, Chem. Phys. 101 (1985) 269. [7] B.L. Fearey, T.P. Carter and G.J. Small, Chem. Phys. 101 (1985) 279. [8] A.P. Marchetti, M. Scozzafava and R.H. Young, Chem. Phys. Lett. 51 (1977) 424. [9] V.D. Samoilenko, N.y. Razumova and R.I. Personov, Opt. Spectrosc. 52 (1982) 346. [10] U. Bogner, P. Schätz, R. Seel and M. Maier, Chem. Phys. Lett. 102 (1983) 267. [11] M. Maier, Appl. Phys. B41 (1986) 73. [12] A.J. Meixner, A. Renn, S.E. Bucher and U.P. Wild, J. Phys. Chem. 90 (1986) 6777. [13] A. Renn, A.J. Meixner, U.P. Wild and F.A. Burkhalter, Chem. Phys. 93 (1985) 157. [14] T.P. Carter, B.L. Fearey, J.M. Hayes and G.J. Small, Chem. Phys. Lett. 102 (1983) 272. [15] H.P.H. Thijssen, R. van den Berg and S. Völker, Chem. Phys. Lett. 120 (1985) 503. [16] C.A. Walsh, M. Berg, L.R. Narashimhan and M.D. Fayer, J. Chem. Phys. 86 (1987) 77. [17] S.A. Canonica and U.P. Wild, Anal. Instr. 14 (1985) 331. [18] S.A. Canonica, J. Forrer und U.P. Wild, Rev. Sci. Instr. 56 1754. A.R. Hoizwarth and H.P. Good, Rev. Sci. [19] (1985) U.P. Wild, Instr. 48 (1977) 1621. [20] H. Baumann, CNDUV99, QCPE Program 333, Indiana University. [21] A.P. Marchetti and M. Scozzafava, Mol. Cryst. Liq. Cryst. 31(1975) 115. [22] M. Berg, C.A. Walsh, L.R. Narashimhan, K.A. Littau and M.D. Fayer, J. Chem. Phys., to be published. [23] R. Silbey and K. Kassner, J. Lumin. 36 (1987) 283. [24] KG. Spears and K.M. Steinmetz, J. Phys. Chem. 89 (1985) 3623. [25] A. Resin and U.P. Wild, Appl. Opt. 26 (1987) 4048. [26] A. Renn, R. Locher, A.J. Meixner and U.P. Wild, J. Lumin. 38 (1987) 37.