Photochemistry of an unsymmetrical polymethine-cyanine dye; solute—solvent interactions and relaxation dynamics of LDS 751

J. Photochem. Photobio!. A: Chem., 84 (1994) 45-55

45

Photochemistry of an unsymmetrical polymethine-cyanine dye; solute-solvent interactions and relaxation dynamics of LDS 751 Ph.

Hkbert,

G. Baldacchino,

Th. Gustavsson

CEA, Cenwe d’Etudes de Saclay, DSMlDRECAMISCMIURA33I (Received

January

3, 1994; accepted

and J.C. Mialocq” CNRS, F-91191 Gif--sur-Yvette Ceder {France)

March 24, 1994)

Abstract The photophysical and photochemical properties of the LDS 751 or styryl 8 molecule have been investigated in various solvents. Our results indicate that the photostability of this laser dye is mostly due to an efficient S,+S, non-radiative internal conversion process which is faster than the radiative process. This radiationless relaxation is governed by a viscosity-dependent process, tentatively assigned to the free rotor effect between the polymethinic chain and the end groups due to the single bond character of the dominant benzenoid structure. Moreover, intersystem crossing to the triplet state and tram+cis photoisomerization are totally inefficient. The shape (relatively small molar extinction coefficient and large bandwidth) and the blue shift of the absorption spectrum in polar solvents show that this unsymmetrical polymethine-cyanine dye is highly polar in the ground state with the cationic positive charge mainly localized on the benzothiazole nitrogen and inducing a large orientational polarization of the surrounding solvent molecules. The absorption transition involves a much less polar Franck-Condon excited state and a charge transfer from the dimethylaniIino nucleus to the benzothiazole nucleus. The shape of the fluorescence spectrum, similar to that of symmetrical polymethine-cyanines and almost independent of solvent polarity, confirms the unpolar character of the first singlet excited state. The sub-picosecond analysis of the timedependent fluorescence Stokes shift thus probes the relaxation of solvent molecules freed from solute-solvent interactions after the Franck-Condon absorption transition.

1. Introduction For

25 years,

polymethine-cyanine

dyes have

attracted constant interest as laser dyes [1,2], Qswitching and mode-locking dyes [3-51, red and near-IR fluorescent dyes [6], probes for polymeric, micellar and biological systems [7,8], sensitizers for photodynamic therapy [9] and molecules suitable for non-linear optics [2]. More recently, in the fast-growing field of molecular electronics, the possibility of using polyrnethine-cyanine dyes as components of electrical circuits has been discussed theoretically [lo]. To be suitable molecular wires, these (Brooker) ions must satisfy several basic physico-chemical criteria: dimensions, correct physical and conductive properties, aud the capability of linking efficiently to the electron donor and electron acceptor at either end [lo]. Another key aspect is the symmetrical or asymmetrical nature of the geometrical structure, since it has been suggested that symmetrical structures should give rise to rapid electron transport and asymmetric structures to applications as molecular switches ‘Corresponding

author.

lOlO-6030/94/$07.00 0 1994 Elsevier SSDI 1010-6030(94)03848-O

Science S.A. AU rights reserved

triggered by small changes in an applied electric field [lo]. For a better understanding of the symmetry effect on the polymethine-cyanines’ properties, we compare in this work the photophysical and photochemical properties of two parent red-absorbing dyes, the symmetrical polymethine-cyanine DTDCI (3,3’-diethylthiadicarbocyanine iodide) [3,4,11-191 and the unsymmetrical polymethine-cyanine LDS 751 (2-(4-(4-dimethylaminophenyl)-1,3-butadienyl)-3-ethylbenzothiazolium pcrchlorate or styryl8) [12,13] (Fig. l), Both dyes are known as laser dyes [14-16,191.

2. Experimental LDS 751 and DTDCI were supplied by Lambda Physik and used without further purification. Solvents, n-alcohols, methanol (Merck UVASOL for spectroscopy), ethanol (Merck, absolute, pro analysi), propanol and butanol (Merck UVASOL for fluorescence spectroscopy), pentanol (Merck, pro analysi), hexanol and heptanol (Merck for synthesis), octanol (Fluka, puriss.), nonanol (Merck

Ph. HPberi et al. / Photochemist~

of an unsymmetrical polymethineqamine

STYRYL 8

MICHLER’SBLUFHOMOLOQ

CH-CH=CH

Cl0

CH,

0 4

Fig. 1. Structures of DTDCI, Michler’s hydrol blue.

LDS

751

and the homologue of

for synthesis), decanol (Fluka, puriss.), dimethylsulfoxide (Merck UVASOL for spectroscopy), ethylene glycol (Merck, pro analysi), acetonitrile, dimethylformamide, acetone, pyridine and tetrahydrofuran {Merck WASOL for spectroscopy), and chloroform (Merck pro analysi) were used as supplied. Ultrapure water was delivered by a Waters Millipore Milli-Q apparatus. The solid matrix material was polymcthyhnethacrylate (PMMA). Plates of LDS 751 and DTDCI were home-prepared according to the following procedure [20]. A PMMA plate (50 x 50 X 2 mm) was immersed in a 10m4 M dye solution in chloroform for 1 h. The dye diffused into the plate along with the chloroform. The plate was then removed from the solution and stored at 80 “C for a few minutes to evaporate the chloroform in excess. A good surface finish was obtained by polishing the plate with altupol no. 2 (Altulor). This infusion technique gave an apparently uniform concentration across the thickness of the plate. The LDS 751 doped plates were found to be photostable. However, the DTDCI-doped plates were photodegradable in room light: their optical density decreased from 2.67 to 0.55 within four days.

dye

UV-visible absorption spectra were recorded with a Beckman UV 5240 or a Gary 3E spectrophotometer and fluorescence spectra corrected for the instrumental response with a Spex Fluorolog 2FlllAl spectrofluorometer run by a personal computer and Spex DM3000F software [21]. The nanosecond laser absorption spectroscopy set-up has already been described [21-231. The signals delivered by the photomultiplier were fed into a DSA 602 Tektronix digitizing signal analyser equipped with two vertical amplifiers, llA32 and llA72. LDS 751 solutions were de-aerated by purging with argon. Picosecond fluorescence decay curves were obtained by using the time-correlated single-photon counting apparatus described elsewhere [22,24]. Briefly, the fluorescence induced by laser pulses of 10 ps duration from a Rhodamine 6G dye laser synchronously pumped by a mode-locked Nd-YAG laser was analysed with an Edinburgh Instruments Ltd Model 199 spectrometer equipped with two ORTEC model 584 constant fraction discriminators, an ORTEC model 567 time-to-amplitude converter and an ORTEC model 7450 multichannel analyser operated at 13.6 psfchannel and interfaced to a personal computer. The conventional photomultiplier was replaced by a Hamamatsu R1564U-11 microchannel plate photomultiplier. The instrument response function was 120 ps FWHM, allowing a temporal resolution limit of better than 50 ps after deconvolution. Fluorescence decay profiles were recorded at 700 nm with magic angle polariz+tion and at a room temperature of 21+1 “C. Sub-picosecond resolution measurements of the fluorescence intensity decay of LDS 751 in various solvents were made using the technique of fluorescence upconvcrsion light gate [25-321. The experimental set-up and results have been briefly described elsewhere [33-351. Since the upconversion technique is polarizationdependent, successive recordings of J,(t) and r,(t) allow the construction of the rotation free decay F(t) by the simple relation F(t) =41(t) + 21, (f) Moreover, as

(1)

the anisotropy

r(t) = [Ill@)-~.l(ol4q~)

r(t) is easily calculated

+ 21, WI

(2)

Data treatment and analysis are described in great detail elsewhere [34]. In order to reveal the temporal evolution of the fluorescence spectrum of LDS 751 in various solvents, we have used the indirect method described by Maroncelli and Flem-

Ph. H&b& e: al. / Photochemistry of an unrymmetricol polymethinewymine dye

ing [36], and fitted the recalculated spectral points for a given time delay to a log-norm function [37], from which the mean frequency v(t) could be calculated. Spectral shifts were evaluated by following the temporal evolution of the mean frequency using the correlation function c(t) = [v(t) - 1I(w )I/[44

- 4 O” >I

The high-performance (HPLC) analyses were described [22].

3. Results

(3)

liquid chromatography performed as previously

and discussion

3.1. Absorption and stea& state fluorescence spectra The absorption and fluorescence spectra of LDS 751 in various solvents are represented in Fig. 2. The absorption spectra are broader and the molar extinction coefficients emay at the wavelength of the maximum absorbance smaller than those of symmetrical polymethine-cyanine dyes. In dimethylsulfoxide for example, the width of the absorption spectrum at half-maximum is 4150 cm-’ and the molar extinction coefficient ls62 nm = (5.1.5&0.10)X 1014 M-l cm-‘, similar to the literature value of 6.15~104 M-’ cm-’ in ethanol [19]. The difference between the E,, values in these two solvents is consistent with the smaller bandwidth (3925 cm-‘) of the absorption spectrum in ethanol. The wavelengths of the absorption and fluorescence maxima are gathered in Table 1, together with the Stokes shifts (the differences in wavenumber between the absorption and fluorescence maxima). The values measured for the symmetrical polymethine-cyanine DTDCI are also given for com1.2r

i

1.0

,

5 43 2 1

I

12345

41

parison. The width of the DTDCI absorption spectrum at half-maximum is only 920 cm-’ in DMSO, which is to be compared to the 4150 cm-’ found for LDS 751, and the molar extinction coefficient is &62 nm = 2.25 +0.15x lo5 M-l cm-‘, in good agreement with the literature value given in ethanolic solution effi3 nm = 2.23 X lo5 M-l cm-’ [19]. In the case of DTDCI, the solvent polarity effect is not important and the smallest Stokes shift v,, - V~= 200 + 50 cm-’ is found in PMMA, where orientational relaxation of the surrounding molecules is inhibited. The Stokes shift increases to 450+ 50 cm-’ in most of the solvents and its highest value is only 700 & 50 cm-’ in water. This indicates a low polarity of both the ground and excited singlet states of DTDCI and a iarge delocalization of the positive charge of the DTDC cation between the two nitrogen atoms of the end groups. Conversely, the absorption spectra of LDS 751 are dramatically affected by the solvent polarity, as shown by the plot of the energies of the absorption and fluorescence maxima of LDS 751 as a function of the corresponding values for DTDCI (Fig. 3). The absorption spectra of LDS 751 are broad and the large blue shift in solvents of increasing polarity (the wavelength of the absorption maximum is 610 nm in chloroform and 520 mn in water) indicates a larger solvation of the ground electronic state than that of the Franck-Condon excited singlet state. Although the blue shift is similar to that of the neutral merocyanine used by Brooker [38] as a solvent property indicator, the styryl 8 absorption maxima do not Absorption and fluorescence maxima

1.2

I 1.0 i

25

0.8

0.6 8

.$

0.6

0.6

nk

0.4

0.4 u 9

A

5

$

5

.$

0.2

0.2

&

0”

0.0

0.0

b2

-0.2

1

’ 400

500

600

Wavelength

700

a00

2

16.

G

15-

1

14.

l

s g

Absorpiion Fluorescence

*- ,

l

*l O

J -0.2

-

14,0

14,5

15,0

15,5

16,0

(nm)

Fig. 2. Absorption and fluorescence spectra of LDS 751 in various solvents. 1: chloroform; 2: methanol; 3: ethyleneglycol; 4: dimethylsulfoxide; 5: water.

FREQUENCIES (103cn?), DTDCI Fig. 3. Correlation curve of the absorption spectra of LDS 751 and DTDCI.

and fluorescence

TABLE 1. Wavelengths of the absorption and fluorescence maxima of LDS 751 and DTDCI in various solvents. Stokes shifts AV and solvents static dielectric constants +*.,. (DMSO: dimethylsulfoxide; EG: ethylene glycol; DMF: dimethylformamide; THF: tetrahydrofuran; PMMA: polymethylmethacrylate) Solvent

41.1.

LDS 751 Absorption

DTDCI Fluorescence

Ar

Absorption

(cm-‘)

&O DMSO EG Methanol Ethanol Propanol Butanol Hexanol Octanol Acetonitrile DMF Acetone Pyridine THF Chloroform PMh4A

80.1 48.9 38.7 33.6 25.1 20.8 17.8 13.9 10.3 38.8 37.6 20.7 12.5 7.6 4.8 _

&)

;lr;,

520 562 575 567 575 585 585 585 585 563 566 562 590 578 610 575

696 721 712 701 699 697 701 701 696 704 714 710 717 711 698 691

correlate properly with the xB values {transition energies of Brooker’s merocyanine). This is probably due to the different nature of the solute-solvent interactions (both in the ground state and in the Franck-Condon excited singlet state) of dyes which are cationic (LDS 751) and neutral (Brooker’s merocyanine). The large blue shift of the LDS 751 absorption spectra with increasing polarity is somewhat surprising if one compares this with the behaviour of symmetrical polymethine cyanine dyes, for example pinacyanol 139,401 or 3,3’-diethyloxadicarbocyanine [40,41]. As already mentioned, LDS 751 is an unsymmetrical cyanine dye, a fact which was first pointed out by Brooker et al. [12,13], who compared the wavelength of its absorption maximum with those of the parent symmetrical cyanine dyes (DTDCI, n =2) and the next higher homologue (n = 1) of Michler’s hydrol blue [12]. According to these authors, from the resonance point of view an unsymmetrical cyanine dye differs from a symmetrical cyanine in the important respect that the two extreme configurations of the former are not identical. Whereas the absorption maximum of an unsymmetrical cyanine dye with two nuclei having the same basicity should occur at a point midway between the absorption maxima of the parent symmetrical cyanine dyes, the LDS 751 absorption maximum in nitromethane is located at 570 nm, a wavelength much shorter than the harmonic mean (676.5 nm) of the wavelengths of the ab-

$) 4850 3925 3350 3375 3100 2750 2850 2850 2725 3550 3650 3700 3000 3250 2050 2900

646 662 660 652 656 658 660 662 663 652 660 653 667 insoluble 668 660

Fluorescence Amar (nm)

AV (cm-‘)

677 683 679 672 676 677 680 679 682 672 680 676 688

700 500 400 450 450 450 450 400 400 450 450 500 450

683 669

350 200

sorption maxima of DTDCI (654 nm) and the homologue (TZ= 1) of Michler’s hydrol blue (701 nm) (Fig. 1). According to Brooker et al., the 106.5 nm deviation is not due to a marked difference of basicity between the nitrogen atoms of both strongly basic benzothiazole and dimethylanilino nuclei, but to the presence of the benzene ring in the chromophoric chain which implies that the benzenoid structure is more stabilized by resonance than the quinonoid structure [12,13] (Fig. 4). Within this picture, which also holds on theoretical grounds [42,43], one expects that the cationic

0

C&

CIO,

Fig. 4. Resonance structures of LDS 751. As explained in the text, the benzenoid form (a) is more stable than the quinonoid form (b).

Ph. Hpbert et aL / Photochemistry

of an unqmmeiricd

charge of the ground electronic state ‘is mainly located on the benzothiazole nitrogen rather than on the dimethylanilino nitrogen. The resulting polar character of the electronic ground state induces a large orientational polarization of the solvent and a lowering of the ground state energy. The solvent-dependent absorption spectra and the blue shift with increasing polarity as shown in Table 1 indicate that the absorption transition involves a less polar Franck-Condon excited state. We thus conclude that the absorption transition involves a charge transfer from the dimethylanilino group to the benzothiazole group which dramatically reduces the localization of the positive charge. LDS 751 (this work) and LDS 750 [28] absorption and fluorescence spectra in solvents of various polarities show the same behaviour. A plot of the absorption and fluorescence frequencies of LD$ 750 [28] versus our measured values for LDS 751 in the same solvents gives a straight line with a slope of 0.91 and a very good correlation coefficient of 0.998, as shown in Fig. 5. The fluorescence spectra of both dyes display the characteristic structure of the fluorescence spectra of symmetrical polymethine cyanine dyes, DTDCI,pinacyanol[39], or DODCI [17,41], feebly sensitive to solvent polarity. In methanol, for example, the width of the fluorescence spectrum of LDS 751 is only 1500 cm-‘, Moreover, if one takes into account the harmonic mean 676.5 nm for the peak wavelength of the absorption transition of a hypothetical “nonpolar LDS 751“, as discussed above, and the average Stokes shift of 20 nm as measured for DTDCI. one finds 697 nm for the theoretical Absorption and fluorescence maxima

I

z<- 21-l -y = 1.283 + 0.914 x i

19 \

/I

FREQUENCIES (lo3 cm-’ ), LDS 750 Fig. 5. Correlation curve of the absorption spectra of LDS 751 and LDS 750.

and fluorescence

poJymethine_cynnine

dye

49

wavelength of the fluorescence maximum of LDS 751. This is in good agreement with the values measured in chloroform, in alcohols and in water as well (Table 1). Such behaviour shows that the relaxed fluorescent singlet state is non-polar (as is the Franck-Condon singlet excited state) and reflects a strong delocalization of the cation positive charge between the benzothiazole and dimethylanilino nuclei. In rigid PMMA, where orientational relaxation of the solvent molecules is inhibited, the wavelength of the fluorescence maximum of LDS 751 is 691 nm, the smallest value observed (although close to the values measured in the other media under study), but the Stokes shift is important, 2900 cm-l, much greater than the Stokes shift of 200 cm-’ experienced by DTDCI. Most of the Stokes shift of LDS 751 thus originates in the blue shift of the absorption spectra through the stabilization of the ground electronic state by the solvent polarization. In PMMA, the absorption maximum of LDS 751 is at 575 nm, as in ethanol or ethyleneglycol. It must be concluded that PMMA shows some polar character and that the effect of traces of chloroform in the plate can be discarded in view of the 610 nm wavelength of the absorption maximum in that solvent. The solubility of LDS 751 in water (3 X lo-’ M in the monomeric form) was enhanced in the presence of P-cyclodextrin. The shape of the absorption spectrum and the wavelength of its maximum remained identical, indicating that complexation with p-CD does not modify the interaction of the polar groups of LDS 751 with the water molecules. Assuming a constant molar extinction coefficient E,, = 5 x 104 M-’ cm-’ for both the uncomplexed and the complexed species, the total LDS 751 concentration dissolved in the monomeric form is 1.1 x 10e4 M and 1.9 X 10e4 M in the presence of lo-’ M and 3 X lo-’ M pCD respectively. However, the 1.9 X lop4 M concentration remains inferior to the 4.6X 10e4 M concentration calculated from the weighted LDS 75 1 powder. As shown above, LDS 751 has the same behaviour as LDS 750. Our conclusion concerning the relative polarities of the ground and the fluorescent singlet states of LDS 751 is in agreement with the decreased solvent stabilization of SI relative to So as found by Castner et al. for LDS 750. 3.2. Nanosecond laser spectroscopy Nanosecond laser excitation of aerated and deaerated ethanolic solutions of LDS 751 has been

Ph. H&ert et al. I Photochemishy of an unsymmetriial plymethine-cyanine

50

performed at 532 nm in a 1 cm path-length cell. The initial 532 run absorbance was 0.3 and the excited volume was 0.4 cm3. Even for a laser pulse energy of 30 ml and in de-aerated solution, we could not observe in the 10 ns to several milliseconds time range any formation of a transient absorbance attributable to a metastable triplet or photoisomer. By exciting an ethanolic solution of DODCI (0.3 absorbance at 532 nm) with 0.5 mJ for actinometry, we measured at 620 nm (ceZ,, _ = 1.8 X 105 M-r cm-‘), which is the wavelength of the absorption maximum of the DODCI metastable photoisomer [17], an optical density variation of 0.05. We find that under the same laser excitation conditions, the optical density variation of LDS 751 solutions is negligible, smaller than 10e3. We thus conclude that the quantum yield of formation of a triplet state or a long livedphotoisomer is below 10e4. However, initially a very short-lived transient formed and disappearing within the laser pulse was observed, characterized by a narrow absorption band with a maximum at 470 nm (Fig. 6). We tentatively assign this feature to the absorption of the S1 excited singlet state. 3.3. Fluorescence quantum yields and lifetimes The fluorescence lifetimes of LDS 751 have been measured using the time-correlated singlephoton technique at 700 nm, close to the wavelength of the fluorescence maxima. In all the homogeneous solvents, after the fast initial relaxation, which will be discussed in Section 3.4, and which occurs within 20 ps [33-351, the fluorescence decay is monoexponential over three orders of magnitude, as shown in Fig. 7. The fluorescence lifetimes r measured in various solvents and the fluorescence decay rate constants kF = UT are gath-

=

0 -3 -6

Fig. 7. Fluorescence decay curve of LDS 751 in hexanol solution. The wavelength of observation was 700 nm. The time-correlated fluorescence photon counting experiment was performed with a resolution of 13.6 pskhannel.

ered in Table 2. The radiative k, values were calculated from the measured fluorescence quantum yields & and lifetimes r using eqn. (4): k, = &lr

400

x0 Wavelength

400

700

800

(nm)

Fig. 6. Normalized absorption spectrum of the LDS 751 ground state and initial differential absorption spectrum after nanosecond laser excitation.

(41

The Einstein probability coefficient for a luminescence transition from an upper electronic state to a lower electronic state is given by eqn. (5): k, = (64v4/3hc3)n3s$IM12

303

dye

(5)

where h is Planck’s constant, c the velocity of light in vacuum, IZ the solvent refraction index, v~ the wavenumber of the fluorescence transition and M the mean electronic transition moment [44-46]. Therefore the radiative rate constants k, are expected to depend significantly on the solvent refraction index and the fluorescence energy [23]. The k, and r= values in methanol, ethanol, ethyleneglycol and dimethylsulfoxide, the solvent refraction indices, the fluorescence wavenumbers and the ratios k,/n3z$ are given in Table 3 for comparison. The ratios kJr2E.1 are equal within the experimental errors, indicating that the electronic

Ph. H&b& et al. I PhotochemLrtty of an unsymmetricd pofymethineqvmine TABLE 2. Macroscopic viscosities 17. fluorescence lifetimes r, deactivation rate constants kF, non-radiative rate constants k,, and fluorescence quantum yields & in various solvents (linear alcohols: C-OH; I’-C&H: isopropanol; DMSO: dimethylsulfoxide; EG: ethyleneglycol; CHsCN: acctonitrile; DMF: dimethylformamide; P-CD: 8_cyclodextrine) IO-’ k,,

;Ps)

0-7

6-7

75 155 270 250 360 500 640 760 890 925 1090

13.3 6.5 3.7 4.0 2.8 2.0 1.6 1.3 1.1 1.1 0.92

13.1 6.3 3.5 3.8 2.6 1.8 1.4 1.1 0.9 0.9 0.7

0.014 0.028

19.9 1400 2.24 0.36 0.84

380 1250 200 55 125

2.6 0.80 5.0 18.2 8.0

2.4 0.60 4.8 18.0 7.8

0.071

0.98

45

22.2

22

;np) C,-OH G-OH C&OH i-C&OH C,-OH Cs-OH Ch-OH C-OH C&H &-OH C,,-OH

0.565 1.14 2.1 2.2 2.8 4.5 4.7 6.2 8.4 10.3 10.8

EG Glycerol DMSO CH,CN DMF Hz0 H,‘J, 3x10-* P-CD

M

&

150

y = 0,165 + 73x

of viscosity)

RA2 = 0,999

/

3 loo-

0

1

2

(CIA

l/q

Fig. 8. Plot of the non-radiative rate constants the reciprocal solvent viscosity in n-alcohds.

of LDS 751 versus

OS!45 2000 T

39,l

=

+ 122,5

q

& 210 (3.58)

7, (ns)

lO+ k,

5.4 5.5 5.3 4.5

0.185 0.182 0.189 0.222

n

1o-3 vp (cm-‘)

I@ k,tn’ vp3

1.33 1.36 1.43 1.48

14.3 14.3 14.0 13.9

2.7 2.5 2.4 2.5

(s-7

transition moment is the same in methanol, ethanol, ethyleneglycol and dimethylsulfoxide. The non-radiative rate constants k,, were calculated from the measured fluorescence decay rate constants k, and an average radiative rate constant k,=O.2~ 109 s-l according to eqn. (6): k,=k,,+k,

= f (inverse

0 =

TABLE 3. Calculated radiative lifetimes rr and radiative rate constants k,, solvent refraction indices n and fluorescence wavenumbers us

Methanol Ethanol EG DMSO

rate constant

51

in n-alcohols

Do-

lo-’ kF

Solvent

Non radiative

dye

(61

The values obtained are given in Table 2 and also plotted as a function of the reciprocal of the macroscopic viscosity 7-l of the linear n-alcohols (Fig. 8). The slight differences between the k, values in the alcohols can be neglected in view of the similar refraction indices and fluorescence

J_

I

0

/ 0

b,

.

2

,8:

4 rl

,

I

6

8

10

12

(cp)

Fig. 9. Plot of the non-radiative lifetimes ofLDS 751 andpinacyanol versus the reciprocal solvent viscosity in n-alcohols.

wavenumbers. A straight line fit gives a slope of &JS~-1=7.3~ 10’ s-l, with an excellent correIation coefficient of 0.999. The straight line passes through the origin, indicating that the radiationless deactivation process is wholly governed by the solvent viscosity. In order to compare the efficiency of the non-radiative deactivation of LDS 7.51 and that of pinacyanol in n-alcohols at room temperature, we have plotted (Fig. 9) the rnr = l/k,, values versus the viscosity of the n-alcohols which give a slope Sr,,,IS~ = 137 ps cP-’ for LDS 751, greater by one order of magnitude than the value found for pinacyanol, &-,,,/Sq=14.0 ps cP_‘. The value for pinacyanol obtained by using the fluorescence lifetimes 7~ T,, given previously [47] is close to the &r/&q =11.7 ps cP_’ value of Akesson ef al. [48] calculated from ground state absorption recovery data. The trend observed in the n-alcohol family is also evident in the other solvents, although

52

affected by specific solute-solvent interactions. lXvo examples of such specific interactions can be given: l The non-radiative rate constant k,, is greater in ethyleneglycol than in the less viscous ndecanol. l The same value of k,, is found in n-decanol and glycerol, the latter being more viscous by two orders of magnitude. The shortest fluorescence lifetime 7=4.5 ps was found in aqueous solution, where LDS 751 is only slightly soluble. In the presence of p-cyclodextrin, the fluorescence decay can be fitted by the sum of two exponentials with decay lifetimes TV= 67 ps and q = 210 ps and respective pre-exponential factors A, = 0.26 and A2 = 3.58. The short lifetime can be attributed to the free LDS 751 species and the long lifetime to the complexed species in equilibrium. Complexation of LDS 751 by p-cyclodextrin is indeed expected to slow down vibronic deactivation by freezing some vibronic relaxation. The general behaviour of the LDS 751 radiationless deactivation is thus quite similar to that observed previously in the case of some polymethine-cyanine dyes, either symmetrical ones, such as pinacyanol [39,47-521, X,1’-diethyl-2,2’dicarbocyanine iodide (DDI) [51,53] or unsymmetrical ones such as 1,3’-diethyl-4, 2’-quinolyloxacarbocyanine iodide (DQOCI) [50,51] and LDS 750 [28]. The effect of solvent microviscosity is dramatic (although difficult to describe properly), but solvent polarity plays no role in the fluorescence deactivation. The specific interactions discussed above for n-decanol, ethyleneglycol and glycerol are probably due to hydrogen bonding, which increases radiationless deactivation. Theoretical models have been put forward. In the case of pinacyanol for example, Kaschke et al. have used a model of internal rotation of large molecular parts within the approximation of classical statistics [52] and Akesson et al. have analysed their results using the Bagchi, Fleming and Oxtoby theory by considering a very fast barrierless relaxation to a twisted state [4&l. Although the viscosity-dependent radiationless deactivation process and the effect of “rigidizing” the molecular structure by cross-linking the heterocyclic rings [50] are well established [47-531, the influence of the end rings (benzoxazolyl, benzothiazolyl, pyridyl, quinolyl, dimethylaminophenyl etc.) is not yet understood, even though the first fluorescence measurements were made in 1936 [ll]. It is a fact, though, that in symmetrical mono-, di- or tri-carbocyanine dyes, the fluorescence lifetime depends strongly on the end moieties. To take a specific example, in ethanol the

fluorescence lifetimes of 3,3’-diethyloxadicarbocyanine iodide (DODCI) [17,39,54] and 3,3’-diethylthiadicarbocyanine iodide (DTDCI) [17] are quite long, = 1.2 ns, while it is only 11.5 & 2.5 ps for l,l’-diethyl-2,2’-dicarbocyanine iodide, DDI [53]. All these molecules possess a five-membered carbon chain (n = 2) but different end groups. For the unsymmetrical LDS 751 molecule with a benzothiazolyl nucleus and a dimethylaminophenyl nucleus, our measured fluorescence lifetime, T= 155 ps in ethanol, shows that the efficiency of the radiationless deactivation process is greater than in DTDCI with two benzothiazolyl nuclei. This suggests that the dimethylaminophenyl nucleus of LDS-751 and the quinolyl nuclei of pinacyanol, DDI and DQOCI have similar behaviour with respect to the free rotor character found in these cyanine dyes. In the benzenoid structure of LDS 751, the end nuclei are connected to the polymethinic chain via single bonds. Conversely, in the quinonoid structure (Fig. 4) they are connected via double bonds, which preclude any rotation. Since the benzenoid resonance structure dominates, a free rotor character is expected. 3.4. Time-resolved solvatochromism and anisotropy measurements in polar solvents lie sub-picosecond analysis of the time-dependent fluorescence Stokes shift (TDFSS) of LDS 751 has enabled us to probe the reorganization of the surrounding polar solvent molecules after excitation by a sub-picosecond laser pulse 133-351. In ethanol, for example, the average wavelength of the fluorescence spectrum was shifted from 683 nm at short time (I= 1 ps) to 702 nm (the fluorescence maximum is at 699 nm) at a time t>20 ps (Fig. 10). The TDFSS was thus 19 nm (400 cm-‘), which is much smaller than the blue shift of the absorption spectrum when going from a non-polar to a polar solvent but consistent with the relative independence of the fluorescence peak wavelength with solvent polarity. To explain the TDFSS, one must remember that the absorption and steady state fluorescence spectra showed that the LDS 7.51 ground state is polar and that the first excited singlet state has a low polar character as a result of a strong delocalization of the positive charge between the benzothiazole and dimethylanilino nuclei. Polar solvent molecules are initially oriented around the ground state solute. After the Franck-Condon absorption transition, the orientation of the solvent molecules around the excited singlet state cannot be maintained any longer, since the magnitude of the dipole-dipole interactions between the excited

Ph. Hkbert et ai. / Photochemirtry

of an un.gmmtriculpoiymethineyanine

s $ l6 -@ 0 12__ z k b c w-W 04 -_

. .

Exeltatron

high

53

fluorescence spectra which evolve on a relatively small frequency scale do not show the existence of two excited species that might be ascribed to a locally excited (LE) state and a twisted intramolecular charge transfer state (TICT). Moreover, the anisotropy decay r(t) of LDS 751 has been measured in various solvents and discussed elsewhere [35]. The main result was that the solvent molecules do not follow the solute in its rotational diffusion.

A

20 __

00 __

dye

-

u

Solute

medium

0

Solvent 0 relaxed

Fig. IO. Energy levels of the ground and excited singlet states of LDS 751, before and at times f = 1 ps and t = 15 ps after subpicosecond laser excitation. solute and the solvent molecules decreases drastically. The solvent molecules thus relax, and at any given time t the fluorescence emission spectrum is the signature of the particular time-dependent organization of the solvent molecules around the S,, Franck-Condon ground state, which is reached in the radiative process. Thus, during the solvent reorganization, the fluorescence transition leads to Franck-Condon ground states characterized by energy levels which increase with time as a result of lower and lower sohation energies. This general description assumes that the curvatures of the free energy surfaces for both the ground and the fluorescent states differ significantly, which is not allowed within the linear response theory, valid only in the limit of small changes in solute electronic structures [55-571. We conclude that both the time-dependent red shift of the fluorescence spectrum and the considerable blue shift of the absorption spectrum in polar solvents arise mainly from the solvation properties of the polar ground state (see Fig. 10). Moreover, the DMABN-like isomerization of the dimethylamino end group, proposed in the literature for the related molecule LDS ‘750 [58], can be totally ruled out in the case of LDS 751. The corrected fluorescence spectra indeed show a single band with a fluorescence maximum almost independent of solvent polarity. The time-resolved

3.5. High-performance liquid chromatography (HPLC) The high-performance liquid chromatography analysis gave a chromatogram with a single peak showing that the LDS 751 compound was pure. After 30 min irradiation using a 1 kW xenon arc lamp, the chromatogram and the absorption spectrum of the eluted compound remained identical. We conclude that LDS 751 is remarkably photostable and that the formation of a stable photoisomer does not occur. 3.6. General aspects of the electronic structure Our findings concerning the reduction of the localization of the positive charge in the excited singlet state are in good agreement with recent MO calculations using the PPP-method for another cationic unsymmetrical styrylcyanine dye (2-{4’N,N-djmethylamino styryl)-1-ethylpyridinium), which showed that the syrnrnetIy deviation parameter Z, defined as the sum of the m-charge densities on one half of the polymethine chain minus the sum of the r-charge densities on the other half [59] decreases from 0.32 in the ground state to 0.02 in the excited state, indicating that the electronic distribution is highly symmetrized by light excitation [60]. Therefore the hypothesis of a twisted intramolecular charge transfer (TICI?) excited state of LDS-751 as suggested in the literature for the closely related LDS-750 molecule [58] can be definitely rejected. 4. Conclusions The LDS 751 absorption spectra in solvents of various polarities and viscosities indicate a blue shift with increasing polaritywhich can be explained by a considerable lowering of the energy level of the ground state in polar solvents. This is due to the strongly polar character of the cationic electronic ground state and to the resulting polarization of the neighbouring solvent molecules, which accommodate the positive charge localized on the

Ph. H&WI et ol / Photochembby

54

ofan uruymmet?i~al pofymethine-cyanine dye

benzothiazole nucleus. Conversely, the FranckCondon excited singlet state is much less polar than the singlet ground state, with the effect that the steady state fluorescence spectra, which behave as those of symmetrical cyanine molecules, are little affected by solvent polarity. Indeed, specific solute--solvent interactions in the excited state seem to play an important role. One must conclude that photoexcitation into the first excited state involves a partial charge transfer from the dimethylanilino nucleus towards the benzothiazole nucleus, which delocalizes the positive charge over the entire molecule, as in both the ground and the excited singlet states of symmetrical polymethine cyanine dyes. Sub-picosecond resolution fluorescence spectra show that the TDFSS (only 400 cm-l in ethanol) due to the reorganization of the solvent molecules initially oriented by the polar ground state solute and delivered after the laser excitation of LDS 7.51 is completed within 20 ps. The TDFSS is simply the result of the time-dependent decrease of the solvation energy of the Franck-Condon ground state reached in the fluorescence process. Moreover, fluorescence lifetime measurements in various solvents show that the deactivation of the excited singlet state of LDS 751 is controlled by a viscosity-dependent radiationless process, as is the case in several other unsymmetrical polymethine-cyanine dyes, LDS 750 [2.8], dimethylaminostyrylpyridinium [60,61] and in symmetrical polymethine-cyanine dyes, such as pinacyanol [39,47-521. This S1 --, So internal conversion process probably occurs via vibrational deactivation through a rotational motion. Our findings suggest a single bond character and a free rotor effect between the benzothiazolyl and dimethylaminophenyl end nuclei and the polymethinic chain, as shown in the benzenoid structure (Fig. 4). Intersystem crossing to the triplet state and trans-cis photoisomerization are not effective in this photostable molecule. Therefore, despite a low fluorescence quantum yield in non-viscous solvents, LDS 751 is a good laser dye.

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