United Kingdom PHOTOPHYSICAL PROPERTIES OF THE DCM AND DFSBO STYRYL DYES CONSEQUENCE FOR THEIR LASER PROPERTIES

The two styryl dyes, 4-dicyanomethylene-2-methyl-6-P-dimethylaminostyryl-4H-pyran (DCM) and 7- 
dimethylamino-3-(p-formylstyryl)-l, 4-benzoxazin-2-one (DFSBO) exhibit similar solvent-induced shifts 
of their absorption and emission spectra related to a large intramolecular charge transfer (ICT) in the first 
singlet excited state. From the Stokes shift values (vA−vF) and a vectorial analysis of their ground state 
dipole moment (μg= 6.1 D for DCM and 5.8 D for DFSBO), and using the Lippert-Mataga theory, we 
have estimated the dipole moments of their fluorescent excited states S1 (μe = 26.3 D for DCM and 27.6 D for DFSBO). Intersystem crossing to the triplet state is totally inefficient in DCM but significant in 
DFSBO. Moreover the absorption of the DFSBO triplet is quite large in the emission band (600–650 nm), 
which makes of DFSBO a poor laser dye. Although DCM trans-cis photoisomerization can be quite 
efficient in non polar solvents (chloroform, tetrahydrofuran), DFSBO does not photoisomerize probably 
due to steric hindrance and to the S1 character which should be more "benzoxazinone" than ethylenic. 
DFSBO is also shown to exhibit rotamerism.


INTRODUCTION
The two styryl dye molecules, 4-dicyanomethylene-2-methyl-6-p-dimethylaminosty- ryl-4H-pyran (DCM) and 7-dimethyl-amino-3-(p-formylstyryl)-l,4-benzoxazin-2- one (DFSBO) have very similar absorption spectrum maxima in the 450-500 nm range which depend strongly on the solvent polarity.It was interesting to compare their photophysical and photochemical properties since DCM is a widely used laser dye 1'2 because of its broad tunability and high conversion efficiency under flash- lamp, 3'4 XeC1, 5 argon ion laser, 6 second harmonic output of a Q-switched Nd" YAG laser 7'8 and copper vapor laser 9'1 pumping.2][3][4] The DCM spectral shift of the absorption and fluorescence spectra with solvent polarity 5-23 is related to the intramolecular charge transfer due to the presence of an electron donor group and an electron acceptor group on each side of the ethylenic bond. 24In a preliminary study, 8 we have determined the dipole moment increase upon DCM electronic excitation using the DCM Stokes shift values VA re) in 25 solvents and the Lippert-Mataga theory: [25][26][27][28] /z -/g 20.2 D. A vectorial analysis enabled us to estimate the DCM ground state dipole moment #g 6.1 D and thus that of the DCM fluorescent singlet state # 26.3 D. TM More recently in an attempt to describe fully the photophysical properties of DCM and the surprising monoexponential behaviour of the fluorescence decay obtained in time correlated single photon counting (TCSPC) experiments with nanosecond flashlamp excitation, 15'16 we undertook TCSPC measurements using tunable picosecond laser pulse excitation. 2'21 '23 They clearly showed the existence of a trans-cis equilibrium in various solvents under ambient light, in good agreement with the findings of Drake et al., who were able to identify the two photoisomers using high performance liquid chromatography (HPLC) and nuclear magnetic resonance spectroscopy (NMR). 15 '17 We thus found that the nonradiative deacti- vation is governed by two competing processes, S So internal conversion (ic) to the ground state and S P* cis photoisomerization (P* is the perpendicular excited state).We found that the efficiency of the latter decreases in more polar solvents as due to the lowering of the S zwitterionic potential energy surface which creates a double well potential model and a higher energy barrier to isomeri- zation.23 '29-32 Our results thus substantiate the theoretical model of Salem et al. [29][30][31][32] who took into account the suggestion of Wulfman and Kumei 33 that the presence of any electron-attracting or repelling ion, molecule or group near either end of the double bond will cause more highly polarizable perpendicular 1A1 (Z) and 1B2 (v) excited singlet states of alkenes, according to the symmetry designations of Mul- liken 34 and Kaldor-Shavitt. 35The doubly excited state A (Z) is replaced by a low energy $2 state in the Orlandi-Siebrand model of the photoisomerization of stil- bene.36 According to these authors, in low viscosity solvents, $2 relaxes towards the perpendicular configuration and then undergoes internal conversion to So with an equal chance to relax towards a cis or a trans configuration.This model does not explain the solvent polarity effect on the DCM photoisomerization.Hsing-kang et al. have interpreted their fluorescence data by considering two different intramolecular charge transfer (ICT) states of DCM in dynamic equilibrium: a short wavelength emission was assigned to a planar conformation and a long wavelength emission to a twisted (TICT) conformation. 16More recently the fluorescence decay of DCM was found nonexponential in dibutylether (5 and -35C) and the main fluorescent state was described as a TICT state. 22e believe that more experiments are needed to clarify the nonradiative processes in DCM: twisting of the dimethylaminogroup, twisting around the double bond, direct internal conversion and solvent relaxation.
The other dye, DFSBO which belongs to the benzoxazinone family also possesses an electron donor group (dimethylamino) and an electron acceptor group (carbonyl). 37,38Le Bris et al. have analyzed the shift of the fluorescence spectrum induced by the solvent polarity using the solvent polarity-polarizability parameter r* of Taft and Kamlet. 37A good correlation was found between the wavenumber of the fluorescence maximum and r* in aprotic solvents.Despite the small overlap between the absorption spectrum and the emission spectrum and the high fluorescence quantum yield, 37 DFSBO did not show a high laser yield under N2 laser or 532 nm pumping. 38n the present paper in the light of our recent DCM results, 23 we present some new data on DCM and extend our study to DFSBO.We thus gained a better understand- ing of the effects of the molecular structure and of the solvent which may help to design suitable styryl dye molecules possessing appropriate photochemical reactivity and laser properties.

EXPERIMENTAL SECTION
The nanosecond laser absorption spectroscopy setup has already been described. 39he DCM and DFSBO triplet states were populated via triplet-triplet energy transfer from naphthalene which was excited by the 4 th harmonic of a Q-switched Nd: YAG laser. 23luorescence decay profiles were obtained using a TCSPC apparatus which has been described elsewhere. 2'2z'23 UV-visible absorption spectra were measured with a Beckman UV 5240 spectro- photometer.Fluorescence spectra were obtained with a Perkin Elmer MPF 66 spectrofluorometer.More recently fluorescence quantum yields were measured with a fully corrected Spex Fluorolog 2FlllA spectrofluorometer using an ethanolic solution of rhodamine 101 as a reference. 4

Absorption Spectra
The absorption spectrum and the uncorrected fluorescence spectrum of DFSBO in methanolic solution are given in Figure 1.The absorption spectrum consists of a first electronic transition So > Sz located at 483 nm and two other electronic transitions DFSBO CHOH 0 200 3)0 400 500 600 700 wavelength ^(nm) XR (kcal.mo1-1) Fixture 2 Energies EA of the absorption transitions So' > Sz of DCM and DFSBO as a function of the XR Brooker's parameter.
at ---320 and 250 nm which are well separated from the first one.This behaviour is very different from that met in DCM which presents badly defined absorption bands below 400 nm. 23The DFSBO absorption spectrum is reminiscent of the absorption spectra of homologous benzoxazinone derivatives which do not possess the p- formylstyryl group in the 3-position. 38Although the DFSBO absorption and fluor- escence spectra are largely red-shifted with respect to those of the 3-methylsubstituted benzoxazinone, they indicate that the first excited singlet state Sz of DFSBO keeps more benzoxazinone than ethylenic character.Such a behaviour has been already demonstrated in the case of naphthyl and phenanthryl stilbene derivatives.4z- 46 Mazzucato pointed out that the rotation around their ethylenic bridge is thus hindered by high activation energies and resulting low rate constants k, for the twisting from the trans (t) to the perpendicular (p) configuration.Mazzucatto concluded that choosing the S energy of a side group was a way to 45 transform a highly photoreactive material in a nonreactive highly fluorescent one.
The energy E, of the DFSBO absorption maximum (Table 1) correlates excellently with the XR solvent property indicator (transition energy of Brooker's merocyanine VII 8'47 as shown in Table I and Figure 2. XR is to be used for the red shift of the absorption maximum of weakly polar merocyanine dyes in solvents of increasing polarity.47 The plot of EA (in kcal mol-) versus the available values ofn is a straight line with a correlation coefficient of 0.97. 24 (DFSBO) 0.386 X/ + 41.47 (I) The difference of the EA values in isooctane and propargyl alcohol is 4.0 kcal.mo1-1 in DFSBO, about the same as in DCM TM but much smaller than the one registered by Brooker for merocyanine VII (AXR 12 kcal.mol-1). 47In the case of DCM, a similar plot gave TM EA (DCM) 0.386 XR + 43.72 (2) The fact that the slopes are equal is fortuitous but it shows that both dyes absorption spectra behave similarly with respect to the solvent polarity.The observed red shift of the absorption maximum arises from the combination of a blue shift due to the stabilization of the electronic ground state induced by the orientation of the solvent molecules around the solute and a red shift due to the stabilization of the Franck- Condon electronic excited singlet state resulting from the electronic polarization of the solvent (dispersion term).Short range interactions such as hydrogen bonding are not taken into account as one can see in Figure 2 for n-alcohols (solvents 20-24)   which induce a slightly different behaviour.The effect of the solvent electronic polarization can be expressed following the theory based on the reaction field of Onsager that Bayliss applied to merocyanines in apolar solvents. 48The plot of the DFSBO transition energy EA (in kcal.mo1-1)versus the (n z 1)/(2n z + 1) function of the solvent refractive index in 9 apolar solvents is a straight line with a correlation coefficient of 0.94 (Figure 3) We have also estimated the effect of the electron acceptor group (CHO) on the absorption transition maximum by calculating in acetonitrile the difference (2.6   kcal.mo1-1) between the energies of the absorption transition maxima of DFSBO, ,max 484 nm, i.e, EA 59.1 kcal.mo1-1(this work) and BOZ-H or 7- dimethylamino-3-styryl-l,4-benzoxazin-2-one, .max 463nm, i.e, EA 61.7 kcal.mo1-1.49It must be emphasized that the simple picture of a charge transfer from the dimethylamino electron donor group to the formyl electron acceptor end group is not correct since the para-substitution of the styryl by a dimethylamino group gives a compound BOZ-NMez which exhibits a greater red shift of the absorption maxi- mum,/max 488 nm, i.e., EA 58.6 kcal.mol-1. 49uorescence Spectra Pure solvents The DFSBO fluorescence spectra are largely red shifted with respect to its absorp- tion spectra. 37The wavelengths of the fluorescence maxima kF in 28 solvents are gathered in Table 1, together with the transition energies EF, Stokes shift values (VA re) (in cm-1) and solvent properties, refraction index n, static dielectric constant and Afvalues of the Lippert and Mataga theories. 5-z8'18 A plot of (VA V) versus Af gives a straight line for solvents 8-32 in the range 0.09 < Af < 0.31, with a correlation coefficient of 0.93 (Figure 4).The non polar solvents (1-7) must be excluded since according to Lippert some additional terms cannot be neglected in equation ( 4) when e n .
We must confess that the wavelengths of the fluorescence maxima are only average of the values obtained for two excitation wavelengths situated on both sides of the absorption band.We shall see later that the DFSBO fluorescence spectrum depends on the excitation wavelength due to the coexistence of rotamers.
From the slope rn of the straight line, we calculated (e g) the difference between the dipole moment of the excited state and that of the ground state using equation ( 5) 2 (e --/g) where a is the radius (in A) of the spherical cavity in Onsager's theory of the reaction field, h is Planck's constant and Co is the speed of light.With a 8.6 A, obtained from the estimation of the ellipsoid half long axis, we found (#e #g) 21.8 D. The DFSBO ground state dipole moment was calculated using a vectorial analysis of the various group dipole moments, z8'24 #G 5.8 D. Therefore/e 27.6 D. These values are different from those obtained by Valeur et al. in a PPP (Pariser-Parr-Pople) calculation #g 8.7 D and #e 20.5 D. 5 Our experimentally estimated value (/Ze #g) is thus much higher than the theoretically calculated value #e #g 11.8 D of Valeur et al.A more precise estimation is not possible at the present time.We however conclude that DCM 18'19'23 and DFSBO (this work) both undergo a large intramolecular charge transfer upon excitation in their fluorescent excited state.

Solvent mixtures
Because of the large shifts of the absorption and fluorescence spectra in increasing polarity solvents, it appeared to us interesting to study the spectral behaviour of DCM and DFSBO in solvent mixtures.We chose methanol-dimethylsulfoxide (DMSO) mixtures since DCM is largely used as a laser dye in both pure solvents.The wavelengths AA, the transition energies EA of the maxima of the absorption spectra, the wavelengths ZF, and the transition energies EF of the maxima of the fluorescence spectra of DCM and DFSBO are gathered in Table 2 together with the volumic percentages of methanol.EA and Eeof both dyes are linearly related to the methanol volumic percentages (X) as shown in Figure 5 and Figure 6.These data indicate that there is no preferential solvation of DCM or DFSBO and that the solvent shell is an homogeneous mixture.Therefore methanol which might induce hydrogen bonding with these solutes does not look like playing a particular role in the binary methanol-DMSO mixtures.We have also calculated the relative fluorescence yield Re of DCM in these binary mixtures by taking q)F (DMSO) as a reference.Re decays linearly with X according to equation ( 6) with a correlation coefficient of 0.965 Re -0.0036 X + 0.953 (6)   0<X< 100 Our Re value for DCM in pure methanol (0.61 + 0.06) is in good agreement within the uncertainty errors with the ratio (0.54) one can calculate from the DCM 59-58-0 ..  These curves are of a particular interest if by using a solvent mixture one intends to shift the DCM absorption spectrum for a better pumping by a copper vapor laser for example or to get more gain at a particular wavelength in the red.DFSBO Fluorescence and Excitation Spectra As we indicated above, the maxima and the bandwidths (FWHM) of the DFSBO fluorescence spectra depend largely on the excitation wavelength (Table 3).The fluorescence and excitation spectra of DFSBO in butyl acetate solution are given as an example in Figure 7.In non polar solvants, isooctane, CC14, trichloroethylene, the number and the intensity of well resolved vibronic bands dramatically depend on the observation wavelength.The DFSBO fluorescence spectra in isooctane solution are given in Figure 8a, for two excitation wavelengths 420 and 490 nm.When exciting at 490 nm, the first fluorescence band at 479 nm completely disappears but the band at 534 nm increases significantly.The DFSBO excitation and absorption spectra in isooctane solution given in Figure 8b and c show also distinct features.'52 The reorientation time constant of the molecules of the low viscosity solvents under study is indeed much shorter (<100 ps) than the fluorescence lifetime which is on the order of one nanosecond.Moreover the observation of different vibronic bands is certainly the best proof that several configurations of DFSBO are involved.
equilibrium between two trans-cis isomers could be attained very rapidly under room light even in methanol although the photoisomerization quantum yield was very low.For solutions which were freshly prepared and kept in the dark, the DCM decay lifetime was found monoexponential.We recently found in methanol solutions that the monoexponential decay time was depending on the presence of oxygen.In methanolic solutions flushed with argon, the DCM fluorescence decay time was x (1.41 + 0.01) ns but in aerated methanolic solutions, it was x (1.36 _+ 0.01) ns 53 in perfect agreement with the value of the trans-isomer fluorescence decay (1.37 +_ 0.01) ns obtained in our previous two exponentials decay analysis of room light exposed aerated solutions. 21The DCM fluorescence lifetime in methanol is thus shorter than that of DFSBO in ethanol x 2.9 ns as measured by Le Bris et al. 37 Photostability under Laser Pulse Excitation The UV-visible absorption spectra of freshly prepared solutions of DFSBO in methanol or DMSO and submitted to laser pulse excitation at 532 nm indicate a decrease of the absorbance around 490 nm and 320 nm and an absorbance increase between 350 and 420 nm.Isosbestic points are found at 317, 353 and 440 nm.The spectral change is found irreversible.In order to understand the nature of the new species formed under light irradiation, we compared the NMR spectra of DFSBO in deuterated chloroform solutions freshly prepared and kept in the dark to those of solutions submitted to the visible light (420 < < 580 nm) of a xenon arc lamp during three hours.Although new peaks could be observed, they did not present the characteristics of a cis-isomer 24 as observed in the case of DCM. 23Moreover the photostability of solutions flushed with argon was largely improved.

Nanosecond Laser Photolysis
Absorption spectrum of the first singlet excited state of DFSBO.
The DFSBO excited singlet state generated in 10 -5 2 x 10 -5 M DFSBO methanolic solutions using nanosecond laser excitation at 532 nm presented a differential absorption spectrum with a minimum at 480 nm (photobleaching of the ground state), a maximum around 380 nm and two isosbestic points at 410 and -515 nm, as shown in Figure 9.The $1 absorbance appeared significant for wavelengths greater than 515 nm but an accurate determination of the absorption spectrum in this wavelength range was made impossible due to the amplification of the analyzing light in the fluorescence band.After the fast decay of $1 within the resolution time of our apparatus (---5 ns), a transient absorption spectrum remained at 200 ns showing the photobleaching of the ground state absorbance, a wide absorption band around 400 nm and an isosbestic point at 440 nm.The absorbance change was however very small.The same behaviour was also found in DMSO (Figure 10).lntersystem crossing to the triplet state 532 nm nanosecond laser excitation of a 10 -5 M DFSBO methanolic solution flushed with argon gave a significant 650 nm transient absorbance which disappeared in aerated solution (Figure 11).The difference absorption spectrum presents an absorbance maximum at 660 nm, a large photobleaching at 490 nm and two isosbestic points at 440 and 535 nm.It decays with a lifetime x (52 + 5) ps.This spectrum was already observed in the nanosecond time range after the $1 decay.At t 400 ps, a small residual absorbance is observed below 450 nm and over 530 nm together with a small photobleaching of the DFSBO ground state absorbance.
In order to ascertain that the 52 ps lifetime transient absorbance was that of the DFSBO triplet state, we also used naphthalene as a sensitizer in triplet-triplet energy transfer experiments as we did before in the case of DCM. 23The kinetics of the energy transfer was analyzed at 480 nm, the wavelength of the absorption maximum of the DFSBO ground state which gave a better signal.The plot of the pseudo first order rate constant k of the DFSBO ground state photobleaching versus the DFSBO concentration gave the bimolecular second order rate constant krr (2.3 + 0.3) x 101 mol-l.dm3.s-1.The energy transfer is thus diffusion controlled.
aN -t-DFSBO kTr XN ; + 3DFSBO Using this method we were able to determine the absorption spectrum of the DFSBO triplet state in methanolic solutions containing 1.8 10 -4 M naphthalene and 2.5 10 -5 M DFSBO.The DFSBO triplet molar extinction coefficient was calculated using the literature molar extinction coefficient of the naphthalene triplet E 415nm 4 10 4 mo1-1 dm 3cm -1 and the naphthalene initial triplet absorbance obtained in DFSBO free methanolic solutions containing only naphthalene.We indeed assumed a complete energy transfer since the naphthalene triplet lifetime was respectively 70 and 1.4 ps in the absence and in the presence of DFSBO.The DFSBO triplet molar extinction coefficient is thus gr 60nm (5.0 -F" 0.5) 10 4 moldm 3 cm-1.The difference absorption spectrum and the calculated absorption spectrum of the DFSBO triplet are given in Figure 12.
'58 Moreover, we found that DFSBO degradates in aerated solutions, probably via chemical reactions involving the triplet state and the excited singlet oxygen 102 3DFSBO + 30 2 1DFSBO + 10 2 102 + 1DFSBO products We did not find any evidence of a trans-cis photoisomerization.Our nanosecond laser absorption studies did not show any absorbance than one may ascribe to a short lived cis-isomer.Moreover, NMR experiments did not show the typical doublet of the ethylenic protons of a stable cis-isomer that we observed in the case of DCM. 23wo major reasons may account for the lack of trans.cisphotoisomerization.The first one is the trivial steric hindrance that one may expect by inspection of the molecular structure of the cis-isomer.The second one is the nature of the first singlet excited state.We indicated above that it had more benzoxazinone than ethylenic character.This behaviour should enhance the energy barrier for the twisting from the trans to the perpendicular configuration, as pointed out by Mazzucato for other substituted styrenyl compounds. 45ow if one considers the "benzoxazinone" character of the first singlet excited state of DFSBO, one must also keep in mind the resonance structures A and B of coumarin derivatives, as pointed out by Drexhage 58 (Figure 13).In their first excited singlet state $1, the polar form B is predominant.The opposite is true in the ground state.A large Stokes shift is thus observed in coumarine derivatives, particularly coumarins substituted by an heterocyclic substituent into the 3-position (coumarin 6, coumarin 30). 58As we indicated above, the simple picture of a charge transfer from the dimethylamino group of the benzoxazinone to the formyl end group is not correct since the para-substitution of the styryl by a dimethylamino group gave an even greater red shift of the absorption maximum.The large Stokes shift of DFSBO has thus a different origin from that of DCM and we conclude that the predominant mesomeric forms of DFSBO and DCM are those represented in Figure 13 and Figure 14.
Let us now come back to the fluorescence and excitation spectra of DFSBO in non viscous solvents.They are dramatically dependent on the observation wavelength.We ascribed the band shifts and the different vibronic bands observed to several configurations of DFSBO.Because we have now rejected the hypothesis of a cis-isomer formation, we believe that these configurations are those of two rotamers resulting from the rotation of the benzoxazinone moiety around the single bond.45][46][59][60][61][62][63][64] The rotamers of compounds which exhibit a variation in the fluorescence spectrum with the excitation wavelength should be almost isoenergetic, 63 i.e., they must show a similar steric interaction.One of the two DFSBO rotamers might be stabilized by intramolecular hydrogen bonding, as indicated in Figure 15.The mesomeric form (B) (Figure 13) probably favours this intramolecular hydrogen bonding.In many cases a two exponentials decay has been observed in the fluorescence of trans- diarylethylenes.42-44'46'61'63'64 The existence of rotamers is reinforced by the recent observation of a two exponentials decay in cyclohexane according to very recent experiments of Valeur et al. 65 Several simultaneous processes including solvent relaxation probably blurred the analysis in more polar solvents.

CONCLUSION
Despite the similar behaviour of their absorption and fluorescence spectra in solvents of increasing polarity indicating a large intramolecular charge transfer, the styryl dyes DCM and DFSBO undergo quite different photophysical processes.The nature of the intramolecular charge transfer is different.In DCM the charge is flowing from the dimethylamino end group to the dicyanomethylene end group.In DFSBO, the charge is probably flowing from the dimethylamino end group to the carbonyl group in the 2-position of the benzoxazinone.Intersystem crossing is totally inefficient in DCM but significant in DFSBO.The trans-cis photoisomerization of DCM is efficient in solvents of low polarity (chloroform, tetrahydrofurane) but we have no evidence of a cis-isomer of DFSBO.However, the existence of conformers of DFSBO is clearly demonstrated.They are probably two rotamers resulting from the rotation of the benzoxazinone moiety around the single bond.
We believe that the different behaviour of DFSBO with respect to DCM is related to the "benzoxazinone" rather than ethylenic character of the first singlet excited state of DFSBO.
We have also explained the poor laser action of DFSBO by the significant quantum yield of intersystem crossing to the triplet state and by the high molar extinction coefficient of the triplet state in the emission wavelength range.

Figure 4
Figure 4 DFSBO Stokes shift (vA vz) as a function ot Af Lippert's parameter.

Figure 5
Figure 5 Transition energies of DCM as a function of the methanol volumic percentage in methanol- DMSO mixtures (a) EA (absorption), (b) EF (fluorescence).

Figure 6
Figure 6 Transition energies of DFSBO as a function of the methanol volumic percentage in methanol- DMSO mixtures (a) EA (absorption), (b) EF (fluorescence).

Figure 9 F|gure 10
Figure 9 Difference absorption spectrum of the first singlet excited state of DFSBO in methanol, << 5 115.

Figure 11
Figure 11 Oxygen effect on the 650 nm transient absorbance of DFSBO in methanol.(a) Aerated solution, (b) Deaerated solution.

Figure 15
Figure 15 Planar molecular structures of the DFSBO rotamers.
Absorption and fluorescence of DFSBO in methanol.

Table 3
DFSBO maximum fluorescence wavelengths (3.era) and bandwidths (FWHM) observed in various solvents for different excita- Using the DODCI photoisomerization quantum yield in