Reversible photochemistry of 4-nitro-substituted diphenylazomethines

J. Photo&em.

Photobiol.

A:

Chem.,

57

(1991)

Reversible photochemistry diphenylazomethines Helmut

235

235-246

of 4-nitro-substituted

Giirner

Max-Plunck-Institut

fir

Strahlenchemie,

W-4330

Miilheim

an der Ruhr

(F.R.G.)

Ernst Fischer Deparrment

(Received

of Structural

Chemisrry,

The

Weizmann

Institute

of Science,

Rehovot

(Israel)

June 21, 1990)

Abstract The photochemical properties of a series of 4,4’-disubstituted diphenylazomethines (R-C~HSCH=NC&IS-R’; R = H, NO>, CH3, 0CH3, N(CH,),; R’ = NO,; 2a, 3,4a, 5a and and thermal 6a respectively) were studied in solution. Direct E + Z photoisomerization Z-+E isomerization were observed in a broad temperature range; the activation energies were in the range 60-75 kJ mol-‘. Fluorescence was observed for 6b (R = NOz, R’ = N(CH,),) and 6a at low temperatures: the quantum yield increased with decreasing temperature. For the others, emission was not detected in fluid solution or in glassy media at - 196 “C. Triplet-triplet absorption (A,,, > 760 nm) was observed only for 6a by both direct and sensitized excitation. No transient could be detected in either the parent N-benzylideneaniline (I) or the other nitro derivatives by laser flash photolysis. The triplet state of high energy donors (e.g. benzophenone or xanthone) in acetonitrile at room temperature was efficiently quenched on addition of the diphenylazomethines. The triplet states of 6a (lifetime ~~<5 ps) and 5a (p < 90 ns) were generated, but no acceptor triplet was observed for the others. From energy transfer energy of 1 is about

measurements

230 kJ mol-‘,

with

a series

with slightly

of sensitizers,

smaller

values

it follows

that

the triplet

for the nitro derivatives.

1. Introduction X) and derivatives (diarylazomethines, N-Benzylideneaniline (C6HSCH=NCsHs, e.g. naphthylanils) exhibit E*Z photoisomerization [l-13]. The Z form (cis) is stable only at low temperatures and isomerizes thermally to the E form [l-9]. Little is known about the pathway(s) for photoisomerization of diarylazomethines. A triplet route for from sensitization meaE+Z isomerization is possible in many cases, as concluded surements with biacetyl and other sensitizers [3, 51. The triplet energy of anils has been estimated by energy transfer using steady state [3-51 and time-resolved 1141 [17], the triplet state of techniques. Unlike the stilbenes [15, 161 and aza analogues anils had not been observed on direct (non-sensitized) excitation until recently, when we reported the triplet-triplet (T-T) absorption spectra of 9-anthryl-CH=NChHd-R (4-R =H, Br, NOz (7-9 respectively)) [18]. For 9 the triplet state is formed in substantial yield on direct excitation and E +Z photoisomerization occurs, at least in part, via the lowest triplet state. For trans-stilbenes [15, 16, 19, 201, rrans-styrylnaphthalenes [21] and trans-azobenzenes [22], it has been demonstrated that the incorporation of a nitro group,

lOlO-6030/911$3.50

0 Elsevier

Sequoia/Printed

in The

Netherlands

236 especially in the 4 position, significantly enhances @;,c. It was therefore of interest to look for such an effect in the diarylazomethines. In this paper, we report an investigation of the following diphenylazomethines. C6H5CH=NC6Hd--NO2 (Za), NOz-CeH4CH=NChHs (2b), NOz- CeH,CH=NCs&-NO* (3), CH3-CC6H4CH=NGH,,-NO2 (4a), NOz-CbH&H=NC6H4-CH3 (4b), 0CH3-CC6H,CH=NC,H4-NO* (5a>, (CH&N-CC6H4CH=NC6H4-NO* (6a) and NO*-CC,H,CH= NC6H4-N(CH3)2 (6b). The results show that substitution of 1 by a nitro group does not significantly change the deactivation pathway of the excited E isomer unless the charge transfer character of the diphenylazomethines is substantially enhanced, as is the case for the two “push-pull” compounds 6a and 6b.

2. Experimental

details

The instruments and procedures have been described elsewhere [18]. The fluorescence decay kinetics at -196 “C were measured by a conventional single-photon counting apparatus (A,,, = 354 nm) with a time resolution of 0.5 ns. Laser flash photolysis was carried out using mainly 353 nm pulses; in some cases excitation wavelengths of 308 and 248 nm were used (width Q 20 ns) [18]. Energy transfer measurements using either A,,, = 248 or 353 nm gave essentially the same results. In both cases selective excitation of the sensitizer was not possible, but the concentrations of the donor and acceptor molecules were adjusted so that most of the light was absorbed by the sensitizer. In order to shift the limit of detection of the acceptor triplet to less than 100 ns, much higher concentrations (corresponding to A35J> 50; path length, 1 cm) were used. Compounds 1, 4a, 5a and 6b were obtained from Aldrich; the rest were prepared by conventional methods. The melting points (“C) were found to be 117-118 (2a), 92-94 (2b), 207-209 (3), 137-139 (4a), 122-124 (4b), 105-106 (Sa), 208-210 (6a) and 227-228 (db). The molar extinction coefficients (M-’ cm-lx lo4 at A,, (nm)) are .53*9= 3.4 for 2a, Ez92 = 2.9 for 2b, E29$,= 2.9 for 3, l336= 2.4 for 4a, +56 = 2.2 for 4b, l347= 3.9 for Sa, e400 = 3.9 for 6a and e4+, = 2.4 for 6b. Some spectra were published elsewhere [23]. All results refer to the E isomers unless the 2 isomers are specifically indicated. The following sensitizers (Merck, Fluka, Aldrich) were used (ET (W mol-‘) given in parentheses): xanthone (310), benzophenone (287), 9-bromophenanthrene (255), 2nitronaphthalene (238), 1-nitronaphthalene (229), fluorenone (223), benzil (222), pyrene (204) and benzanthrone (197). Most solvents were obtained from Merck; methylcyclohexane (MCH) and decalin were purified by passing through grade I basic alumina coIumns (Woelm); toluene, 2-methyltetrahydrofuran (MTHF), butyronitrile (Fluka) and ethanol were distilled. The other solvents were Uvasol products (2,2-dimethylbutane-n-pentane (8:3), acetonitrile) or analytical quality (cyclohexane, glycerol triacetate (GT)). The solutions were non-degassed in some cases (e.g. thermal 2-E conversion, fluorescence) and purged with argon in others (e.g. continuous irradiation, T-T absorption)_ 3. Results 3.1. Absorption spectra and E + Z photoisomerization Substitution of 1 by a nitro group in the 4 and/or 4’ positions shifts spectrum to longer wavelengths. The solvent also has some influence

the absorption on the shape

237

of the spectrum at low temperature, as shown in Figs. l(c) and l(d). The longwavelength part is enhanced on going from the alcohol mixture to toluene and the spectrum becomes broader in MCH. The effect of irradiation is shown in Figs. l(a)-l(c) for compounds 2a, 2b and 3. The results are ascribed to E -2 (trans+ cis) photoisomerization and the establishment of a photostationary state (PSS). The position of the PSS varies with the irradiation wavelength in accordance with the relative absorbances of the two isomers. This is illustrated for 2b in MCH at -90 “C, where the PSS contains more of the 2 form when hirr= 366 nm than when hirr= 405 nm (Fig. l(a)). A similar behaviour is observed for the two push-pull compounds (Fig. 2). Compared with compounds 2a-Sa, ‘the absorption maxima of the E isomers of 6a and 6b are significantly red shifted to 400-430 and approximately 450 nm respectively. The spectra of the Z isomers in Fig. 2 were calculated from the PSS with 436 nm irradiation, assuming 75% conversion to Z for 6a and 60% for 6b. The curves denoted “Z” in Figs. 1 and 2 represent a rough estimate of the spectra of the Z isomers. In the absence of simple methods for measuring or calculating these spectra, we followed from the PSS attained at our earlier method [2-51 of extrapolating the “Z” spectra conversion is at a maximum, e.g. 436 an irradiation wavelength at which the E 4Z E/Z ratio. This assumption is limited nm in Fig. 2(b), and assuming a “reasonable” by two basic requirements. The extrapolated spectrum cannot achieve negative values and the absorbance of Z at hirr cannot be zero. If this were the case the PSS would

oo-

--- “Z” -303nm -

08-

I

400

I

I

I

300

I

-nm

I

400 I

I

Fig. 1. Absorption spectra of 2b in MCH (a), 2a in MCH (b) and 3 in ethanol-methanol (1:l) (c) at low temperature prior to (full lines) and after irradiation (& and temperature as indicated). The “2” curves are the spectra of the Z isomers estimated by extrapolation [3, 41. (d) Solvent dependence of the absorption spectrum of non-irradiated 3.

238

nm

nm

Fig. 2. Absorption spectra of 6b in decalin at - 80 “C (a) and 6a in 2-propanol-ethanol (3:l) at - 120 “C (b) prior to (full lines) and after irradiation at 436 and 366 nm (PSS). The “Z” curves are estimated using &= 436 nm and assuming 60% and 75% conversion to Z for 6b and 6b respectively.

0

A

T 0

AA i 0

350 400 h (nm) Fig. 3. Absorption spectra (full lines, Amax= 1.5, path length, 1 cm) and difference spectra (0, AA”==O.l) recorded at the end of the pulse (Lx,=353 nm) for 2a (a), 4a (b) and Sa (c) in MCH at 25 “C. 250

300

contain only the Z isomers, i.e. the “436” curve in Fig. 2(b) would represent pure Z. Our estimation is about midway between the two extremes. The isosbestic points, e.g. 260 nm for 2a in MCH, are also markedly red shifted, e.g. to 370 nm for 6a in the alcohol mixture (compare Figs. l(b) and 2(b)). For 6b in decalin (Fig. 2(a)) a second isosbestic point is observed at longer wavelengths (510 nm) (as for 2a and 2b in MCH). This is due to a smaller decrease in the Z absorbance with increasing waveIength compared with that of the E isomer. E + Z photoisomerization at room temperature was observed by laser flash photolysis [18]. Examples of the difference spectra after the 353 nm pulse, together with the in Fig, 3 for 2a, 4a and Sa in absorption spectra of the E isomers, are presented MCH. These spectra, apart from small spectral changes due to the effect of temperature, are in agreement with those recorded at low temperatures after continuous irradiation. The wavelength for LU = 0 in the difference spectrum, corresponding to an isosbestic

239

point in the conventional to 295 nm for Sa.

absorption

spectrum,

increases

from

about

262

nm for 2a

Thermal Z -+E isomerization The absorption spectra of the irradiated solutions revert thermally back to the original E spectra at rates which depend on the temperature. The kinetics of this reaction were measured with the Gary spectrophotometer at low temperatures and with the laser flash photolysis set-up at elevated temperatures [18]. The recovery follows first-order kinetics; the lifetimes of the 2 isomers (l/k,,,) range from about 1 ms for 3 in acetonitrile to l-2 s for 6b in MCH, both at 25 “C (Table 1). For 3 it was ensured that the presence or absence of oxygen had no effect on the spectral or kinetic changes. Linear dependences of log kZ_E vs. T-l were obtained in all cases examined. From the Arrhenius plots the respective activation energies E, and A factors were calculated; the results are summarized in Table 1. The rate kZ_E at 25 “C, E, and A are virtually independent of the solvent and (within experimental error) not influenced from approximately 60 kJ mol -’ for 6a to approximately 75 kJ by &,,. E, increases for 4b. The A factor varies only moderately with the substituent. The values mol-’ are of the order of 10’2-10’3 s-l, i.e. they are close to the A factor usually found for diarylazomethines [3-91. 3.2.

3.3. Emission spectra and fluorescence properties No emission could be detected for compounds 2a-5a at room temperature (acetonitrile, ethanol) or for 2b, 3 and 4b in rigid glasses (MCH, MTHF, ethanol). However, a weak broad emission in the 480-550 nm range was observed for 2a, 4a TABLE

1

Thermal 24 E isomerization exponential factors” R

R’

in solution:

Compound

rate

constants

Solvent

kz+r

at 25 “C, activation

(25

(s-7 H NO2

NO, H

NO2

NO,

CH3 NO2

NO2 CH3

OCH3

NO2

N(CH&

NO2

NO,

N(CH3h

2a 2b 2b 3 3 4a

4b 5a 6a 6a 6b 6b 6b

MCH MCHb Acetonitrile MCH Acetonitrile MCH MCH MCHb MCH Alcohol’ MCH MCHb AlcohoF

300 3 3 550 800 500 2.4 400 600 > 100 1 2 Gl

“C)

energies

E, (W mol-*) 64.9 71.2 68.7 61.1 60.7 63.2 75.4 64.1 60.3 >60

and pre-

~XllY’

s-l)

50 6 2 20 25 43 26 47 16

66

ranges “In non-degassed solution, A,,, = 353 nm, unless otherwise indicated. The temperature were 30-100 “C for MCH and 25-70 “C for acetonitrile. b&,, =308 nm; values for 2b and 5a were similar to those at A,,= 353 nm. c2-Propanol-ethanol (3:1), A,,,=436 nm, temperature - 100 to - 80 “C and - 30 to - 10 “C for 6a and 6b respectively.

240

and Sa in MTHF or ethanol at - 196 “C. This emission is identified as phosphorescence because of its long lifetime (greater than 1 ms). The phosphorescence arises from 4nitroaniline which remains as a trace impurity (less than 1%) even after several recrystallizations. Hydrolysis could not be prevented in the experiments by any means as previously noted 191. Apart from the “impurity phosphorescence”, measurable emission was observed for 6a and 6b, especially at low temperatures. This emission is ascribed to fluorescence for energetic reasons and because of the short lifetime (no emission in the phosphorescence time range, e.g. after 10 qs). Examples of the fluorescence and excitation spectra at - 196 “C are shown in Fig. 4 and values of the fluorescence maxima Af and fluorescence excitation maxima A?” in several glassy media are compiled in Table 2. The shape of the excitation spectrum supports the assignment to the fluorescence of the substrate and excludes an impurity as the origin. The main effect on hgX is a red shift on going from 6a to 6b, as expected from the absorption spectra at room temperature. The solvent has only a slight influence on h r”; for 6b in MCH a shoulder appears in the excitation spectrum. The fluorescence emission spectra at -196 “C show that Ar has a larger value for 6b than 6a in a given solvent (Fig. 4 and Table 2). In addition, A, of both compounds exhibits a red shift with increasing polarity of the medium. At higher temperatures, however, there is no general trend in the fluorescence properties, apart from a decrease in intensity. In non-polar MCH, the temperature has little influence on Al of both compounds. In MCH-toluene, a medium of low polarity, hf at - 100 “C is smaller for 6b (but larger for 6a) than at - 196 “C. A red shift with increasing temperature is observed for 6a in GT but is less pronounced for 6b. This effect on hr is significantly stronger for 6a in butyronitrile, but weaker and in the opposite direction for 6b. On increasing the temperature, a red shift is observed, followed by a blue shift for 6a in ethanol. These observations are only preliminary due to the small quantum yield of

h(nm) too

”

28

24 -

500

20 i7 (cm-’

600

16

a00

12

x lo3 )

Fig. 4. Corrected fluorescence emission and excitation of 6a (a) and 6b (b) in MCH (. . - . .), MCH-toluene (-) at - 196 “C.

spectra ( - --),

(A,,, -436 nm, Al= 550-600 nm) MTHF (- --) and butyronitrile

241 TABLE

2

Excitation

and emission

Solvent

maxima

and quantum

Temperature (“C)

Z,Z-Dimethylbutanen-pentane (8:3) MCH MCH-toluene

MTHF CT

Butyronitrile Ethanol

“In non-degassed

solution,

A,, = 436

of fluorescence

A;”

Al

@ml

(W

421

430

432

431

in various

solventsa

6b

da

- 196 - 100 - 196 25 - 100 - 196 - 196 25 -80 - 196 25 - 196 2.5 -80 -120 - 196

yields

@f

500

0.004

495 500 500 522 505 525 620 548 545 680 545 565 590 640 520

< 0.001 0.01 < 0.0001 0.003 0.1 0.5 0.004 0.4 0.9 0.0007 0.7 < 0.0002 0.0005 0.008 0.4

hf

lf

(nm>

GW

450,

468

459

465

489

@f

560

0.002

565 560 530 535 582 600 620 605 606 530 616 520 554 592 604

< 0.0001 0.005 < 0.0001 0.0006 0.04 0.08 0.001 0.05 0.1 < 0.0001 0.1 < 0.0001 0.002 0.1

nm.

fluorescence @r at temperatures higher than approximately - 150 “C (except for a few cases, e.g. in GT). @of 6a in glasses at - 196 “C increases with increasing polarity from approximately 4x low3 in 2,2-dimethylbutane-n-pentane (8:3) to 0.01 in MCH, 0.1 in MCH-toluene and 0.5-0.9 in MTHF, GT and butyronitrile. An analogous solvent dependence and generally lower @r values (typically 0.1) are observed for 6b at - 196 “C (Table 2). The temperature dependence of the relative @f values is shown in Fig. 5 and Table 2 for several cases. The curves are similar for both compounds in a given solvent, @r shows an Arrhenius behaviour in the accessible temperature range which levels off at low temperatures, i.e. in the highly viscous region. This indicates an activated step in the radiationless deactivation pathway of the excited singlet state. The activation energies are approximately 14 kJ mol-’ in MCH-toluene and ethanol and around 25 kJ mol-’ in GT (Fig. 5). The steeper increase in @r with T-’ in GT relative to ethanol indicates that the viscosity also affects the main process which competes with fluorescence. The fluorescence lifetimes rf of the two “push-pull” compounds at - 196 “C were obtained from monoexponential fitting of the decay curves in the region of hf in some cases, better fitting was achieved with two components, e.g. in MCH and MCH-toluene (1:l). The r,values are 1.U2.3, < l/2.4,3.1,3.1 and 3.2 ns for 6a in MCH, MCH-toluene, GT, butyronitrile and ethanol respectively. The corresponding values for 66 are < l/2.9, 2.2i3.8, 3.1, 3.6 and 3.4 ns.

242 0

t(‘c)

-170-196

-140

-100

10-l

10-3 3

5 lO’/T

7 (K-‘)

9 -

13

500

600

700

800

A tnm)

Fig. 5. Temperature dependence of Pfe’ for 6a (open symbols) and 6b (filled symbols) in GT (triangles), MCH-toluene (l:l, squares) and ethanol (circles) (A,,=436 nm (not corrected for changes in absorption with temperature)). Fig. 6. Transient absorption spectra of 6a at room temperature on direct excitation (353 nm, end of pulse) in cyclohexane (. . . - +), acetonitrile (-) and ethanol (- . -) (a) and in argonsaturated acetonitrile under benzophenone-sensitized conditions 20 ns (e) and 1 ps (0) after the pulse (b).

Laser flash photoIysis Apart from a weak transient absorbance (&I) at high laser intensities in some cases, which may arise from trace impurities (or may even be caused by shock waves), no transient is detected on direct excitation of compounds 1-B and 6b in acetonitrile solution at room temperature using h,,,= 248, 308 or 353 nm. However, 6a exhibits a weakly absorbing transient in various solvents with A,,, in the red spectral range (Fig. 6(a)). This transient is assigned to a triplet state because it is formed within the pulse width, its decay follows essentially first-order kinetics, its lifetime (71.=&S-1) is quenched by oxygen and ferrocene and the same transient can be generated on xanthone or benzophenone sensitization (Fig. 6(b) and Table 3). Assignment of the observed transient to the triplet of 4-nitroaniline, recalling that this is a trace impurity in type a nitro compounds (see Section 3.3), is excluded on the basis of the strikingly different T-T absorption spectra of 4-nitroaniline (A,,<500 nm) and 6a. For 6a in butyronitrile at low temperatures, a similar T-T absorption spectrum is observed as at 25 “C; the yield is similar to that at room temperature but the lifetime is much longer (50 ms at -18.5 “C). A search for the triplet of 6b under these conditions was not successful. The rate constant for quenching of the triplet of 6a in acetonitrile by oxygen is 2 X lo9 MS1 s-l and that for quenching by ferrocene is about 5 x lo9 M-’ s-l. From the relative AA values of the triplet of 6a (Fig. 6(b) refers to about 70% energy transfer) and triplet benzophenone in argon-saturated coefficient of 1.6 X lo4 acetonitrile ( es*0 - 7 x lo3 M- ’ cm- ’ [24]), a molar extinction M-’ cm-’ is estimated for 6a. In a second experiment, @ii,, was obtained using the above value with reference to benzophenone (@is== 1). The results are @ir,=O.O8,0.05, 0.01 and 0.005 for da in MCH, acetonitrile, GT and ethanol respectively. 3.4

243 TABLE

3

Properties Compound

of the

transients

on direct

Sensitizer

and sensitized

excitation’

Solvent &)

;: 5a 6a

Xanthone Benzophenone Xanthone None None

Acetonitrile Acetonitrile Acetonitrile MCH Cyclohexane

353 353 353 353 248

No No 440-500 820 > 800

None

Toluene

353

> 780

None

Acetonitrile

248

780

None None’ None Xanthone Benzophenone

Acetonitrile Butyronitrile Ethanol Acetonitrile Acetonitrile

3.53 353 353 248 353

“In argon-saturated solution at room temperature bNo transient for 3 after 50 ns and for 6b after ‘At -185 “C.

I/ 200

/

3

‘:6

unless 200 ns.

>, 780 > 780 740 > 780 > 780

otherwise

< 0.05 < 0.0s - 0.09 0.08 0.08 0.2 =3

=5 5x104 31 54 Z4

indicated.

Xanthone Benzophenone J-Bromophenanthrene Z-Nitronaphthalene I-Nitronaphthalene Fkorenone Bend Pyrene Benzanthrone

250

300

ET (kJ/mol) Fig. 7. Semilogarithmic plots of the rate constants of triplet for 1 (circles) and 5a (triangles) in argon-saturated acetonitrile filled symbols refer to h,, =353 and 248 nm respectively).

quenching VS. the triplet energy at room temperature (open and

On addition of one of the other compounds (l-5a) to xanthone in argon-saturated acetonitrile, the lifetime (/cobs-l) of triplet xanthone (A,,= 620 nm) is reduced, but no new or residual transient with T~>O.S ps is observed. From the linear dependence concentration, the rate constant for quenching k, of kobs vs. the diphenylazomethine was obtained. For 1, 3 and Sa, kq is close to the diffusion-controlled limit: (1-2)X 1O1* M-l s-l. Similar results were obtained with benzophenone. For 1 and 3, at the largest concentrations possible, no transient could be detected after quenching the xanthone triplet to about 50 ns. This indicates that the triplet lifetime of the diphenylazomethines

244 is shorter than this limit and/or the extinction coefficient in the 400-700 nm range is very low (less than 100 M-’ cm-‘). The argument for rr<50 ns does not hold for 6b and 5a. For the latter, a remaining transient with a lifetime of about 90 ns is observed in the 440-500 nm range when the xanthone triplet is reduced to 30 ns. This indicates that rr of 5a is around 100 ns, taking into account that self-quenching may contribute to a certain extent. Cur experimental limit for 6b is rr< 200 ns. In order to estimate the triplet energy&of the diphenylazomethines, “Herkstroeter” plots were determined using various sensitizers. Plots of log i&, vs. ET are shown in Fig. 7 for 1 and Sa in acetonitrile. They exhibit a steep increase in the 190-230 kJ mol --” range. At higher ET values the curves level off, approaching k, = 1.5 X 10” M-’ s -1 .

4. Discussion Diarylazomethines show E * 2 photoisomerization but generally no emission [3-51. This holds for the parent compound 1 [3] and also for its derivatives with one (2a and 2b) or two (3) NO, groups, NO* and CH3 (4a and 4b) and NO1 and OCH, (5a) as suhstituents. The E-, Z photoisomerization is the dominant deactivation pathway of the excited E isomers. This can be observed either at low temperatures on a long time scale (Figs. 1 and 2) or at ambient temperature on a short time scale (Fig. 3). The corresponding Z-+E photoisomerization is revealed by the existence of a PSS and its dependence on the wavelength of irradiation (Figs. l(a) and 2(b)). For 1 and several diarylazomethine dyes, the quantum yields for both E-*Z and ZdE photoisomerization are substantial over a large temperature range, even as low as -180 “C [3-51. The thermal Z+E back reaction of 1 is characterized by an activation energy of 69 kJ mol-’ and an A factor of 1.2X 10” s-l, corresponding to a rate constant - 1 s-l at room temperature [6, 91. The effect on 1 of substitution by electronk, dtniting or electron-attracting groups at either the 4 or 4’ position has been studied previously [9]. In the present work, there are several cases where both positions are substituted. The data in Table 1 reveal a pronounced effect on kzdE at 25 “C for the two types of nitro compounds: those where the nitro group is attached to the phenylimino part (2a,4a, Sa, 6a) and those where it is attached to the benzylidene part (2b, 4b, 6b). The former (type a) show larger CCz_E values (shorter lifetimes in the millisecond range) than the latter (type b), which have lifetimes in the second generally have smaller E, values (60-65 kJ mol-‘) than range. Type a compounds type b (66-75 kJ mol-‘). Compound 3 behaves more like type a than type b. The behaviour of 6a and 6b, in terms of E +Z photoisomerization (Fig. 2) and the thermal back reaction (Table l), is very similar to that of the other cases examined. It is therefore remarkable that these two compounds show fluorescence at low where temperatures (Fig. 4 and Table 2). For some cases, e.g. 6a in butyronitrile becomes the dominant deactivation pathway. For d+=O.7 at - 196 “C, fluorescence 9-anthrylanil (7) and its derivatives 8 and 9, where a bromine or NO2 comparison, of fluorescence group is introduced, exhibit no emission at - 196 “C 1181. The occurrence in 6a but not in 5a (where the N(CH& group is replaced by OCH,) is probably due to the stronger charge transfer (CT) interaction between the para-substituted NO2 and N(CHs)* groups in the excited singlet state of 6a. Some information about CT in the ground state is available [12] but the influence of NOa and N(CH,), substitution on the excited state properties is not known.

245

The following fluorescence effects are apparent in the two push-pull anils: (i) the large Stokes shift in polar solvents (see Table 2); (ii) an increase in @t at - 196 “C with increasing polarity of the glass; (iii) an activation barrier in the processes competing with fluorescence (Fig. 5). This is compatible with the presence of a CT state for the E configuration which may be populated only in these cases. In the simplest case, @r/~~ should be constant when the environment is changed. However, the values for Tf (see Section 3.3) and Qf (Table 2) at - 196 “C do not correlate at all for the two push-pull compounds. For example, af of da increases by a factor of 70 on going from MCH to butyronitrile, whereas Tf changes by less than a factor of three. This indicates that the fluorescence does not originate directly from ‘E* but more likely from an additional state with CT character. For 4-nitro-4’dimethylaminostilbene, a relevant push-pull compound, the influence of CT interaction on the fluorescence properties has recently been discussed 1221. For 6a and 6b a detailed analysis is not yet possible due to the small Qf values at room temperature. A further remarkable result is the formation of a triplet state for 6a (Fig. 6 and Table 3). Gii,, of diarylazomethines seems to be small [3, 41 or virtually zero. Even the non-substituted anthrylanil 7 shows no triplet on direct excitation; ai,= is also small for 8 (R=Br) but markedly enhanced for 9 (R=NOz) [18]. Although, in the iatter case, the incorporation of a nitro group is sufficient for triplet population, this is not so for 2a-5a. Strong CT interaction is required for enhancing ai5;, of diphenylazomethines. The reason why rT is smaller for 6b than for 6a is not clear. A prediction about @;,, and TV, based on the absorption and fluorescence properties of the two push-pull . anils, does not seem possible_ Based on energy transfer, using a steady state method, a triplet energy of between 205 and 230 kJ mol-’ has been reported for 1 [3]. The latter value is in good agreement with the results obtained by the time-resolved method in acetonitrile at room temperature (Fig. 7). ET of 3 and Sa is only a few kilojoules per mole smaller. A precise Er value is difficult to obtain from this method due to the possible contribution of “non-vertical” energy transfer, as known for other anils [14] and for stilbenes [15]. The much smaller value (ET= 170 kJ mol-‘) for 7 compared with ET=230 kJ mol-’ for the diphenylanils is due to the introduction of the anthracene chromophore in 7 [18].

Acknowledgments We thank Professor D. Schulte-Frohlinde for his generous support, the late Mrs. Y. Frei for early experiments, Mr. M. Kaganowitch for synthesis of several compounds, Dr. H. J. Kuhn for synthesis of 4b, and Mrs. N. Castel, A. Keil, E. Hiittel, S. Roos and Mr. L. J. Currell for technical assistance.

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