Absorption and Emission Spectra and Triplet Decay of

Absorption and Emission Spectra and Triplet Decay of some Aromatic and N-Heterocyclic Compounds in Polymethylmethacrylate BY M. A. WEsT,* K. J. McCALLUM, R. J. Woons

Dept. of Chemistry and Chemical Engineering, University of Saskatchewan, Saskatoon, Saskatchewan, Canada AND

S. J.

(in part)

FORMOSINHO

t

Davy Faraday Research Laboratory of the Royal Institution, London W. 1. Received 22nd December, 1969 Polymethylmethacrylate (PMMA) is a convenient solvent for studies of the fluorescence, phosphorescence, and triplet-triplet absorption of a number of aromatic hydrocarbons and N-heterocyclic compounds. Theoretical assignments of some triplet states are discussed briefly. Triplet lifetimes of the solutes have been determined at 77 and 293 K by phosphorescence decay measurements and by kinetic spectrophotometry. Non-exponential decays of compounds with triplet lifetimes less than 0.1 s at 293 K have been attributed to diffusion-controlled quenching reactions occurring in low viscosity regions in the polymer.

Polymer glasses are convenient matrices for studies of excited states, particularly triplet states. 1 -s They can be used over a much wider range of temperature than frozen organic solutions such as EPA (ether-isopentane-ethanol) glass, which has been widely used as a solvent for triplet studies and, compared with glucose 6 and boric acid 7 • 8 glasses, which have also been used as matrices, have the advantages of being insensitive to moisture and of being prepared at relatively low temperatures at which solute decomposition is generally negligible. Polymethylmethacrylate (PMMA) appears particularly suitable as a polymer matrix : glasses containing a wide variety of organic, and a more limited range of inorganic, 9 solutes are readily prepared from methyl methacrylate solutions by gentle heating (at 60-80°C for 10-50 h). The glasses are readily cut to shape and polished, are transparent between "'350 nm and the near infra-red, are useable at temperatures up to "'90°C, and (for triplet studies) are semi-permanent when stored under normal conditions. An incidental advantage of the thermal polymerization of methyl methacrylate is the conversion of dissolved oxygen to a combined form which does not appear to be an effective triplet quencher; the monomer need not, therefore, be degassed nor polymerized in vacuo. Oxygen does not diffuse into the polymer very rapidly and we have found that cylindrical samples (25 mm diam. x 25-40 mm) can be handled and stored in air for several months without serious reduction in the triplet yield upon excitation. In the present study, triplet-triplet absorption spectra and triplet lifetimes of a number of aromatic hydrocarbons and N-heterocyclic compounds in PMMA solution have been measured by kinetic spectrophotometry. Fluorescence and phosphorescence spectra of the *present address: Davy Faraday Research Laboratory of the Royal Institution, London W.l. ton leave from Dept. of Chemistry, University of Coimbra, Portugal. 2135

2136

AROMATIC AND N-HETEROCYCLIC SPECTRA

solutes in PMMA were obtained using a spectrofluorophosphorimeter a nd the triplet lifetimes confirmed by measurements of the phosphorescence decay. The decay of triplet states in solid solution is generally considered to occur without significant material diffusion, and non-exponential decay observed with solid solutions has been attributed to the presence of impurities/ 0 • 11 aggregation of the solute, 12. 13 solute-solvent interaction/ 4 interaction of spacially isolated triplets/ 1 • 15 -17 the presence of more than one metastable state/ 1 and the presence of oxygen. 5 We have observed non-exponential decay of triplet states of a number of aromatic and N-heterocyclic compounds in PMMA that cannot be readily explained by any of these mechanisms and suggest an alternative explanation for the non-exponential decay observed in samples of this polymer prepared thermally in the absence of a catalyst (i.e., in polymer that generally contains a significant amount of residual unsaturation). EXPERIMENTAL MATERIALS

Methyl methacrylate (British Drug Houses and Aldrich Chemical Company) was rapidly distilled at atmospheric pressure through a 45 em helix-packed column and then stored in a refrigerator until required. Analysis by g.l.c. indicated the presence of less than 0.1 mol % impurity other than impurities that may have eluted with the major peak. Polymer prepared from the distillate did not phosphoresce. Styrene was distilled through a similar column under reduced pressure and also stored in a refrigerator. Benzoyl peroxide (Bt;itish Drug Houses) was used as supplied. Solutes were used as supplied (by Aldrich Chemical Company, British Drug Houses, Eastman Organic Chemicals, Koch-Light Laboratories or James Hinton) except as described below. 1,10-Phenanthroline and 1,2,3,4,6,7-tribenzophenazine were crystallized from benzene and 5,6,7,8-dibenzoquinoxaline from ethanol. Quinoxaline was sublimed. The following were zone-refined under argon (20-100 passes) after preliminary crystallization, anthracene, naphthalene and phenanthrene from ethanol, acridine and 7,8-benzoquinoline from Skelly C, phenazine from glacial acetic acid, and phenanthridine from aqueous ethanol. In all cases, the melting point of the solute used, determined with a Perkin-Elmer Differential Scanning Calorimeter, agreed with melting points reported in the literature for the compound. PREPARATION OF SAMPLES

Solutions (I0- 6 -I0- 2 M) of aromatic hydrocarbons and N-heterocyclic compounds in distilled methyl methacrylate were polymerized in air at "'80°C and the polymer samples cut and polished to give cylinders approximately 25 mm diam and 2-45 mm long. Initiator ( "'0.001 wt %benzoyl peroxide) was added only when samples otherwise failed to polymerize. Solute concentrations refer to those of the solute in methyl methacrylate prior to polymerization. Residual unsaturation in the polymer samples was estimated from their absorption at 1621 nm 18 determined with a Cary 14 recording spectrophotometer. Styrene solutions were polymerized in vacuo at temperatures up to 180°C, 19 and the polymer cut into cylinders and polished. APPARATUS

Fluorescence and phosphorescence emission spectra were determined at room temperature with a Parker and Hatchard type spectrofluorophosphorimeter. 2 0 Phosphorescence-decay curves for long-lived triplet states were obtained by disconnecting the disc drive and feeding the photodetector output directly to an oscilloscope. Flash excitation was also used to produce phosphorescence. Triplet-triplet absorption spectra at 77 K were determined on a Cary 14 recording spectrophotometer in a manner similar to that described by Henry and Kasha. 21 .Spectra at room temperature were measured following flash excitation by conventional kinetic spectrophotometry.

2137

M. A. WEST, K. J. MCCALLUM AND R. J. WOODS

Determination of triplet-triplet absorption spectra and triplet decay kinetics involved flashing individual samples a number of times. Only acridine and phenazine samples showed signs of slight decomposition after repeated flashing, becoming pale brown. No samples showed evidence of new absorption bands between 350 and 550 nm after repeated flashing. RESULTS

SINGLET ABSORPTION AND EMISSION SPECTRA

The principal singlet absorption (in ethanol) and emission (in PMMA) bands above 300 nm for some of the less-common solutes studied are given in table 1. PMMA absorbs strongly below "'300 nm so that absorption and emission measurements in this polymer are restricted to wavelengths longer than this. With the exception of acridine, which showed maxima at 317, 329, 343 and 380 nm in PMMA compared with reported maxima at 339, 351, and 380 nm in ethanol, 22 the solutes examined gave singlet-singlet absorption spectra in PMMA which agreed closely with TABLE I.-SINGLET ABSORPTION MAXIMA (IN ETHANOL) AND EMISSION MAXIMA

solute

singlet absorption in ethanol

l{nm

J.fnm

5, 6-benzoquinoline

1,2, 3,4-di benzophenazine 5,6,7,8-dibenzoquinoxaline 4,7-diphenyl-1,10-phenanthroline 2,3-diphenylquinoxaline 1,1 0-phenanthroline 5-phenyl-1 ,1 0-phenanthroline 4-phenylquinazoline 1,2,3,4,6,7-tribenzophenazine

* fluorescence spectra determined

307 314 321 329 337 345

fluorescence inPMMA

22

349 365 367 382 404 421*

(in PMMA)

perviously reported fluorescence solvent J.fnm

345 363 372 381 403 "'420

309 351 360 369 377 390 305 338 379 365 354 374 none observed "'310 343 486* 309 324 404 329 338 "'300 "'330 "'450* 316 400* 491 523 351 360 396 423 369 380 389 401 436 411

ref.

cyclohexane

23

ethanol

24

ethanol

24

with a Hitachi-Perkin-Elmer spectrofluorimeter (MPF-2A).

their absorption spectra in ethanol. The absence of additional absorption maxima with relatively concentrated PMMA solutions (other than those containing acridine) suggests that solutes dissolve in the polymer without forming dimers and without significant solute-solvent interaction. Acridine was a fairly effective inhibitor of methyl methacrylate polymerization and it seems probable that some chemical reaction involving the solute occurs when monomer containing acridine is polymerized.

2138


The fluorescence maxima reported in table 1 were obtained by directing th exciting light on to one plane end of the polymer sample and viewing the fluorescen e from the same face ; considerable distortion of the fluorescence spectrum occurr~ when the sample was illuminated through the curved side and viewed through th end. The phototube-monochromator combination u ed in obtaining the fluorescenc c data was calibrated using a standard quartz-iodine lamp and appropriate correctione made to the spectra obtained. The spectra showed maxima at positions close to those reported previously for fluorescence in liquid solutions but with a slight displacement (1-4 nm; ,..... 50-300 cm- 1 ) towards lower energies; Gaecintov, Oster and Cassen 5 have reported a similar red shift of200-300 cm- 1 when the solvent is changed from EPA glass (at 77 K) to a polymer (at room temperature). Pure PMMA showed a characteristic weak, bluish-white fluorescence at high light intensities. However the fluorescence of the polymer was of much lower intensity than that of the solute~ and did not interfere with the fluorescence measurements. TRIPLET ABSORPTION AND EMISSION SPECTRA

The principal triplet-triplet absorption maxima in the region 390-650 run are listed in table 2 for some of the less-common solutes studied in PMMA solution. Values given at 77 K were obtained directly using a recording spectrophotometer TABLE 2.-TRIPLET ABSORPTION solute

3,4-benzopyrene

"direct" determination (77 K) J.fnm

MAXIMA IN

"indirect" determination (293 K) J.fnm

419 442 470

fl.uoranthene acridine 5, 6-benzoqu inoline 7,8-benzoquinoline 5,6,7,8-dibenzoquinoxaline phenanthridine 1,10-phenanthroline phenazine quinoxaline 1,2,3,4,6,7-tribenzophenazine

,.....450 no pronounced absorption 420 418 440 438 480 440 470 500 433 460 490 460 490 387 429 460 440 465 520 560 421 "'440 450 426 no pronounced 440 absorption 460 no pronounced "'415 absorption 525 525

PMMA previously reported absorption maxima J.fnm

solvent

ref.

444 468 477 405

EPA*

25

420 440

benzene

26

EPA*

25

EPA*

25

benzene

26

EPA*

21

"'452 482 517 "'440 465 500

430 445 475 417

• EPA = 5 parts ether, 5 parts isopentane, 2 parts ethanol.

,

~ r

~~~~

II

'I

;

~

= · =

i

llli'!~

~~

~ iiii:!·

;:::!111 -··

acri dine 440 nm

ant hracene 425 nm 20 m / di

10m /di v

~-:T Ell

-·I"

[-

,·

I

\

r

!Iii:! iii:!

·~

-··

110'!

i

phenazine 440 nm 5 m / di

l( .

u:

!~ ;

~

~

-- -~1 ~

~~

phenanthridine 435 nm 10m / di

..... lliiiii::: ~~

phenanthrene 490 nm 500 m /di

--

naphthalene 415 nm 500 m / div

\

~

~~ ~ iiii:::! :::::!""

7,8-benzoquinoline 490 nm 500 m /di PLATE

1.- 0 cillo cope trace

To face page 2139.]

· 1,2,3,4,6, 7-tribenzophenazine

500 nm

200m / di

bowing the deca of triplet-triplet ab orption of olute molecule in pol ymeth ylmethacr late (293 K).

2139


while those at 293 K were derived indirectly from triplet-absorption decay curves at a number of wavelengths. Triplet absorption spectra determined by the " direct " and" indirect" methods agreed well with each other and with pectra recorded in the literature (the references given in tables 1-3 are not exhaustive). Acridine in PMMA showed a triplet absorption band at 480 nm by the " direct" method that was absent from the " indirect " spectrum. This is believed to be due to a photodecomposition product produced by the intense illumination of the mercury-xenon arc used in the direct determination; solutes other than acridine appeared to be stable under the conditions used. Table 3 lists the principal phosphorescence emission bands observed with PMMA solutions at room temperature. The observed maxima again appeared to be shifted towards lower energies by 1-10 nm ( rv50-500 cm- 1 ) when compared with phosphore cence maxima in EPA and other organic glasses at low temperatures. TABLE 3.-PHOSPHORESCENCE MAXIMA IN PMMA phosphorescence inPMMA (293 K)

solute

J.fnm

7 8-benzoquinoline 1,2,3,4-dibenzophenazine 5,6,7 ,8-dibenzoquinoxaline 4,7-diphenyl-1,10-phenanthroline 2,3-diphenylquinoxaline 1,1 0-phenanthroline 5,-phenyl-1,10-phenanthroline 4-phenylquinazoline quinoxaline 1,2,3,4,6, 7-tribenzophenazine

466 498 544 582* 441 460 494 522* 533* 461 489 544 530* 528* 472 5QO 531 "'580 *determined in PMMA at 77 K using an Aminco-Kiers

previously reported phosphorescence maxima (generally in EPA at 77 K) ref. A/nm

458 467 495 543 559 588 434 446 457

27 24 24

457 494 531

28

470 503

29

spectrophosphorimeter.

The intensity of phosphorescence shown by solutes in PMMA depends to a small extent on the amount of residual unsaturation (measured by the absorption at 1621 nm 18) in the polymer, samples estimated to contain 10-15 % of the original unsaturation giving lower intensities than samples containing little residual unsaturation. The phosphorescence intensity was reduced if the polymer samples were warmed above room temperature and it was also reduced, irreversibly, if they were exposed continuously to the mercury-xenon arc. After standing several months in air, polymer samples developed a surface region several mm thick that did not phosphoresce, although the material in the interior of the samples retained its phosphorescent properties and the entire sample fluoresced normally. Samples prepared and stored in vacuo did not show the surface quenching effect, which has been associated with the low diffusion of oxygen into the polymer. 30 - 32 TRIPLET DECAY

Typical oscilloscope traces showing the decay of triplet absorption at 293 K are given in plate 1. Analysis of the traces shows that they fall into two groups, those in which the decay is exponential over the entire interval examined (e.g., anthracene, acridine, phenanthridine and phenazine in plate 1) and those which show nonexponential decay initially which is followed, generally after about one half-life, by essentially exponential decay (naphthalene, phenanthrene, 7,8-benzoquinoline and 1 2,3,4,6,7-tribenzophenazine in plate 1). One compound examined, quinoxaline,

2140

AROMATIC AND N-HETROCYCLIC SPECTRA

showed entirely non-exponential decay. Phosphorescence measurements gave deca curves that did not differ significantly from those obtained by measurement of tripl~ absorption. At 77 K , the decay of triplet absorption and phosphorescence wa completely exponential and lifetimes, and the initial absorption and phosphore cence intensities observed, were appreciably larger than those at 293 K. Triplet lifetimes at 77 K and 293 K are given in table 4. Values at 293 K for which showed initial non-exponential decay at this temperature were estimated assuming the terminal decay to be exponential. This assumption is not completely valid and in some cases the lifetimes depended on the interval over which the decay was measured; these values should therefore be regarded as approximate. Lifetime were generally reproducible to ± 10 % with different samples and for determination made at intervals duri?g s~ver~l mont~s. ~e found no evi~ence of lifetimes being shortened by oxygen dtffuswn mto the mtenor bf samples wh1ch were exposed to air. TABLE

solute

4.-TRIPLET DECAY OF

triplet initial absorption decay at 293 K (*) J.fnm

SOLUTES IN

triplet lifetime t( = kJ.')fs

77K

424 470 580

exp exp non

0.046 2.6

0.026 0.071 0.87

fluoranthene naphthalene phenanthrene

450 415 490

non non non

0.76 2.3 3.8

0.4 1.3 1.3

triphenylene

430

non

14

5.3

acridine 5, 6-benzoquinoline 7,8-benzoquinoline 5,6,7,8-dibenzo quinoxaline phenanthridine 1,10-phenanthroline phenazine quinoxaline 1,2,3,4,5,6, 7-tribenzophenazine

400 500 490

exp non non

429 435 450 440 415

non exp non exp non

0.26

~0.05

non

0.74

0.32

525

0.94 0.87

previously reported lifetimes (solvent in parentheses if not PMMA)

t/s

77K

293K

anthracene 3,4-benzopyrene chrysene

3.3 1.9

PMMA

0.019 1.2 1.1 0.20 0.025 0.43 0.009

0.040

33

2.0 33 2.4 2 2.4 2 3.4 2 16 (EPA) 34

-293K

0.035 33 1.2 2, 3 "'0.3 1 1.5 2, 3 1.62 2.5 3 8.6 3 9.4 34

3.0 (ethanol) 35 2.1 (EPA) 25 0.95 (ethanol) 35 1.5 (ethanol) 3 5 0.25 (durene) 36

*exponential (exp) or non-exponential (non).

A few experiments carried out with polystyrene as the glassy matrix showed severe quenching throughout the sample after a few days exposure to air, suggesting rapid diffusion of oxygen into the interior. 10 No changes were observed in the triplet decay when solutions were flashed repeatedly at 77 K or, provided the samples were not allowed to become warm, at 293 K; both triplet absorption and phosphorescence intensity were reduced when warming occurred while the proportion of " nonexponential " decay was increased. Lifetimes were independent of the solute concentration and of the intensity of the exciting light. Representative results for phenanthrene are given in table 5. The effect of temperature upon the lifetime of triplet phenanthrene in PMMA is also illustrated in table 5, which includes representative results obtained at 77 K

M. A. WEST, K. J. MCCALLUM AND R. J. WOOD TABLE 5.-LIFETIME OF TRIPLET PHENANTHRENE IN cone. phenanthrene in monomer

[ J/M

2.3 x w-2 4.9x 10- 3 1.9x I0- 2 1.5 x w- 4 1.9 X 10- 2 5.4 X 10- 3 5.4 X 10- 3 6.7x 10- 4 6.7x 10- 4 sample heated at 105°C for 120 h 10- 3 ; + "'5 "% methyl isobutyrate

method •

temp. T/K

" exponential " T( = k"j 1)/s

abs phos pbos phos phos phos phos abs

77 77 77 293 293 323 353 293

3.4 3.25 4.2 1.7 1.65 0.82

abs

293

abs

293

0.35

1.4 0.95 0.95t 1.05 0.95t

2141

PMMA interval after excitation over which decay measured

t/s

2-12 0.75-3.75 0.75-3.75 0.5-3.5 0.5-3.5 0.1-2.5 0.3-0.55 0.4-1.6 0.1-0.35 0.1-0.35 0.1-0.4 0.1-0.4

0.45

* triplet decay followed by triplet-triplet absorption (Abs) or by phosphorescence decay (Phos) t intensity of exciting light reduced by approximately half.

e c:

0 ...D

~

...D 0

1

1·1

0

0·2

0·8 time, s

Fro. 1.-First-order plots of the decay of 1,2,3,4,6,7-tribenzophenazine triplet absorption in polymethylmethacrylate at 293 K. Polymer samples were heated at 105°C for the following times (the approximate content of unsaturated material, based on the absorption of the sample at 1621 nm, is shown in parentheses) : a, not heated (15 %) ; b, 0.25 h (11.4 %) ; c, 1 h (8.4 %) ; d, 4.5 h (2.1 %). The concentration of the tribenzophenazine in methyl methacrylate prior to polymerization was 1.1 X 10-4 M.

2142


and between 293 and 353 K. Anthracene in PMMA gave similar results. The data suggest a rapid increase in triplet lifetime with decreasing temperature from from 353 to 293 Kanda much smaller increase in lifetime between 293 and 77 K. 5,34,37 Polymer samples were normally prepared by heating methyl methacrylate solutions at ,...., 80°C until solid (12-48 h), to give PMMA which generally contained a significant amount of residual unsaturation (cf. fig.1 ). However, comparison of samples prepared in this manner with others prepared using an initiator (0.001 % benzoyl peroxide) to ensure more complete polymerization showed no significant differences in triplet lifetimes estimated from the exponential portions of the decay curves. Residual unsaturation was reduced by heating the polymer samples at 105°C for 12-120 h again without apparent effect _upon the (exponential) triplet lifetime (table 5). Reducing the residual unsaturation in PMMA by heating at 105°C reduced the extent of the initial non-exponential decay where this occurred. This effect is illustrated in fig. 1 for tribenzophenazine in PMMA ; the unsaturation present was estimated from the absorption at 1621 nm assuming it to have the same extinction coefficient at thi wavelength as methyl methacrylate. All samples, whether heated at 105°C or not, showed increased " non-exponential " decay if examined above room temperature. The presence of unsaturated material had little effect upon triplet lifetimes at 77 K, where phenanthrene had phosphorescence lifetimes of 3.6, 3.9 and 4.8 sin (aerated) methyl methacrylate, polymethylmethacrylate and EPA respectively. At the same temperature, tribenzophenazine had similar lifetimes (0.7 ± 0.2 s) in each of the three solvents. Polymerization of 10-3 M solutions of phenanthrene in methyl methacrylate containing 5 to 10 %methyl isobutyrate gave polymer samples showing no significant absorption at 1621 nm. Phosphorescence and triplet absorption decayed rapidly (within 0.05 s) .with a sample containing 10 % methyl isobutyrate. The triplet lifetime was reduced to about half its normal value in the polymer containing 5 % methyl isobutyrate (table 5). Weak delayed fluorescence was observed from concentrated solutions of phenanthrene and naphthalene on excitation with high light intensities at room temperature : the ratio of the peak intensities of delayed fluorescene to phosphorescence was less than 1 : 500. DISCUSSION SPECTRAL ASSIGNMENTS

Theoretical studies of the triplet states of the more important aromatic hydrocarbons have been described. 38 - 40 Since the introduction of a heteroatom in an alternant hydrocarbon produces small changes in the n energy levels, it is possible to correlate the states of the heteroconjugated molecule with the states of the corresponding isoelectronic hydrocarbon 41 and · to assign the levels of the triplet-triplet transitions. The introduction of an n orbital produces n-n* excited states for which triplet splitting is much smaller than the splitting for the n-n* states. The energy of the n molecular orbitals is a weaker function of the molecular structure and size than the energy of the n molecular orbitals. As the energy of the n-n* states decreases with an increase in molecular size, the lowest triplet state will generally be 3 n-n* for large molecules. 42 This is true for the solutes studied here. NAPHTHALENE AND QUINOXALINE

Defining the axes as shown, 40 the observed triplet-triplet transition in naphthalene is assigned as 3 B3 9 ~ 3 Biu- 38 • 40 • 43 The vibrational structure of the two bands 44 47 (with a separation of 1450 cm- 1 ) is identical with other published data. -


2143

Quinoxaline has a C 2 v symmetry and both triplets involved in the transition are the phosphorescence being 3 B2 ~ 1 A 1 • One explanation for the observed short 1J~time of this n-n* state is as follows. The intensity of an n-n* band is proportional to the electron density in the lowest n* orbital of the substituted carbon atom.

B

I

--69-I

1

I I

If there is more than one nitrogen atom, each will given rise to an n-n* band, and the intensity of these bands will depend on how many of these transitions contribute to the observed band. 48 • 49 With naphthalene, the electron density of the lowest vacant n-orbital is maximum at the alpha carbon atoms. The n-n':' band of quinoxaline will be reasonably allowed and the 3 n-n* spin orbit coupled with this state will have a shorter lifetime than other heterocyclic derivatives of naphthalene. ANTHRACENE, ACRIDINE AND PHENAZINE

The triplet-triplet absorption spectrum of anthracene, in the spectral region studied, agrees well with other published data. A 3 B39 +- 3 Btu transition 38 • 40 • 43 is obsttrved at 424 nm with two vibrational bands. The phosphorescence spectrum (which was not observed) is 3 Btu~ 1 A;. The spectrum of acridine is similar with the appearance of another vibrational band. With a symmetry C 2 v, the corresponding triplet-triplet transition is assigned as 3 B 2 +- 3 B 2 and the phosphorescence 3 B 2 ~ 1 A 9 • Introduction of two nitrogen atoms into positions 9 and 10 of anthracene (to produce phenazine) does not alter the symmetry D 211 and the assignments for phenazine are the same as for anthracene. The contribution from the n-n,.,' singlet state to the spin orbit coupling between the lowest 3 n-n* state and the ground state should be important . ince positions 9 and 10 are those with a maximum of n electron density. By a similar argument, the triplet lifetime of phenazine should be less than that of anthracene, and acridine, in agreement with our observations. PH ENA

THRENE,

5,6-BENZOQUINOLINE, 7 ,8-BENZOQUINOLINE, PHENANTHRIDINE AND 1,1 0-PHENANTHROLINE

The lowest triplet state of phenanthrene is 3 BI .40 • 50 The observed triplet-triplet transition is 3 A1 +- 3 Bt and the phosphorescence is 3 Bj ~ 1 A1. 1,10-Phenanthroline has the same symmetry, C 2 v, as phenanthrene. The other azaphenanthrenes have a Cs symmetry and all the states involved in the transitions transform as A'.

cP:Jc (l

d

The aza substitution at position a, where the n electron density is maximum is responsible for the short lifetime of phenanthridine. 1,10-Phenanthroline has a shorter lifetime than both 5,6-benzoquinoline and 7,8-benzoquinoline. In this compound, the n-n* singlet includes a contribution from the two nitrogen atoms but then electron den ity at po ition e i not a high as at po ition a.


2144

TRIPHENYLENE AND

5,6, 7 ,8-DIBENZOQUINOXALINE

As in all the aromatic hydrocarbons, the lowest triplet state is 3 La (or equivalentl A2 in the D 3 h group or symmetry, choosing the ~-axis perpe~dicular to the plane the molecule). We are not aware of any theoretical calculatiOns of the triplet level of triphenylene. The possible orbital states are A!, A2 and E'. A symmetryallowed transition is possible between 3 A2 and 3 E'. The phosphorescence (3 A2. ~ 1A') 1 is orbitally forbidden and the lifetime is very long. 5,6,7,8-Dibenzoquinoxaline has a C 2 v symmetry. By loweri!1g the ymmetry with crh~crv(yz), the lowest triplet state becomes 3 B 2 and the phosphorescence 3 B 2 ~ 1 A 1 is now orbitally allowed. Following this suggestion, the E' state splits into A 1 3 3 3 3 and B 2 and both the transitions A 1 ~ B 2 and B 2 ~ B 2 are allowed.

J

3

1 ,2,3,4,6, 7-TRIBENZOPHENAZINE, 3,4-BENZOPYRENE AND CHRYSEN E

Both tribenzophenophenazine and benzopyrene have a Cs symmetry and all the states transform as A'. Chrysene has a C2 h symmetry; the triplet-triplet transition is 3 A;~ 3 B;i and the phosphorescence is 3 B;i~ 1 A;. 36 • 46 TRIPLET DECAY

The compounds examined show two kinetically distinguishable types of triplet decay in PMMA prepared by thermal polymerization at "'80°C and containing residual unsaturation. At 77 K, and at higher temperatures for those compounds whose triplet states have short lifetimes ( < "' 0.1 s), the decay is exponential and is accounted for by the following first-order reactions: 3 p ~ 1 p + hv (phosphorescence) (1) 3 P~ 1 P (internal conversion) (2) 3 1 P+solvent~ P+solvent, (3) or impurity impurity or product.

ep and

3

p represent solute molecules in singlet ground state and lowest triplet tate respectively.) Reaction (3) represents any pseudo-first-order quenching reaction involving polymer or impurity molecules ; it is included to account for the shorter lifetime of anthracene triplet at 293 than at 77 K. The second type of decay, observed near room temperature and above with compounds having longer triplet lifetimes ( > ·"' 0.1 s), shows initial non-exponential triplet decay which, after about one half-life (the actual time depends on the composition of the PMMA matrix and the temperature), becomes essentially exponential (fig. 1). Triplet-triplet annihilation could be responsible for the observed nonexponential decays but most of our experiments were conducted with solute concentrations between I0- 4 and I0- 2 M and the average separation between randomly distributed solute molecules was appreciably greater than in the I0- 2 to I0- 1 M solutions examined by Azumi and McGlynn 15 and by Parker 17 who observed triplet-triplet annihilation processes. The absence of any significant effect upon the decay kinetics when the solute concentration was changed by several orders of magnitude (from 10-6 to 1o- 2 M for phenanthrene) and the light intensity reduced by a factor of two also implies that the observed non-exponential decay is not the result of interaction of two triplet molecules. One explanation for the observed decays would be· a series of first-order and pseudo-first-order reactions occurring in kinetically separate regions of the polymer.

'


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The sum of such reactions would give a triplet decay curve initially non-exponential but becoming essentially exponential if the decay in one of the regions was long compared with that in the other regions. The model that this suggests for the present system is one in which the solute molecules are divided between regions of extremely high viscosity, where material diffusion is negligible and triplet decay occurs by reactions (1) to (3) as in low-temperature decay, and regions of relatively low viscosity where decay occurs by the same reactions but with a considerably enhanced, diffusion-controlled, contribution from triplet-quencher reactions (eqn. (3)). The model proposed accounts for our experimental observations if PMMA is considered as made up of " solid " regions of close-packed polymer molecules in which trapped solute molecules are essentially immobile, and regions of varying, but relatively low, viscosity composed of unpolymerized and partially polymerized material in which solute molecules are able to diffuse. At 77 K, material diffusion will be extremely limited in all regions and triplet decay will be by reactions (1) to (3) with quenching (eqn (3)) limited to reactions of triplet molecules with their near neighbours and, possibly, to quenching over a longer distance by an energy-transfer process. At 293 K, triplet molecules with short lifetimes will decay in the same manner before sufficient diffusion is possible in the low-viscosity regions to affect greatly the rate of decay (a " short" lifetime in this context is one for which firstorder decay is considerably faster than diffusion-controlled quenching). Triplet states with " long " lifetimes at 293 K will react predominantly by quenching in the low-viscosity regions of the polymer and will decay by the " low-temperature " processes in the solid regions. If the rate of decay is markedly slower in the solid regions of the polymer, the overall decay will tend to become exponential as the total triplet population falls. The polymer is likely to contain " low-viscosity " regions with a wide range of viscosities, so that the overall decay curve will be the sum of a large number of exponential curves corresponding to the different viscosities present and may not become closely exponential until a large proportion of the triplet molecules have decayed. Since all the decay processes are first-order or pseudo-firstorder, the decay should be independent of solute concentration and light intensity, as observed (it is assumed that the concentration of quenching molecules is not dependent upon these variables). Non-exponential decay was most apparent with polymer samples that contained appreciable amounts of unsaturation or of methyl isobutyrate, i.e., with samples that would be expected to contain a larger proportion of lowviscosity material. Decay also tended to be non-exponential when polymer samples were heated above room temperature, probably as a result of a reduction in viscosity in regions in which diffusion would otherwise be slow. Quinoxaline appears to represent the situation where the rate of first-order triplet decay is about the same as the rate of the diffusion-controlled quenching reactions, and no " exponential " decay is observed. Other authors have also suggested the presence of low-viscosity region in PMMA and Shaw 32 has estimated the effective viscosity of the region through which oxygen diffuses into the polymer to be 40 poise, compared with a macroscopic viscosity for PMMA at 293 K greater than 1011 poise. The nature of the quenching species in reaction (3) is not established by our experiments. Molecular oxygen is an efficient triplet quencher and is apparently the cause of the phosphorescence quenching observed in the outer layers of samples that have been exposed to air for some months. However, our triplet-decay measurements were made on material in the core of the cylindrical samples where only combined oxygen is believed to be present. Unsaturated compounds present in the polymer might also act as quenchers, although at 293 K methyl isobutyrate appeared as effective as residual unsaturation in PMMA in enhancing rapid, non-exponential 68

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triplet decay. Unsaturated compounds appear also to have little effect under conditions where diffusion is limited. At 77 K, for example, changing the solvent from polymetbylmethacrylate to methyl methacrylate had only marginal effect upon the lifetimes of triplet phenanthrene and triplet tribenzophenazine and, at 293 K, the terminal, exponential, decay of these two triplet states in PMMA proceeds with the same rate constant regardless of the amount of residual unsaturation present in the sample (fig. 1). We thank the National Research Council of Canada for financial assistance and Prof. R. L. Eager and Prof. J. W. Hunt for much useful discussion. S. J. Formosinho thanks the Comissao Coordenadora da Investiga9ao para a NATO (Portugal) for the award of a post-graduate scholarship. Dr Ott and Denzer of Sandoz Pharmaceuticals kindly supplied us with a sample of 4-phenyl quinazoline. Note added in proof· Manville and Woods (unpublished results) have recently carried out an investigation of the n.m.r. absorption changes accompanying the polymerization of solutions of naphthalene in methyl methacrylate. The experiment show that the solute is mobile until the liquid starts to gel and then is progressively trapped as the monomer polymerises. The naphthalene ~.m.r. signal disappears completely when the polymerization is complete. 1

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