ELECTRONIC SPECTRA OF AZA-AROMATICS IN POLYMER MATRICES

Photochemistry and Photobiology. 1968. Vol. 8. pp. 26 1-272. Pergamon Press. Printed in Great Britain

ELECTRONIC SPECTRA OF AZA-AROMATICS IN POLYMER MATRICES?

E. H. PARK, A. H. KADHIM and H. W. OFFEN Department of Chemistry, University of California, Santa Barbara, Calif. 93 106, U.S.A.

(Received 27 February 1968; in revisedform 30April 1968)

Abstract -The absorption and fluorescence spectra of acridine, phenazine, their cations and phenazine-di-N-oxide are studied in several polymer matrices. There is a one-to-one correspondence between previously measured spectra in fluid media and those reported here for plastic media. The matrix-induced spectral shifts are dependent upon the polarity and hydrogen-bonding ability of the polymer substrate. Environmental perturbations are also produced by high pressures (0-27 kbar) applied to the polymer matrix. Specific interactions between polymer substrate and the electronically excited solutes are evident from pressure-induced red shifts of the fluorescence and changes in quantum yields under pressure. These interactions between solute and its microscopic environment are subtle and varied for this group of nitrogen heterocyclics, although their effects are smaller than those encountered in the adsorption of related dyes.

I N T R O D U C T I O N

POLYMER matrices offer a new solvent ‘system’ for studies in molecular electronic spectroscopy. Although plastic scintillators, prepared by trapping organic solutes in the polymer matrix, have been used for some time, utility of polymer films in spectroscopic studies has only recently been demonstrated. For example, studies of triplet- triplet absorption, quantum yield, polarization, intramolecular and intermolecular energy transfer, and triplet state phosphorescence [ 11 have been carried out in plastics. Oster et a1.[2] have compa-ed the role of various polymer matrices in the phosphorescence process at room temperatures. Diffusion of oxygen in plastics has been studied by observing the time-dependence of the phosphorescence intensity of a solute in the plastic[3].

Less information is available about the electronic spectra of solutes embedded in polymer matrices. The present work demonstrates the usefulness of several plastics as optical media for the study of the absorption and fluorescence spectra of acridine and phenazine, their salts and one N-oxide derivative. The emphasis here will be on the photophysical process rather than photochemical reactions which also occur in polymer matrices for this class of compounds. The effects of chemical perturbations produced by protonation of the nitrogen heteroatom are compared with environmental perturbations produced by different polymer matrices or by the compression of a given matrix. A few results on the effect of high pressures on the electronic absorption spectra of anthracene. acridine and phenazine have already been published[4]. It is found that the electronically excited aza-derivatives are very susceptible to the microscopic environment surrounding the solute molecules.

E X P E R I M E N T A L Acridine was purified by column chromatography using aluminum oxide and silica

gel columns in a nitrogen atmosphere with pentane as the eluent[5]. Several repetitions

+This work was supported in part by the Office of Naval Research. 26 I

262 E. H. PARK, A. H. KADHIM and H. W. OFFEN

of the above process yielded very pale yellow crystals (M.P. = 110-1 10.5"C). Acri- dine purified by chemical methods had a slightly lower melting point and was not used. Phenazine was similarly purified by physical methods. Phenazine-di-N-oxide was formed after 16 hr of refluxing through the direct oxidation of phenazine with 30 per cent hydrogen peroxide in glacial acetic acid at 50°C[6]. The recrystallized product of orange-brown needles melted at 190°C compared to literature values of 195"C[7] and 204"C[6].

Plastic films for optical studies are easily prepared by dissolving the polymer in a suitable solvent, followed by evaporation. Each polymer is commercially available in a different state (powder, sheet, rod, beads, etc.) and required different solvents for embedding a solute in the matrix. Rods of polymethylmethacrylate (PMMA) and polystyrene (PS) were prepared from the purified monomer by thermal polymerization in the absence of any additives. The methylmethacrylate monomer is subjected to three alkaline washings (1 0 per cent NaCl, 20 per cent NaOH), dried by passing through a silica gel column, and twice distilled over phosphorous pentoxide at 47°C and 1 10 mm Hg. The middle portion is then refrigerated until used. Styrene monomer is purified by two distillations at 30°C and 13 mm over anhydrous calcium chloride. The other plastics were used as purchased. Dissolution of cellulose acetate (CA) or polyvinyla- cetate (PVAC) in acetone and reprecipitation by the addition of water did not yield improved transmission properties in the U.V. (see Fig. 1). The plastic CA (Eastman Organic Chemicals) is only 39.5 per cent acetylated. Similarly, polyvinylalcohol (PVA) dissolved in water and precipitated from methyl alcohol gave the same results as the orginal PV A powder.

The transmission of plastic films 0-004-0.01 5 in. thick is shown in Fig. 1. It is observed that the nine polymer matrices are suitable for optical studies above 300 nm, although the percent transmittance is reduced by partial crystallinity in polypropylene (PP), polyethylene (PE), and in the ionomer 'Surlyn A' developed by Du Pont. The

Wavelength (nm 1

Fig. 1 . Light transmission of plastic films.

Electronic spectra of aza-aromatics in polymer matrices 263

company states that this ionomer is formed between carboxyl groups attached to polyethylene chains and metal ions from Group I1 elements. Information about the transmission properties of 'Surlyn A' as well as C A and PVAC could not be found. The U.V.

absorption edges of the other plastics are in good agreement with the literature[8-13]. The very weak structure above the absorption edge can frequently be related to the carbonyl chromophore. The physical properties of some polymer matrices relevant to spectroscopic studies are tabulated elsewhere [ 141.

Plastic samples containing solutes are prepared by the common solvent evaporation technique. A transparent film is produced upon solvent removal from the solute-polymer slurry. The solute at nominal concentrations of 10-2-10-3 M for the aza-aromatics is assumed to be homogeneously dispersed to give a solid solution. Spectroscopic studies can be made with plastic films dried in air or with films placed under high vacuum for removal of oxygen and residual solvent. The common solvent used in the sample preparation is benzene for PMMA and PS, perchloroethylene for PE and 'Surlyn A'; N,N'-dimethylformamide for polyacrylonitrile (PAN). decaline for PP, and acetone for CA and PVAC. Frequently, elevated temperatures are required for dissolution and solvent evaporation. PVA samples are prepared at 50°C by adding solute dissolved in acetone to water containing dissolved PVA powder. The cations of acridine and phenazine are prepared by adding aqueous HCl to films of PVA, CA or PVAC containing the neutral parent compounds. Dilute acid yields the green-yellow acridine cation and the yellow phenazine mono-cation, while the addition of concentrated acid gives the red-purple di-cation of phenazine.

All absorption measurements are made in the Cary 14 spectrophotometer. The high pressure apparatus for use with the Cary 14 has been described[l5]. The fluorescence is measured at room temperature and 1 atm, at 77°K and 1 atm and at higher pressures (298°K) [ 161.

SPECTRA IN PLASTICS

The similar features of the long-wavelength absorption bands of anthracene, acridine, and phenazine are shown in Fig. 2. The spectra of these solutes in PMMA are in agreement with spectral measurements in nonpolar liquid solutions with respect to location, relative intensity, and vibrational structure[7, 17-2 I]. The T-* electronic transitions have been assigned as indicated in Fig. 2. Anthracene shows only the L, band, while the azaaromatics clearly show the L, + A band with high intensity at shorter wavelengths. The nlr* bands lie between the La and Lb band in acridine[7,22], and below the extended tail of the L, band in the phenazine molecule[7,23]. The latter type of orbital promotion has usually small intensity and is in this case entirely sub- merged under the band envelopes of the stronger ~ l r * transitions. The fact that the lowest excited singlet state in phenazine is a result of n +. T* orbital promotion is responsible for the presumed absence of fluorescence [ 71. Fluorescence was not detected for phenazine in several plastics at 298°K and 77"k with our apparatus.

It is seen from Fig. 3 that mono-protonation in acid media produces new features in the absorption spectra. Both cations show vibrational structure in the new absorption at longer wavelength, in correspondence with polar fluid media[7, 17, 191. The bands below 26 kK in energy are shifted toward longer wavelength (red shift) relative to the unprotonated azaaromatic. Solvent shift studies and the intrinsic intensity of the longwavelength absorption in the visible spectrum support an lL,(m, a*)'A

264 E. H . PARK, A. H. KADHIM and H. W. O F F E N

I2

1.0

3 0 8 a m (L 0 m m a

0.6

0.4

I - ANTHRACENE 2 - ACRlDlNE 3- PHENAZINE

Fig. 2. The absorption spectra of anthracene, acridine, and phenazine in PMMA at 1 atm and 298°K.

assignment [ 191. The charged heteroatom has then severely distorted the .rr-electron density of the short-axis polarized transition. The acridinium and phenazinium cations fluoresce very brightly in polymer matrices at room temperature. In the case of phenazine, protonation presumably reverses the energies required for n-n" and T-T* orbital promotion in order to explain the high quantum yield from the lowest energy .rr-r* state. Figure 4 illustrates the relation between the absorption and emission of acridine HC1 in PVA. For both cations, a one-to-one correspondence with the literature in fluid media is obtained[7,18,20,241.

P O L Y M E R M A T R I X E F F E C T S

The effect of the polymer matrix on absorption spectra[25] is illustrated in Fig. 5 for phenazine in PE and PVA. It is seen that the Lb band is narrower and structured in the nonpolar plastic relative to the polar PVA matrix. The long-wavelength shoulder (La band) of phenazine shows a larger red shift than the remainder of the absorption band in plastics with increasingly polar groups. The cations show smaller solvent effects


0.8

0 6

a 0 E

0

0.4 v) n a

0.2

0 0

Wavelength ( nm )

Fig. 3. Absorption spectra of the acridinium and phenazinium cation in CA.

I.(

W V 2

2 8 0.1 a

m U

0.;

ACRIDINE-HCL in PVA

I 1 I 1 I

350 450 550

WAVELENGTH (nm)

Fig. 4. The absorption and fluorescence spectra of acridine -HCI in PVA at 1 atm and at high pressures.

266 E. H. PARK. A. H. KADHIM and H . W. OFFEN

0.8

0.6

W u z

n cn

a m 0 0.4 m a

0.2 I

I I

I

\

I I I I \\. WAVELENGTH (nm)

Fig. 5. Absorption spectrum of phenazine in PVA and PE.

than the parent compounds. The band-maxima of acridine-HC1 are displaced relative to C A toward lower and higher energies in PVA and PAN. respectively. The fluorescence band maxima of this compound are compared in Table 1 for several media. Phenazine-di-N-oxide shows a 300 cm-I blue shift of the L, band origin (20.5 kK in CA) in passing from CA to PVA plastic (Fig. 6). This result in solid solvents is consistent with the data of Harrell[7] who observed a large blue shift in water relative to methyl- cyclohexane. The shift direction is ascribed to increased hydrogen bonding in PVA which would tie up the oxygen electrons, decrease the size of the rr-conjugative system and thereby produce a blue shift.

The effect of the polymer matrix on the fluorescence of acridine is illustrated in Fig. 7. The 0-1 band is located at 24.27 kK in PE and at 23-92 kK in PVA, i.e. a red shift of - 350 cm-I results from the increased polarity in the hydrogen-bonded plastic. The origin of the long-wavelength fluorescence, particularly evident in PE, is unknown but may arise from aggregates such as excited dimers. These results are reproducible and agree with the study of Ladner and Becker[26] who conclude that acridine has an


Table I . Fluorescence band maxima of acridine-HCI in three plastics at 77°K

Ca 22.12 21.19 PVAC 22.22 21.28 20.00 PVA 22,22 21.28 19.95 FPAt 22.22 2 1.28 2040 19.01

t Reference[26].

WAVELENGTH (nm)

Fig. 6. Absorption spectrum of phenazine-di-N-oxide in CA and PVA.

inherent fluorescence [ 71. However, the possibility. that residual solvent molecules are trapped near acridine molecules in the polymer matrix and form 'complexes' which assist the luminescence process[27], cannot be eliminated. The spectra at 77°K relative to 298°K are slightly blue shifted and show minor sharpening of the vibrational structure. If liquid monomer is cooled to 77"K, considerable fine structure is seen, as expected for frozen solutions of small solvent molecules. For example, the emission from phenazine-di-N-oxide consists of one broad band in PMMA and several sharp vibronic bands in frozen methylmethacrylate at 77°K.

PRESSURE EFFECTS The most easily measured spectral parameter at high pressures is the frequency

shift of the bands. Polymer matrices at room temperature are easily subjected to pres-

268

0 .

E. H. PARK, A. H. KADHIM and H. W. OFFEN

I , I I I 1 I I I

sures exceeding those available in apparatus for spectral studies of compressed liquid solutions. High pressures produce reversible effects in the spectra, such as band broadening, spectral shifts, and alterations in band intensities. Figure 4 illustrates such effects for the acridine cation in PVA. In a previous report[41 it was shown that the magnitude of the red shifts in the absorption spectra of anthracene, acridine, and phenazine depend upon the type of orbital promotion and the type of orbital promotion and the type of matrix. At 27 kbar the monotonic red shifts varied between 200 and 900 cm-l. Table 2 summarizes the measured spectral shifts for these azaaromatic compounds. Some of the data is illustrated in Fig. 8.

One feature illustrated by Table 2 is the difference in pressure effects on the two absorption bands of the charged and neutral nitrogen heterocyclics. It is observed that the red shift at the same pressure Av is greater for the L, band than for the Lb band of

Table 2. Red shift" (-Av. cm-') of aza-aromatics in two polymer Matrices at 23-25 k b d

CA PV A ABS. FLUOR. ABS. FLUOR.

Acridine L,, 130 L, 140 L,, 360 240 L,, (sh)

La 240 170 L,, 110 380 Acridine-HCI Lh 400 L, 310

Phenazine La 340 - Lb 160 - Phenazine-HCI Lb 550 L, 330

L,, 250 510' L,, 150 -0 Phenazine-di - N-oxide 60 300 170 200

"The red shift in absorption is measured for the first vibronic band of the indicated electronic transition. In fluorescence, the red shift of the band maximum is measured.

bEstimated error is - 100 cm. eNondegassed sample.


-400 -

- -200 E

2 -400

V -

-400 -

L I 1 I I I

5 15 25

PRESSURE (KBAR)

Fig. 8. Pressure shifts (Av, cm-') of several aza-aromatics in CA. I-acridine, Lb absorption; 2-acridine fluorescence; 3-acridine, L, absorption; 4-phenazine-di-N-oxide absorption, long wavelength band; Sphenazine-HCI, L, absorption; 6-phenazine, LB absorption; 7-phenazine- HCI, L b absorption; 8-acridine-HC1 fluorescence; 9-acridine-HC1, L, absorption: 1 O-acridine-

HCI, Lb absorption.

acridine [Av(Lb) < Av(L,)]. The pressure response of the cations of acridine and phenazine is the opposite, i.e. Av(Lb) < Av(LL,). The La band which was very sensitive to protonation (compare Figs. 2 and 3) now shows the smallest net environmental influence. The data also illustrate the difference between polymer matrix shifts at 1 atm and the pressure shift in a given polymer. The N + 0 di-substituted compound showed relatively large blue shifts in polymer matrices with increasingly polar substituents while the effects of compression is a small red shift (Fig. 8).

A third point that emerges from Fig. 8 and Table 2 is the difference in the pressure shift of the longest-wavelength absorption and the corresponding fluorescence. Elec- tronically excited polar molecules .are very susceptible to the microscopic environment. Not only the quantum yield but also the magnitude of the red shift depend upon the sample preparation and history. This is illustrated for acridine in Fig. 9 where the removal of most residual solvent and dissolved gases by pumping results in smaller shifts and a more gradual change in quantum yield. The solvents used here would freeze below - I5 kbar when compressed in bulk at room temperature. A similar solidification of the solvation shell from residual solvent in the polymer matrix might explain the strong fluorescence quenching observed for acridine (Fig. 9) as well as its

270 E. H. PARK, A. H. KADHIM and H. W. OFFEN

I,,,, 5 15

PRESSURE (KBAR)

Fig. 9. The red shift and intensity ratio of the acridine fluorescence (CA matrix) for undegassed (---) and degassed (-) samples.

cation at the higher pressures. The high pressure fluorescence changes may also indi- cate stronger adsorption on the polymer substrate, similar to the binding produced by amino-acridines[28]. These experiments are qualitative in that the precise content of solvent in the matrix is unknown. For example, some solutes may have a strong affinity for polar solvents and this may not be removed by ordinary pumping of the films at room temperature. The reversal in shift direction of the acridinium cation fluorescence at high pressures may be due to incomplete homogeneity in microscopic environment. The magnitude of the fluorescence shift may be greater or smaller than the absorption solute molecule. Such sensitivity in the fluorescence shift Av, compared to the absorption shift A V , ~ was not observed for nonpolar aromatic molecules such as anthracener291 or pyrene [ 161 which invariably show AvF > AvA at the same pressure.

The absorption intensity is also affected by pressure. Since window and base line corrections of the observed absorbance are crude, only qualitative statements can be made. Increasing pressures enhance band intensities of neutral aza-aromatics and reduce band intensities of the corresponding protonated species. Extreme values observed for the pressure effect on absorption intensity at band maxima are + 30 per cent for phenazine in CA and -30 per cent for acridine-HCl in CA at 20-25 kbar relative to atmospheric pressure.

DISCUSSION

This paper has demonstrated the convenience of introducing several solutes into polymer matrices. These simple aza-derivatives show a one-to-one correspondence between the spectra in solid polymer and fluid solutions. Spectral shifts are observed

Electronic spectra of aza-aromatics in polymer matrices 27 I

when the environment is changed by pressure or by choosing plastics with different substituents along the polymer chain. The magnitudes of the red shifts can be discussed in terms of solvent shift theories[4,25,30] in which the role of the liquid solvent or polymer in electronic transitions is characterized by the refractive index and dielectric constant of the medium[31]. Such theories are useful starting points for discussing solvent or density effects, although this becomes less worthwhile for polar solutes. A number of matrix and pressure effects have been measured in the absorption spectra of aza-aromatics. The observed response under these environmental perturbations is extremely varied. Evidently the medium is capable of strong, specific interactions with the electronegative heteroatom which influences orbital energies and vibronic intensity distributions. The n-rr* transitions were too weak to be observed in the neutral compounds, but the presence of the lone pair electrons undoubtedly contributes to the observed matrix and pressure shifts of 7 r q * transitions. If the lone pair becomes bonded to the proton, the charge distribution in the r-orbitals is modified and hence the inter- action with its environment. In balance, the interactions in simple aza-aromatics are much weaker than those observed for dyes adsorbed on polymeric substrates, where the polarity and amino groups strengthen the interactions considerably[32,331.

Specific interactions are not only evident in the absorption spectra of these polar solutes, but much more so in their fluorescence properties. The microscopic environment is decisive in determining the emission properties of electronically excited molecules in condensed media. Both fluorescence shift and quantum yield depend upon the detailed structure of the solvent cage. Hence other photophysical and photochemical processes would also be expected to be sensitive to the microscopic structure[34]. The heterogeneous medium consisting of polymer and residual solvent provides an interesting environment for studies of energy dissipation in polar solutes such as the aza-derivatives. This work demonstrates that an awareness of the local environment is essential for luminescence studies. Even though the solvation shell of solutes embedded in plastics by solvent evaporation is ill-defined it may approximate biological environ- ments. Plastic films yield little new spectroscopic information at low temperatures because inhomogeneous medium broadening prevents spectral resolution of vibrational structure. However, these media can be used at temperatures above ambient. Then one has a choice of polymer, residual solvent and dissolved gases to study photobiological processes under controlled conditions. Of course, viscous solutions containing polymers[34,35] may be used to complete the spectrum of possibilities.

This work has explored the medium effects in several polymers for some simple molecules. Such information is preliminary to photoreactions of complex molecules on heterogeneous substrates[ 33,361. The investigations employing polymer films com- plement dye adsorption studies on silica gel where strong substrate effects on luminescence have been observed[37]. Additionally, the diffusion of the quencher oxygen and its influence on photobiological processes can be studied in these heterogeneous media[ 3 3 ,3 8 ,3 91. Polymer media offer controlled conditions for studying the various factors influencing the structure and reactivity of electronically excited molecules. One important parameter for changing the local environment and binding in a specific manner is pressure. The next step is to demonstrate the usefulness of the pressure parameter in elucidating the mechanisms of heterogeneous photoreactions and energy relaxation in polymer matrices. The specificity of the pressure response found here is encouraging for such an extension.

772 E. H. PARK. A. H. KADHIM and H. W. OFFEN

1.

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3.

4. 5. 6.

7.

8.

9. 10. 1 I . 12. 13. 14. 15. 16. 17. 18. 19. 20.

21.

? 7 --. 23. 24. 25. 76. 27.

28. 29. 3 0.

31.

32. 33. 34. 35. 36. 37. 38. 3 9.

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ELECTRONIC SPECTRA OF AZA-AROMATICS IN POLYMER MATRICES