Photochromic luminescence in crystalline aromatic hydrocarbon derivatives

SpectrochimicaActa, 1963,Vol. 19, pp. 1865 to 1870. Pergamon Press Ltd. Printed in Northern Ireland

Photochromic luminescence in crystalline aromatic hydrocarbon derivatives B. Department

STEVENS

and T.

DICKINSON

of Chemistry, The University, (Received 23 ApiZ

Sheffield 10

1963)

Abstract-Microcrystals of I-chloroanthracene and 9-cyanoanthracene exhibiting either a blue-structured or green-structureless fluorescence spectrum have been obtained either by fractional microsublimation or by recrystallization. In each case ultraviolet irradiation of the green-fluorescent crystals produced the blue-fluorescent form, after which the green-fluorescent form could be recovered by resublimation. Both forms of the same compound exhibit the same fluorescence spectrum in dilute solution. Crystalline 9,10-dichloroanthracene exhibits only a structureless green fluorescence which is stable to ultra-violet radiation. It is suggested that both the green- and blue-fluorescent forms of the mono derivatives have the same anthracene type lattice but that the former have an appreciable surface concentration of dimeric exciton traps which can be removed by photodimerization. The crystal structure of 9,10-dichloroanthracene has a different configuration which favours the formation of dimeric traps; however the photochemical destruction of these may be prevented by steric hindrance.

been suggested that the position and appearance of the fluorescence spectrum emitted by a crystalline aromatic hydrocarbon is related to the molecular orientation in the lattice which in turn depends on the shape of the individual molecules [ 1,2]. Elongated molecules such as naphthalene, phenanthrene, anthracene and chrysene exhibit weak coupling in a (type A) lattice where the overlap of adjacent parallel planes at an interplanar separation of >4 A is restricted both by the molecular separation and the steep angle of inclination of the molecular to the crystal axis; the fluorescence spectrum of the crystal is only slightly shifted with respect to the molecular spectrum and the vibrational structure is preserved. Disk-shaped molecules of pyrene, perylene, coronene and ovalene on the other hand prefer a lattice in which there is a large overlap of adjacent aromatic planes at an interplanar distance of -3.5 d; such a lattice (type B) is characterized by a structureless emission shifted to the red by some 5000 cm-l relative to the structured molecular spectrum observed in dilute solution. The emitting site in this case is believed to be a sandwich-like pair of parallel molecules present either as a unit of the B, type lattice of perylene and pyrene, or as a lattice or surface imperfection in the type B, lattice of coronene and ovalene [I]. Since the position and appearance of its emission spectrum in relation to the molecular spectrum is a most sensitive criterion of crystal purity, the possibility that the red-shifted structureless emission band of a type-B lattice is due to the IT HAS

[l] B. STEVENS, Spectrochim. [2] J. M. ROBERTSON, Proc.

Acta 18, 439 (1962).

Roy. Soo. (London) A20’7, 101 (1951). 1865

1866

B. STEVENS and T. DICKINSON

presence of an impurity cannot be eliminated. However, support for the explanation offered is provided by the work of TANAKA [3] who has obtained crystals of a green structured fluorescent modification of perylene with an A-type lattice, together with the normal B, type crystals, which emit a structureless orange band, from the same solution. If the molecular shape determines the position and appearance of the crystal fluorescence spectrum it should be possible to produce marked changes in the latter by introducing substituents to the molecule. This communication describes the emission spectra of crystalline 1-chloro-, 9-cyano- and 9,lOdichloroanthracene. EXPERIMENTAL

Samples of the substituted hydrocarbons, chromatographed on alumina, were donated by Dr. E. J. BOWEN. These were subjected to fractional microsublimation in a current of dry, O,-free nitrogen as described by MELHUISH [4]; the maximum temperature in the sublimation tube was adjusted to some 5°C below the melting point of the compound in each case. Fluorescence spectra of the sublimates were recorded photoelectrically in reflection on an Aminco-Keirs spectrophotophosphorimeter converted for use as a fluorimeter, with the appropriate length of Pyrex (5 mm. o.d.) sublimation tube placed vertically in the cell compartment, and the recordings converted to relative intensities by comparison with quinine bisulphate and m-dimethylaminonitrobezene standards [5, 61. Fluorescence spectra of the sublimates dissolved in spectroscopic grade solvents were obtained in the same way using the standard fluorescence cell and attachment. In all cases the slit system was that recommended for the highest resolution. RESULTS 9-Cyanoanthracene

m.p. 170-172’C

Fractional microsublimation produced: (i) A band of green-fluorescent needles near the hot end of the tube. (ii) A blue fluorescent sublimate towards the cooler end of the tube. The fluorescent spectrum of a very dilute solution of (i) in acetone was indistinguishable from that of a similar solution of the unsublimed material. However, the spectrum of (ii) in dilute acetone, although similar in appearance to those of the other solutions, was blue-shifted by some 20 m,u; moreover resublimation of the needles (i) did not produce the blue fluorescent sublimate (ii) which must therefore be due to an impurity in the original material. It was found that after several hours exposure to light of wavelength 365 rnp from a mercury arc, the spectrum of the green-fluorescent needles was completely replaced by a blue structured emission shown in Fig. 1. Resublimation of the blue fluorescent crystals produced a single band of green-fluorescent needles with the original spectrum (shown in Fig. 1) and which could again be converted to the blue [3] [4] [5] [6]

J. TANAKA. I.S.S.P. report A78, Tokyo (1963) W. H. MELHUISW, Nature 184, 1933 (1959). W. H. MELHKJISH,J. Opt.Sot.Am. 52, 1256 (1962). E. LIPPERT,~. NAGELE,~. SEIBOLD-BLANKENSTEIN,U. STAIGER and W.Voss, Chem. 170,1 (1959).

2. anal.

Photochromic

luminescence in crystalline aromatic hydrocarbon

The spectra fluorescent form on irradiation. solution were in~stinguishable (Fig. 1). -

of both

forms

1867

derivatives

in dilute

acetone

~-C~~or~n~~r~c~~e m.p. Sl-8f’C Fractional

microsublimat,~on

400

of the chromatographed

500

sample

produced

the

6CQmu

Fig. 1. Fluorescence spectra of I)-oyanoanthraeene; - - - green-fluorescent needles; -. - . blue fluorescence obtained from green-fluorescent needles after spectrum of both forms in dilute acetone exposure Hg 365 m,u radiation; ___ solution.

following three bands in order of position along the tube, i.e. of decreasing temperature : (i) A yellow-green structureless-fluorescent band. (ii) A blue structured-fluorescent band (iii) A very small impurity band. The fluorescent spectra of bands (i) and (ii) are shown in Fig. 2 together with the spectra of the same sampIes in dilute hexane solution. On exposure of the yellow-green-fluorescent sublimate to light at 365 rnp from a mercury arc, the yellow-green fluorescent band was completely repIaced by a weaker blue fluorescent band after s few hours; the spectrum of this band is shown in Fig. 2 together with the spectrum of its dilute solution in hexane. Slow evaporation of a concentrated solution of l-chloroanthracene in toluene produced both the yellow-green-fluorescent and blue-fluorescent crystals with the same spectral distribution as those of forms (if and (ii) obtained by microsublimation. Each of these crystals produced bands (i) and (ii) with no trace of the imthe impurity presumably remains in purity band (iii) when microsublimed; solution, Rapid evaporation of a solution in toluene produced only the yellow-greenfluorescent form as did precipitation from a solution in acetone when added to

B. STEVENSand T. DICEIXSON

1868

water; on the other hand the rapid evaporation of a solution in hexane produced only the blue-fluorescent solid. The ye~ow-green-fluorescent solid could also be obtained by rapidly cooling the fused blue-fluorescent crystals.

:ri

;! i. :I i 1 :t iil i. :I i‘. :..‘.

,I-\ \

h

I

I 4co

I 500

\ \ \ i

\ \ \ \ \ \ \ \ I \ \ \ \ \

\ \ \\

\ I 600 mu

Fig. 2. Fluorescencespectra of l-chloroantbracene; - - - yellow-green-fluorescent solid; -*-= blue-fluorescentsublimate: . . . . blue fluorescenceemitted by yellowin-fluorescent solid after exposure to Hg 365 rnp radiation; __ spectrum of different solid forms in dilute bexane solution.

Fig. 3. Fluorescence spectra of 9,10-dichloroanthracene; - - - crystals; __ dilute solution in hexane.

9,10-Dichloroant~racene

mp.

209-10°C

A single green-fluorescent band was obtained when this compound was subIts emission spectrum, together with that jected to fractional microsublimation. of its dilute solution in hexane, is shown in Fig. 3. No change in the spectrum of the solid was observed even after irradiation by a mercury arc for several days.

Photochromic

luminescence in crystalline aromatic hydrocarbon

derivatives

1869

Discussion If the crystal fluorescence spectrum is diagnostic of molecular orientation in the lattice [l], then it appears that both g-cyan0 and 1-chloro-anthracene exhibit dimorphism, the blue and green fluorescence bands originating from type-A and B lattices respectively. According to an X-ray analysis, 9-cyanoanthracene crystallizes [7] with an A-type lattice, and from the criterion of molecular shape [2] a similar structural type should be observed for 1-chloroanthracene although no structural data have been reported for this compound to our knowledge. It is ditlicult to accept that the conversion of one crystal form to another can be aocomplished by exposure to ultra-violet radiation, however, and the following alternative explanation is therefore offered. Assuming that both these compounds crystallize only with an A-type lattice, nevertheless the change in molecular shape due to substitution in the l- and g-positions promotes the formation of dimeric defects which aot as exciton traps, particularly at the crystal surfaces under conditions of rapid deposition. Following exoiton capture these traps may then either: (a) Emit the characteristic structureless red-shifted excimer band [8] usually observed in solutions of compounds of this type [9, lo] (b) IJndergo a photochemical dimerization to a dianthraoene derivative in which the lowest excited rr-electron states lie above the crystal exciton band [ll]. If these are competing processes then prolonged ultra-violet irradiation gradually removes the dimeric exeiton traps by process (b) and the normal structured shorter wave emission characteristic of a type-A lattice is restored. Both 9-cyanoand I-chloroanthracene are known to undergo photodimerization in solution at room temperature [12, 131. The crystal structure of 9,lOdichloroanthracene may be classified as type B since an X-ray analysis reveals [I.41considerable overlap of almost parallel adjacent molecules at an interplanar separation of 3.52 A; however the adjacent molecules are rotated by some 90” about the interplanar axis and the formation of defect dimer traps might be expected to involve some rotation about this axis in addition to a slight lateral ~splacement of one or both molecules. The absence of photochromism in this case indicates that photo~merization does not compete with trap emission although the photochemical removal of excitor1 traps should not in any case lead to the appearance of a structured emission band at shorter wavelengths characteristic of a type-A lattice; photodimerization of 9, IO-dichloroanthracene has not been reported to our knowledge and may be sterically hindered. If the spectral changes observed in crystals of I-chloro- and 9-cyanoanthracene [7] H. RABAUD and J. CLASTRE,AC&ZCVY& 12, 911 (1959). [S] 33. STEVENS,Nazzm 192, 725 (1961). [Q] TEL FORSTER and K. KASPER, 2. Elektrochem.59,977 (1955). [lo] 5.R. BIRKS~~~L.G.C~XSTOP~OROU, Nature l&442(1962).

[ll]C. A.COULSON,L.E. ORUEL,~. TAYLOR and J-WEISS,J.Chem.Soc. [12] R.CUS and R.L~DB, Bult.Soc. china. Prance 763 (1959). [13] R. LALANDE and R. &LAS, BJZ. 806. chim. France 144 (1960). [ 141 J. TROTTER,Acta Cry&.11,564 (1958).

2961(1955).

1870

B. STEVENSand T. DICKINSON

are due to the presence of an impurity, then in view of the similarity between the spectrum of the blue-fluorescent form and the molecular spectrum, the green band is the more likely to be an impurity emission. In this case since (a) Complete evaporation of solutions of the green fluorescent form in certain solvents produces a blue fluorescent residue (b) The green-fluorescent form is obtained by resublimation of the blue-fluorescent modification an explanation of the effects observed would require that the impurity may or may not be an efficient exciton trap depending on the conditions of deposition. In view of the similarity in shift and lack of structure of these spectra relative to the molecular spectrum, and the appearance of the excimer band in concentrated solutions of molecules of this type [9, lo], it is suggested that the emitting centre is not a chemical impurity but rather an orientational impurity with an excimer configuration. We gratefully acknowledge the gift of hydrocarbon derivatives from Dr. BOWEN, and the maintenance of T. D. by the D.S.I.R. who also provided a grant for the purchase of the spectrophotophosphorimeter.