INTRAMOLECULAR PROTON TRANSFER REACTIONS
4473
Intramolecular Proton Transfer Reactions in Excited Fluorescent Compounds
by David L. Williams and Adam Heller* The Exploratory Research Laboratory, Bayside Research Center of General Telephone & Electronics Laboratories Inc., Bayside, NEW York 11380 (Received July 17, 1970)
Intramolecular hydrogen transfer reactions in excited fluorescent molecules lead to unusually large Stokes shifts. The photochemical Btability of compounds undergoing such reactions is also improved. As a result fluorescent materials of desirable properties are obtained. In crystalline 2-(2’-hydroxyphenyl)benzothiazole and in its derivatives, a proton is transferred in the excited state from an oxygen to a nitrogen atom. The compound and its derivatives have typical Stokes shifts of 7000 em-1 and quantum yields to 0.57. The photochemical stability of the compounds is greatly improved relative to Ai-methylated and other derivatives in which no hydrogen transfer can take place.
Introduction Many crystalline “azole” compounds having the general structure H-0
are bright, uv-stable fluorescers where X = S, 0, or ISH giving, respectively, benzothiazoles, benzoxazoles, or benzimidazoles. We have prepared over twenty of these azole derivatives and studied their solution properties and solid-state spectral characteristics. The compounds are generally colorless, but yield visible fluorescencc with long-wavelength uv excitation, the particular spectral characteristics being dependent on the nature and position of the various substituents, R and It’. It is our purpose to present a concerted model of the behavior of these azoles subsequent to excitation and, in particular, to elucidate the role played by the intramolecular hydrogen bond in stabilizing the molecules toward ultraviolet degradation.
Experimental Section 2 . Materials. All azoles were prepared by esseniially the same procedure, a one-step condensation reaction between a salicylic acid derivative and o-aminothiophenol (or derivative). The chemicals, obtained from chemical suppliers, were mixed in equimolar amounts in hot toluene. An equimolar amount of PC13 was added dropwise, and the solution was refluxed for 2 hr. The crystals formed on cooling were washed, filtered, and recrystallized from acetic acid. Yields for most azoles were about 50%. For 15, the N-methyl derivative (see Table I for identification of compounds), 10 ml of a 1: 1 molar mixture of 2-(2’-hydroxpheny1)benzothiazole and dimethyl sulfate were heated in an oil bath at 115” for 30 min. The clear, light-amber solution that resulted was cooled and about 75 ml of dry ether was added, and then 30 ml of distilled water. This solution was then made basic with sodium hydrox-
ide. The deuterated azoles were made from the parent compounds by exchange and recrystallization from deuterated acetic acid. Pure crystalline powders were used in most measurements. When plastic solutions were used, liquid solutions in partially polymerized methyl methacrylate containing 10% butyl phthalate were prepared. Polymerization was completed by adding 0.2% of benzoyl peroxide and heating in a sealed, evacuated ampoule at 60” for 3 hr. 2. Stability Studies. The crystalline samples, homogeneously dispersed over a support material, were placed in quartz tubes which were set in a circular holder that slowly rotated around a Hanovia 977B-1 1000-W mercury-xenon arc lamp. Between the lamp and the samples was a circular quartz cooling jacket through which distilled water was pumped. The entire system was encased in an aluminum tank in which air was continuously drawn for cooling and removing ozone. This arrangement ensured that all samples were equally and uniformly irradiated despite minor fluctuations in lamp output or inhomogeneities in radiation distribution. Operating conditions for the nominal 1000-W lamp were 28 A and 14 V, leading to an actual input power of 392 W. Approximately 30% of the lamp output occurs below 4000 8, and the lamp is assumed to be 20% efficient. This leads to a 23.5-W output of ultraviolet radiation, or 0.075 W/cm2 on the samples which were a t a distance of 5 cni from the arc. 3. Fluorescence Xpectra. Spectra of the crystalline solid powders were recorded with a Jarrell-Ash 0.5-m Ebert scanning spectrometer fitted with a llS0 lines/ mm grating blazed a t 5000 8. The excitation source was a Hanovia 977B-1 1000-W mercury-xenon arc lamp coupled with a Schoeffel Instrument Company miniature quartz p$sm monochromator; excitation was usually at 3663 A. Fluorescence was detected by an 1TT FW130 focusing photomultiplier (S-20 response) operated at a potential of 1600 V. Prior to striking
* T o whom correspondence should be addressed. The Journal of Physical Chemistry, Vol. 74, ~VO. 26, 1970
4474
DAVID L. WILLIAMS AXD ADAM HELLER
Table I : Emission Characteristics of Azoles
h a x ,
Compd
mr
Relative Quantum photoefficiency, chem. % stabilitya
14 15 16 17 18 19
I. X = S Benzothiazoles 2‘-OH 517 30.7 10 2‘-OD 516 21.0 2’-OH, 5’-OCHs 575 13.5 36 2’-OD, 5’-OCHa 576 8.3 2’-OH, 3’-OCHa 529 21.8 43 2‘-OH, 5’-XH2 543 37.6 2’-0H, 5’-Br 523 2.2 50 2’-OH, 5’-C1 527 25.4 45 2’-OH, 3’-NOz 537 -1 0 2’-OH, 3’-C1, 5’-Cl 536 36.5 30 2’-OH, 5’-NO2 518 6.5 0 2’-OH, 3’-NOz, 5’-iY02, Thermochromic and 5-KO2 photochromic 2’-OH, 8’-OCHa, 5-C1 533 5.8 2’-011, 5’-CH3 532 57 97 2’-OH, 3’-C(CHa)a, 523 36.9 8.5 6‘-CHa 2‘-OH, 3’-biphenyl 550 28.9 55 2‘-0-, S+-CHa 604 5.4 0 No substituent No fluorescence 3‘-OH S o fluorescence 4‘-OH No fluorescence 2’-OCHa No fluorescence
20 20D
2’-OH 2’-OD
21
111. X = Tu” Benzimidazole 2’-OH 462 3.5
1 1D 2 2D 3 4 5 6 7 8 9 10 11 12 13
11. X = 0 Benzoxazoles 506 503
42.0 34.0
38
Per cent residual brightness after 26 hr of illumination at 0.075 W/cm2 of ultraviolet radiation. a
the photomultiplier, the emission was chopped a t 200 He, and the modulated signal was amplified by an Electronics, ;\Iissiles, and Communications, Inc. Model RJB Lock-In amplifier and was recorded. The spectral response of the system was determined by using a standarized XBS tungsten filament lamp of known output. A computer program was written which corrected the spectra and plotted out the true fluorescence emission curves. The empirical curve was processed with a Calma VIP 480 digitizer. The fluorescence spectra were obtained in an apparatus allowing front surface excitation. The exciting radiation was diffused to a hemispherical reflecting surface with a hole (for viewing the sample) in its center. The sample was placed in the focal point of the hemisphere. In measurements down to liquid nitrogen temperatures cold nitrogen, obtained by boiling off the liquid, was passed through the quartz chamber in which the sample was held. For measurement of spectra a t lower temperatures a special liquid helium Dewar with windows at 90” was used. Helium boiling off the liquid was used for cooling. The Journal of Phgsical Chemistrg, Vol. 743No. 26, 1970
4. Quantum Eficiencies. Quantum efficiencies of the solid powders were determined by reference to a known NBS standard luminescent compound, magnesium tungstate, and to sodium silicate whose efficiency is wavelength independent from 2537 to 3550 A. This, coupled with measurements of relative excitation intensities a t 3550 and 3663 A, enabled a determination of quantum efficiency of the crystalline azoles at 3663 based on a value of 43.4% for sodium silicate. 5. Excitation and Infrared Spectra. The excitation spectra of the powders were taken on a Hitachi NPF2A fluorescence spectrophotometer. The infrared spectra were taken on a Perkin-Elmer 627. 6 . Decay Rates. A capillary (1 mm i.d.) lamp filled with 20 atm of hydrogen, and a 0.5-em gap between the electrodes was used to obtain pulses of light for the excitation of fluorescence. The decay time of the pulse to l / e of the peak intensity was about 1 nsec, but the light pulse had a “tail” of about 8 nsec, limiting the range of our measurements to decay times longer than sec. The lamp was pulsed at loa cps. The light was filtered by a filter combination limiting the radiation reaching the sample to the ultraviolet. The emitted light from the sample was focused on a fast photomultiplier (less than 1-nsec rise time) directly connected to a fast Tektronics sampling scope, Model 556 with a 1S1 sampler. The rise time of the sampling scope system was less than 100 psec. Results and Discussion i. Proton Transfer. The phenomenon of proton transfer in excited states has been firmly established by studies on the thermochromic and photochromic properties of anils of o-hydroxybenealdehydes,’-6 all having the basic substructure
N -2
Cohen and Plavian‘ proposed such a transfer process for the luminescence of solutions of two azoles, 2-(2‘hydroxypheny1)bensothiazole (1) and 2-(2’-hydroxypheny1)benzoxaeole (20). The stable ground-state configuration of the benzothiazole shown above has the proton predominantly on the oxygen, whereas the “stable” excited singlet configuration, s‘*,has the proton on the nitrogen. The molecule initially formed in the excited shate, S*, can be regarded simply as a vibrationally excited form of (1) M. D. Cohen and S. Flavian, J . Chem. SOC.B , 317 (1967). (2) & D. I.Cohen and G. M . J. Schmidt, J . Phys. Chem., 66, 2442 (1962). (3) M . D. Cohen, J . Pure A p p l . Chem., 9, 567 (1964). (4) R . S. Beclcer and W. F. Richey, J . Amer. Chem. Soc., 89, 1298 (1967). (5) Tir. F. Riches and R. S. Becker, J . Chem. Phys., 49, 2092 (1968) I
(6) R . Potashnik and ,M.Ottolenghi, i b i d . , 51, 3671 (1969).
INTRAMOLECULAR PROTON TRANSFER REACTIONS
us& di& H-0
0-
-
-
H-transfer+
S"
s'"
lisorption
t
H- 0,
SO
Visible fluorescence
H I
0,-
SO'
Basic Mechanism for Fluorescence of 1
S'*. Vibrational relaxation by loss of the excess vibrational energy to the environment is rapid. The same type of argument can be made for the ground state wherein S',,is viewed as a vibrationally excited form of
so. The transfer of the proton in the excited state is evidenced by a large Stokes shift for the azoles (-7400 cm-l). The change in the electronic distribution effecting the transfer has been demonstrated by studies of pH changes in excited states.'-'O For excited singlets, phenols become considerably stronger acids and N-heterocyclics become stronger bases; changes for excited triplets are several orders of magnitude less. lo 2. Uv Stability. Instability toward ultraviolet radiation is simply a measure of the probability of photochemical bond breaking upon excitation by uv light. If, in fact, excess energy is localized in one of a few vibrational modes for a finite length of time, the particular bond may be expected to rupture. However, if the process is reversible, no permanent degradation occurs; such a reversibility is built into an intramolecular hydrogen bond. An internal mechanism is thus provided to stabilize a molecule against uv degradation. Recent discussions of photochemical reactions",'* have indicated that exccss electronic energy is preferentially coupled into stretching vibrations with high vibrational energies, in particular, the stretching modes involving hydrogen with carbon, nitrogen, and oxygen. It has been theorized that acceptance of electronic energy may incipiently involve only one stretching vibration of a hydrogen atom, and also that proximity to the center of electronic excitation is a factor influencing energy transfer.'* Furthermore, the larger the a n h a r m ~ n i c i t yof ' ~ the vibration or the displacement of nuclei upon excitation,'* the greater the Franck-Condon overlap, and thus the larger the probability of a stretching mode being activated. I n light of these ideas, it should be expected that excess electronic energy would flow, a t least initially, into the stretching vibration of an intramolecularly bonded proton between an oxygen and nitrogen in molecules such as the azoles under discussion. For an n --b T * transition, the proton is close to the center of
4475 electronic excitation, and a vibration between two different locations would be expected to be exceedingly anharmonic. Furthermore, because of the changes of the acidities of the phenol and the nitrogen, the hydrogen nuclei change their equilibrium position upon excitation. Hence, stability toward ultraviolet radiation can be incorporated into a molecule by a judicious choice of potential energy acceptors. 3. Spectral Characteristics of Crystalline Azoles. Table I lists the various azoles studied, their fluorescence wavelength maxima, quantum efficiencies, and relative photochemical stability. The actual stability figures are given in terms of per cent residual brightness after 26-hr illumination at 0.075 W/cm2 uv radiation. This would correspond to 1600 days using a 15-W black light a t a distance of 36 in. The fluorescence spectra of all the azoles are similar; that for 2-(2'-hydroxyphenyl)benzothiazole (1) is given in Figure 1. An important point is the lack of any observable fluorescence emission for 16, 17, 18, and 19 (see Table I), none of which has a hydroxyl group in the 2' position. 17 and 18 have no hydroxyl group and 19 has a 2'-methoxy group. From these observations, it appears that a hydroxyl group in the 2' position is a necessary requirement for fluorescence. There is an indication in almost all the azole spectra that there are actually two broad overlapping bands, the second band being located some 800-1000 cm-l to the red relative to the first. This is best manifested in the spectrum of 3 fhown in Figure 2. Certain derivatives, namely, 7, I), and 14, show almost 110 indication of a second peak. The origin of this second peak will be indicated later. The oxazole, 2-(2'-hydroxyphenyl) benzoxazole (20), has a higher quantum efficiency and greater uv stability than 1, its thiazole counterpart. Its fluorescence emission, shown in Figure 3, is more structured than that of the thiazoles, having four bands instead of two. The spectrum of the imidazole, 2-(2'-hydroxypheny1)benzimidazole (21)) is given in Figure 4. It is an unsymmetrical single peak shifted well into the blue from either 1 or 20. It is a relatively weak emitter. An excitation spectrum of 1 is given in Figure 5. It is representative of the excitation spectra of all the azoles listed in Table I, having a maximum a t approximately 3750 8. (7) A . Weller, Z. Elektrochem., 60, 1144 (1956) (8) A . Weller, ibid., 61, 956 (1957). (9) N. Mataga, Y. Kaifu, and M. Koizumi, Bull. Chem. SOC.Jap., 29, 373 (1956). (10) G. Jackson and G. Porter, Proc. Roy. Soc., Ser. A , 260, 13 (1961). (11) N. C. Yang, S. P. Elliott, and B. Kim, J . Amer. Chem. SOC.,91, 7551 (1969). (12) A. Heller, Mol. Photochem., 1, 267 (1969). (13) W. Siebrand and D. F. Williams, J . Chem. Phys., 49, 1860 (1968). (14) R . Englman and J. Jortner, J . Luminescence, 1,2, 134 (1970).
The Journal of Physical Chemistry, Vol. 74, S o . 26, 1970
4476
45'30
DAVID L. WILLIAMS AND ADAMHELLER
5900
6000
55C0 WA'!
E L EbIGT t1
650C
7300
?
Figure 1. Fluorescence spectrum of 2-(2'-hydroxyphenyl)benzothiaxole (1).
WndELENCTH
r%)
Figure 5 . Excitation spectrum of 2-(2'-hydroxypheny1)benzothiaxole (1). 'IMVELEYGTH
(8)
Figure 2. Fluorescence spectrum of 2- (2'-hydroxy-3'-methoxyphenyl)benzothiazole (3)
I
Lj03
50C0
5503 'WJELENSTtl
6C33
6500
?GO3
(8)
Figure 3. Fluorescence spectrum of 2-(2'-hydroxypheiiyl)benzoxazole (20).
ponent (phosphorescence) at either room or liquid nitrogen temperatures in the crystalline materials. Solutions of the compounds in plastics, in which phosphorescence is usuaIly observabIe a t room temperature, did not show phosphorescence. 6. Mechanisms of Fluorescence and Uv Xtabilixation. The combination of large Stokes shift and unsymmetrical fluorescence band shape are consistent with the basic proton transfer mechanism illustrated previously. Also, the lack of fluorescence in molecules lacking a hydroxyl group in the 2' position confirms the importance of a proton being associated with the nitrogen as part of the luminescence mechanism. To further illustrate the function of the intramolecularly bonded proton, an N-methyl derivative of 1 was prepared.
6- 13-Methyl-2 (3H)-benzothiazolylidene]cyclohexa-2,4-dienone
MIELENGTH
'A)
Figure 4. Fluorescence spectrum of 2-(2'-hydroxyphenyl)benzimidazole (21 )
4. Decay ChaTacteristics. The emission of each compound investigated decays with a rate exceeding IOs sec-', the fastest rate that we could measure on our apparatus. There \\;as no evidence for any slower comThe Journal of Physical Chemistry, Vol. 7dVNo. 26, 1970
This compound, 15, exists in two distinct forms (vide infra), a yellow modification stabilized by water and a dry-stable orange modification. The yellow, wetstabilized form fluoresces yellow-green, and the orange, dry-stabilized form, fluoresces orange. However, neither form is stable toward uv radiation, both degrading very rapidly. This is a further indication that the facile transfer of a proton from oxygen t o nitrogen is the key to stability. It is, however, not absolutely essential for luminescence. Lacking any substituent on the nitrogen in the excited state, the molecules do not fluoresce. The fluorescence of the N-methyl derivative
INTRAMOLECULAR PROTON TRANSFER REACTIONS
4477 H
H
I
I
S”
0
1600 WAVELENGTH (cm-’)
Figure 6 Infrared spectra of the two modifications of 15: (a), recrystallized from water; (b)- - --, same as (a) after having dried in spectrometer; (c)-.-.- .-, recrystallized from anhydrous CHICN.
-
demonstrates that the substituent on nitrogen need not be hydrogen. The configuration having the proton bonded to the nitrogen may be written either as an ionic entity (the previously designated S’ state) or in the form
Evidence for the existence of a quinoid species is derived from the properties of 15, the N-methyl derivative. Its two modifications, the wet-stabilized yellow form and the dry-stabilized orange form are completely interconvertible. Infrared spectra were taken of each form, and the results are given in Figure 6. The yellow modification has a strong band at 1635 cm-’ which is assigned to a C=O stretch. Upon drying, the band becomes greatly reduced in intensity and the compound turns orange. A very dry, recrystallized sample of the orange modification shows no band a t 1635 cm-l. (An intense band a t 1605 cm-1 is found in both the yellow and orange modifications and is assigned to the aromatic phenyl-ring vibrations.) The structures and mechanism proposed to explain the behavior of 15 are shown in Figure 7. Separation of charge is facilitated by a medium of high dielectric constant, and thus the yellow modification is assigned the “trans” configuration with the quinoid form predominating. The dry, orange modification is assigned the “cis” configuration with the ionic form predominating. The possible close approach of the positive and negative charges in the “cis” ionic form is the stabilizing factor for the orange modification. The reason the ionic and quinoid forms are distinct entities and not simply resonance forms of each other can be seen with the aid of molecular models. The ionic form is planar. However, even though the quinoid form can be planar, it is less strained in a slightly bent configuration, the bend occurring along an axis formed by CHa, N , and S. To further confirm the above hypothesis, the following two compounds were prepared and studied
15a
S’ Quinoid (Q’)
which may be thought of as either a tautomer of the ionic form S’, or if there is a slight change in molecular configuration, as a distinct quinoid species derived from S’. In addition, with the molecule in the S’ form, there is the possibility of rotation about the C2-C1’ bond yielding a “trans” species labeled S”. This S” state will also have a corresponding quinoid form
S” Quinoid (Q”)
15b
W
15a cannot have “cis” and %ans” forms. The possible configurations are shown in Figure 8. The crystals show a very slight change in color upon drying under vacuum, going from a yellow-orange to a yellow-brown. However, this change is far less dramatic than for 15 itself. The infrared spectrum shows a strong maximum at 1600 cm-’ attributable to the aromatic ring and a very weak shoulder a t 1630 cm-’ when the compound is very dry. The shoulder is more pronounced The Journal of Physical Chemistry, Vol. 74, N o . 38, 1070
447 8
DAVIDL. WILLIAMSAND ADAMHELLER u-n
W E T I IDRY
YELLOW
I
I ;,
j/
I '
I"
iI
AESORPTION
I
\j
VISIBLE FLUORESCENCE
I!
//
//
I VISIBLE
I ABSORPTION
i~
I 1
I1 ti
1I
II 1 1
II !i li
ORANGE
Figure 7. Structures and mechanism of 15.
2-
ids'-
I
H-TRANSFER
a$+=J0-CHI
CHI
CH3
1
I
a y - 0
IONIC-PLANAR
QUI NOID-PLANAR
==z OUINOID-NONPLANAR
Figure 8. Structures of E a .
IONIC-PLANAR
Q U I N O I D - PLANAR
OUINOID- N O N P L A N A R
/
-0
0
Figure 9. Possible configurations for 15b.
when the compound is wet. Both have intense bands a t 1250 cm-l, the region where aromatic ethers and phenols absorb (C-0). This would seem to indicate that there is very little quinoid form present, most probably that which can be detected being a small amount of the nonplanar configuration in equilibrium with the ionic form. The possible forms for 15b are shown in Figure 9. The "trans" forms are all improbable for steric reasons, and indeed, only an orange-red modification is found, similar in color to the orange, dry form of 15. There is a strong band a t 1619 cm-l which is ascribed to aromatic ring vibrations and not C=O stretching. Also, at 1250 cm-1 there is a very intense absorption, most probably due to C-0. Evidence therefore, indicates the dominant form t o be the "cis" ionic configuration. With the above information, i t is possible to construct a more comprehensive picture of the processes occurring in the azole compounds. This is presented in Figure 10. The ground-state mo1ecule, So, absorbs a photon in the ultraviolet and assumes an excited state, S *, having essentially the same molecular configuration as So. In this state, the hydroxyl proton is more acidic and shifts to reside predominantly on the nitrogen. This is the excited molecule S'*. For most The Journal of Phl/skd Chemistry, Vol. 7q9No. 16,1970
b
5'
a"
SO
Figure 10. Reaction sequence and species involved in the luminescence of 2-(2'-hydroxylpheny1)benzothiazole and related azoles. The transitions indicated by solid lines have been established; those indicated by broken lines are tentative.
of the azoles, it is this state which emits the visible fluorescence, decaying to the ground-electronic state S' with the proton remaining on the nitrogen. However, in the ground state, the configuration with the proton situated on the oxygen is the stable form, and the molecule thermally relaxes back to So, thereby completing the cycle. As has been shown, there are other configurations possible, and depending on activation energies and stabilizing influences, either intramolecular or intermolecular, the azole system may follow a different route. There is the possibility of a rotation about the C2-C1' bond to form a "trans" state although the barrier for this would be expected to be considerable in the solid state. However, stabilizing factors could make the process energetically favorable, and solid-state isomerism has been postulated for related systemsU2 Such isomerism is postulated to explain the behavior of 15, the N-methyl compound. The S'" state may also assume the nonplanar quinoid form, again as postulated as part of the mechanism for 15. The stability of this form or the activation energy for its formation are difficult to assess. The processes of Figure 10 may be depicted in terms of potential energy curves as is shown in Figure 11. The diagram was constructed with the following points in mind. First, the shapes of the absorption and the emission spectra suggest that the excited-state minima are located vertically above their corresponding groundstate minima. Second, the energy differences between the ground states and the excited states are shown to correspond to the large Stokes shift between the ultraviolet absorption and the visible emission spectra. Insufficient information is available to calculate actual barrier heights. However, the data do allow some qualitative conclusions to be reached. The greatest uncertainty is the size of the barrier between S* and S'* (and also between S' and SO),and an attempt to
INTRAMOLECULAR PROTON TRANSFER REACTIONS
4479
4800
5300
5800
WAVELENGTtl
6300
6800
(i)
Figure 12. Normalized fluorescence spectra of 1 for the solid and 0.04 M solution in CHCL.
E
__I.
5. REAClIOlr COORDliiATL
Figure 11. Potential energy diagram for the various states involved in the luminescence of azoles.
define the barrier led to a study of deuteration and temperature effects on fluorescence. 6. Deuteration and Temperature Efects. Deuteration of the 2’-hydroxy group produces compounds with lower quantum yields of fluorescence implying that either the rate of crossing from S* to S’* is reduced, or there is an increase in radiationless relaxation from one of the excited-state forms. Since such an increase is contrary to the usual deuterium effects, we must assume that there is a measurable barrier between S* and SI”, and that the difference in zero-point energy between -0-H and -0-D vibrations reduces the crossover process. Fluorescence spectra of 1 and 1D were obtained a t 7”K, and there was a decrease in the overall quantum yield of both, but only of about 35%. I n addition, the ratio of the two peaks for each compound (at 5470 and 5170 A) changes so that the peak that is major a t room temperature (5170 A) is reduced by about 43% a t 7”K, and the secondary peak becomes dominant. This indicates that temperature changes are aff ect’ing the populations of the S”* and/or the quinoid states relative to that of the S’ state. 7 . Barrier Heights. If the barrier between S* and S’* were of sufficient magnitude that differences in zero-point energies were responsible for the deuteration effect, there should have been a drastic reduction in quantum yield a t 7”K, a t least for the deuterated compound. Nevertheless, deuteration does reduce the fluorescence output. Based on the facts available, we conclude that there is a poten tial-energy minimum and that the transfer from S* to SI* is a tunneling effect. This is consistent with viewing the S* form as a vibrationally excited state of SI*. From the S’ states, it is possible to go to “trans”
states (solid line --) or nonplanar quinoid states (dotted line . . . .). As mentioned previously, the barriers to the trans quinoid states, Q‘ and & I * , would be expected to be high in the solid state, although they should be considerably reduced in solution. The spectrum of a 0.04 M solution of I in chloroform superimposed on the spectrum of solid 1 is shown in Figure 12. As can be seen, there is considerably more red emission in the solution spectrum. The presence of the second band in many of the fluorescence spectra can be attributed to emission from a state other than SI*, and it appears that this band is enhanced in solution as well as at low temperatures. The barrier from the SI’* state (dashed line ---) is expected to be of the same order as that between S‘* and &I*. The actual energy level of the S”* state (or S” state) can presumably be lowered by the addition of water or some other high-dielectric medium as discussed for 15 since the S” states involve a separation of charge. 8. Thermochromic and Photochromic Azoles. Azole 10, 2-(2‘-hydroxy-3‘,5‘-dinitrophenyl)-5-nitrobenzothiazole is a yellow compound which is both thermochromic and photochromic (unlike the anils of Cohen, et al., whose thermochromicity and photochromicity are mutually exclusive), Upon irradiation with ultraviolet light, 10 fluoresces green, but gradually turns into a brown, nonfluorescent form. Also, 7 and 9, the 3’-nitro and 5‘-nitro derivatives, show some thermochromism and photochromism. The presence of the nitro groups on the phenyl ring considerably enhances the acidity of the phenol, thus enhancing the stability of the forms in which the proton is situated on the nitrogen. This undoubtedly stabilizes the S’ state with respect to the So state. The colored nature of 10 indicates that its ground state most likely is predominantly an SI- or &’-type molecule, that is, a molecule Tvith the proton bonded to nitrogen. The thermochromicphotochromic state is stable. This denotes a measurable barrier between the colored state and the ground state, leading one to suggest that the thermochromicphotochromic molecule is a “trans” species. This is supported by the fact that regeneration of the ground state is best accomplished by first treatment with a base (ammonia) followed by treatment with an acid (hydrochloric). For cis-trans isomerization, the molecules The Journal os Physical Chemistry, Vol. 74, N o . 26, 1970
EDWARD P. KIRBYAND ROBERT E’. STEINER
4480
must be in the ionic form. Hence, the need for a base signifies that the thermochromic-photochromic state is the trans quinoid species, Q”. 9. Triplets. Thus far, no mention ha8 been made of the possible role played by triplet states in these systems. The decay time of less than sec is evidence that luminescence originates from nothing but singlet states. Such a result is consistent with the studies of
Richey and B e c k e ~who , ~ estimated the decay time for the related ani1 salicylideneaniline a t 20 i 10 nsec. Intersystem crossing from the excited singlet, St*, would produce a vibrationally excited triplet. The stable form of the triplet in all likelihood is with the proton on the oxygen. Such a species, as we have seen previously, does not luminesce, and therefore decays by a radiationless process.
The Influence of Solvent and Temperature upon the Fluorescence of Indole Derivatives1 by Edward P. Kirby* and Robert F. Steiner Laboratory of Physical Biochemistry, Naval Medical Research Institute, National Naval Medical Center, Bethesda, Maryland go014 (Received July 20, 1970)
The temperature dependence of fluorescence quantum yield for indole in water can be rationalized in terms of two nonradiative deexcitation processes. One of these is temperature independent and the other is temperature dependent. The temperature-dependent process may be associated with electron ejection from the excited indole. When D20 is used as the solvent, or nonaqueous solvents are added to HzO solutions, the magnitude of the temperature-dependent process is decreased. Indole derivatives which have a lower quantum yield than indole generally exhibit a decreased apparent activation energy, but this may be due to the introduction of a second, unresolved, temperature-dependent deexcitation process with a lower activation energy.
Introduction Although a considerable volume of data has been accumulated on the luminescence properties of indole, tryptophan, and their derivatives, no definitive picture of the mechanisms responsible for the dependence of emission characteristics upon environment and chemical structure is currently available. For an indole derivative under a particular set of conditions, the quantum yield and excited lifetime of fluorescence are primarily dependent upon the competition between the direct emission of radiation by the excited state and various radiationless deexcitation processes. The nature and relative significance of these deactivation processes are still the subjects of considerable controversy. Intersystem crossing is generally conceded to account for a portion of the radiationless deactivation observed, but this mechanism would not be expected to exhibit the profound temperature and solvent dependence observed in indole and its derivatives. Several other deactivation mechanisms have been proposed, including “tunneling” from the excited to the ground stateleaelectron ejection to the solvent,zbproton or hydrogen transferlat4 and intramolecular electron transfer to a quenching group.5 The Journal of Physical Chemistry, VoE. 74, No. 26, 1970
The characteristics of the emission spectrum, the quantum yield, and the fluorescent lifetime of the indole derivatives have all been shown to be very dependent on the s o l ~ e n t . ~ ~Walker, fl Bednar, and Lurnry2b,’ have postulated that a stoichiometric complex is formed by the excited indole with molecules of a polar solvent. They have introduced the term “exciplex” for such complexes. They also investigated the temperature dependence of the fluorescent lifetime for several indole derivatives in water and have observed that at least one temperature-dependent deactivation process is present.* A
*
To whom correspondence should be sent a t Department of Biochemistry, University of Washington, Seattle, Wash. 98105. (1) From Bureau of Medicine and Surgery, Navy Department, Research Task MR005.06-0005. The opinions in this paper are those of the authors and do not necessarily reflect the views of the Navy Department or the naval service a t large. (2) (a) J. Eisinger and G. Navon, J . Chem. Phys., 50, 2069 (1969). (b) M. 5. Walker, T. W. Bednar, and R. Lumry, ibid., 47, 1020 (1967). (3) R. W. Cowgill, Biochim. Biophys. Acta, 133, 6 (1967). (4) L. Stryer, J . Amer. Chem. SOC.,88, 5708 (1966). (5) R. F. Steiner and E . P. Kirby, J. Phys. Chem., 73, 4130 (1969). (6) B. L. Van Duuren, J . Org. Chem., 26, 2954 (1961). (7) M.S. Walker, T. W. Bednar, and R. Lumry, J . Chem. Phys., 45, 3455 (1966).
