Spectroscopic criteria for wavelength shifting, fast, and red-infrared scintillators

Radial. Phys. Chcm. Vol. 41, No. l/2. pp. 331-349, 1993 Printed in Great Britain. All rights reserved

0146-5724193 SS.OO+O.OO Copyright (Q 1993 Pergamon Press Ltd

SPECTROSCOPIC CRITERIA FOR WAVELENGTH SHIFTING, FAST, AND RED-INFRARED SCINTILLATORS Alexander Sytnik* and Michael Kashat Institute of Molecular Biophysics and Department of Chemistry Florida State University, Tallahassee, FL 32306-3015

ABSTRACT An outline is presented of the distinguishing characteristics of polyatomic molecule electronic excitation, including internal conversion, intersystem crossing, spio-orbitally-determloed transitions, and orbital character of excited state configurations, with the resultant effects oo Euoreaceoce quantum efficiency. The mean lifetime of fluorescence in its frequency dependence and the corresponding oscillator strength of the related absorption is analyzed in terms of the Einstein coefficient A.&r ratio. A solvent cage spectroscopic model is discussed in relation to internal torsional modes of certain molecules, as well as twang-intra~lecular-charge-barer (TICT) and excited-state-intramolecular-proton-transfer (ESIP’I’). The role of the latter as largemagnitude wavelength-shifters is stressed. Criteria for wavelength-shifting, short mean lifetime, red-to-infrared molecular fluorescence are given, with special consideration of solid phase effects as a guide to plastic sciotillator development.

KEYWORDS Sciotillator; molecular excitation; fluorescence lifetime; wavelength shifter; red fluoresceoce; solvent cage effects; proton-transfer fluorescence.

POLYATOMIC MOLECULE ELECTRONIC EXCITATION Molecular sciotillators, as radiation detectors, require careful spectroscopic selection and specification if the triple restrictions of radiation hardness, long-wavelength emission to avoid Wand scattered light interferences, and ultra-short fluorescence decay times are to be met. The spectroscopy of large polyatomic molecules stands in sharp contrast to the behavior of electronic excitation behavior of atomic and diatomic (or small polyatomic) systems. The full understandlog of the spectroscopic characteristics of polyatomic molecules ls essential to the rigorous requirements of the radiation-detecting sclotillator. We shall describe brlefIy the most important characteristics of polyatomic molecule excitation.

As applied to polyatomic molecules, this behavior represents the rapid radlationleas decay of electronically excited upper states to the lowest excited state, leading to the speuma@c criterion (Kasha, 1950): the emitting state (of a given multiplicity) is the bwes: excited state of that mu&&&y (Cf. Fig. 1). Thus, if a higher electronic state is excited, e.g., S, + S, , only St -+ $ fluorescence will be observed. There are almost no exceptions to this general behavior. The relatively large energy gap S, - St usually precludes or limits S, --> S, internal conversion, the radiationless process. *Workdone under ProjaetNo. RGFY9241between the lkas National Raaean%bboratoq Commbbm and Florida State University. tWork done underContact No. DEFGO5-87ER60517between tba Office of Healthand l?m&mmentalRasearch, U. S. Departmentof Energy,and the FloridaState University. 331

332

ALEXANDER

SYTNIK and MICHAELKASHA

Associated with the manifold of singlet states observed for all molecules with closed shell ground state electron configurations, is observed a corresponding set of triplet states, each T-state for a given configuration lying below the S-state for that electron configuration. Intersystem crossing is dejked (Kasha, 1950) as the spin-orbitally controlled radiationless transition between states of different multiplicity. In most cases intersystem crossing occurs Tom the lowest excited singlet state to the nearest triplet state lying below it (Fig. 1). The T, + $ emission can then be observed

Internal Conversion

Intersystem Crossing (spin - orbitally determined)

Fig. 1.

Radiationless transitions in polyatomic molecules.

as a phosphorescence in rigid glass media (Lewis and Kasha, 1944). There are two main factors which lead to enhancement of spin-orbital coupling, with parasitic loss of Sr state excitation (Kasha, 1950, 1960, 1961), leading to diminution of fluorescence quantum yield: (a)

heavy atom exo-substituents such as Cl, Br, I ; heavy atom endo-substituents such as S, Se, Te ; heavy atom chelated ions such as Fe+3,Cu”, En+*; and La+3I Gd*’, Et? , LJI+~-

(b)

hetero-atom groups, such as axa =N-, nitro -NO, nitroso -NO, carbonyl C&O, quinone G=G --- C=O, etc. in small aromatic molecules.

These. latter groups (b) introduce (Kasha, 1950,1960,1961) n 4x* excitation (non-bonding lone pair-electron to pi-antibonding), leading to greatly enhanced intersystem crossing (Kasha and Raw& 1968a; Kasha, 1968b), and in small molecules (benzene, naphthalene) virtual quantitative replacement of fluorescence by phosphorescence. The latter is observable generally only in solid matrices at low temperatures, and the lifetimes in the range of millisecond precludes their use as scintillators of short pulse time. In large (many ring) aromatic molecules, the x - at* excitation dominates. . * -(Levshin’s

law of mirror image symmetry) (Levshin, 1931.1935)

Assuming that Boltxmann or thermal excitation is absent for a given vibrational sequence, absorption could he observable only from the xero-point vibrational level of the ground state Se. The Franck-Condon integrals in absorption would be

where the vibronic wave functions ~0” and x,’ correspond to the vibrational quantum numbers v,” and v,’ of Fig 2 respectively (using standard spectroscopic designations). Intramo1ccu1ar vibrational relaxation (IVR) rapidly brings the upper state S, to thermal equilibrium, populating

Spectroscopic

Fig. 2.

criteria

Vibrational envelope inversion: absorption vs. fluorescence.

the v,’ level. Then fluorescence emissions originate in turn from this xero-point vibrational level of the emitting state Sr , to various vibrational levels of the ground state, with corresponding Franck-Condon integrals

Thus, the absorption and emission contours exhibit a vibrational-enwkye inwrsion, and a molecule is essentially transparent to its own fluorescence emission (as noted early by Einstein (Einstein, 1905)), except for some 0” - 0’ vibronic overlap (which generally is diminished by St dielectric relaxation in the medium). In so far as the spacing3 in the S, state for v/ to v,,” and the S, state from v,’ to vi am not too different, and the corresponding F-C integrals S, and S, ate in rough cotreapondence, there is a “mirror-image symmetry” about the 0” - 0’ center (Levshin’s law), qualitatively apparent between the Franck-Condon contour for absorption vs. that for fluorescence, exact symmetry breaking down if quantitative examination is applied. A typical example of vibrational-envelope inversion can be seen for the absorption and fluorescence of a solution of the dye violanthrone (vide inpa) in Fig. 3 representing a prototype good case of Levahin’s law, which actually was formulated from qualitative observations of dye molecule fluorescence and absorption. In small molecule cases, with better-resolved vibrational spectra,

Wavelength Fig. 3.

[nm]

Absorption (1) and fluorescence (2) of dilute solution of violanthrone in o-dichlorobenxene at 298K

the “mirror-image symmetry” breaks down in terms of both frequency and intensity (FC) correspondence.

333

ALEXANDER

334

SYTNIK and MICHAELKASHA

EINSTEIN ABSORPTION AND EMISSION COEFFICIENTS, B,, AND 4,,

The Einstein Blz coefficient &fining absorption probability can be expressed in experimental parameters as sdln;,

so that a plot of molecular absorption coefficient E - ( 1 /c I ) log,,,( 4/I )(wherein c - solute conceztration in moles/liter, I- length of optical path in cm, log, (I& measures optical density) vs. lnv defines the Einstein B, coefficient between electronic states i and j by the area under a discrete absorption band (plotted as Evs. In;). The molecular absorption coefficient is found to cover a very large decadic range (Kasha, 1968b) over the molecular structural types known, from I values of 10 - 100 for “forbidden” electronic bands, and several hundred to about 50,000 for strongly allowed bands. Dye molecules have e values from 100,000 to 300,000. However, the B coefficient is not simply proportional to the e values. Thus, anthracene has a S,, + Si band with e - 9,000, whereas many dyes have a values 5 100,000, even though in both cases allowed n-electron transitions are involved and the absorption band ureus may be similar. Thus, the Franik-Condon envelope for anthracene as a typical condensed aromatic (annelated) hydrocarbon Av has a full band width of about r: 9,000 cm-‘, whereas many dyes may have full band-widths Av s 2,000 cm-‘. Obviously, a full band integration must be utilized in order to evaluate B,* (for the S, + S, absorption), the vibronic structure contributing to the need for band integration.

The Einstein radiation equilibrium derivation (Einstein, 1917) was carried out for two arornic states leading to the A/B relation:

4,, = 8xhv -3 0

g1 g3 B,,.

A series of derivations by R. Ladenburg, R. C. Tolman, F. Perrin, and by G. N. Lewis and M. Kasha (Cf. Lewis and Kasha, 1945) led to the integrated formula

(5) (where

X$is the intrinsic lifetime of state 2 N is the Avogadro number, c is the velocity of light, n is the refractive index of the medium, g, and gz are the state multiplicities, e is the molar absorption coefficient, and v the frequency of the transition in cm-‘).

The integration of the absorption band is over the Franck-Condon envelope, and v before the integral is arbitrarily taken as the band origin (0,O band).Obviously, the above relation requires an averaging over the ratio of the Franck-Condon integrals (Eq. 1, 2) for the absorption and fluorescence bands, in order to approach a correction for broad molecular vibronic bands compared with the atomic-line states of the originai Einstein derivation. Strickler and Berg carried out the appropriate averaging and produced the equation (Sttickler and Berg, 1962)

(where [ ($z,lml-

/

Spectroscopic

335

criteria

The comparison of Strickler and Berg measured fluorescence lifetimes with those calculated by integrated absorption were found to correlate within 10% error on the average, although for several molecules much larger errors (40%) obtained. Recently Speranka Svirschevsky has carried out a theoretical analysis which indicates (Svitschevsky, 1992) that the shupes of the Fksnck-Qmdon envelope should correlate with the deviations of the integrated absorption value of meanlifetime from the experimentally observed value. When tested, this could provide the means for a further refinement of the Au/B, relationship for molecular vibronic transitions.

The classical oscillator strength of a transition is an indication of the number of optically active electrons involved in a transition (dispersion electrons) and is given by s dv

2303 rn2 f

12=--

N

ne2

(where m = mass of electron, e = charge on electron). The larger the molecule and the larger the number of electrons, the greater is the oscillator strength. The oscillator strength may be defined for a single electronic band, or can be summed over all electronic bands. The absorption intensity of an electronic band increases with increasing molecular size (in dye molecules f-values > 1 are observed). Thus, one might anticipate that the calculated mean lifetime (by Eq. S or 6) would proportionally shorten. However, as the molecular size increases, the wavelength of the first So 4 S, electronic transition simultaneously shifts to longer wavelength for molecules of a homologous structure series.~Thus, in competition with the increase in oscillator strength is a concomittant inverse dependence on (G,’ (Eq. 5). Thus, mean lifetime ukcreuses with increasing integrated absorption (f-value) and the mean lifetime increases with decrease in v’ :

As the theme of this paper is to favor fluorescence emissions which can be observed in the red or infrared regions, the question becomes “Can the oscillator strength empirically grow sufficiently with molecular size (and wavelength of absorption/fluorescence) to offset the inverse square Rower frequency dependence of the mean lifetime (Eq. 5)?” The magnitude of the problem can be illustrated by comparing a fluorescence emission centered on 400 nm at the violet onset to one at 700 nm at the red limit of the visible. The inverse ratio of frquency squared would then be given by

$

I

(5!Q)2

I

(G)’

-

3.06,

w-8

which would indicate the mean lifetime lengthening if the oscillator strengths were qual for the two cases. However, it is not uncommon to find the increase in oscillator strength to of&et and even exceed this frequency dependence. Thus, carefully selected light absorbers of very high f-vslue in the red-to-infrared range should provide fluorescence mean lifetimes as short or shorter than the common violet/blue fluorescence scintillatom.

ALEXANDER

336

SYTNIK and

MICHAEL

KASHA

correlating an expected or observed fiuoreaccnce mean lifetime is to in&grate over a band which consists of more than one electronic transition. For example, the naphthalene absorption bands (Jaff6 and Orchin, 1962) if carelessly integrated from 320 to 240 mn, the calculated mean lifetime would be orders of magnitude too short (K&a rmd Nauman, 1949), ss this region includes two electronic transitions.

The moist common error in

In many cases, the vibrational envelope inversion (Levshin’s law) between absorption and fluomscence would provide a clue on the complexity of an absorption band. Such is the case of our &xu.xoxe molecule series (wide tifie), in which four vibronic bands appear in the absorption, but only two in the fluorescence band. Solvent behavior of the absorption confirms the presence of two overlapping electronic transition (S,,-S,, S,,+!$), of which only the lowest emits fluorescence ( S1 + S,) after internal conver&x~ (S, -> S,). A special case occurs for polyenes and dphenylpolyenes. Figure 4 illustrates an apparent correlation of the lowest absorption spectrum with the fluorescence spectrum. This molecule presents two properties which would make it an ideal ultra-rapid scintillator. The absorption intensity

100 0.4 : % P L 0.2 iii

1

h F!i 2

50

/h4

i$ z!

z

0

Wavelength Fig. 4.

[nm]

Apparent first absorption band (1) of diphenyloctatetraene and the apparentlycorresponding fluorescence band (2), although the bands actually correspond to S,, --, S, absorption and S, + S, fluorescence. The medium is methylmethactylate solid polymer.

of diphenylpolyenes is very high, with the apparent first absorption band exhibiting e values (Jafi% and Orchin, 1962) of over 100,000 for chains containing n = 6 or more double bonds. With absorption peaks in the 400 run region (for n = 6), one could calculate very short lifetimes, c lOa sec. Furthermore, with virtually no overlap between absorption and fluorescence, as shown for diphenyloctatetraene in methylmethacrylate polymer matrix in Fig. 4, an ideal plastic scintillator could be contemplated (albeit with fluorescence in the blue-green region). Figure 5 indicates that the energy level diagram for diphenyloctatetraene involves an unsuspected complexity.

The absorption band cotresponds (Hudson and Kohler, 1972,1973; Schulten and Karplus, 1972) to a highly dlowed S,, -D S, (g + u) electronic transition, whereas the fluorescence band is from a forbidden state S1 * S, (g -, g), made allowed by perturbation mixing. Thus the Braintegration over the very strong absorption band (S,, - Sa) does not correspond to the S, + S, fiuoret#nce emission, which would have a mean lifetime longer than that calculated from the apparent first absorption.

Spectroscopic

s, ---

337

criteria

___ _T 1,

..__

g

tt

Fig. 5.

Schematic electronic energy level diagram for diphenyloctatetraene.

MOLECULAR TORSIONAL PCYTENTIALS: FLUORESCENCE ACTIVATION IN SOLVENT GAGES

In a molecule in which molecular vibrations involve stretching and bending normal modes, molecular displacements near a potential minimum ate normally far removed from solvent cage perturbations. For example, normal mode 0 -, 1 vibrational transitions are remarkably constant in going from gas to condensed phase, and upon changing the solvent in which a solute ls studied. Only nearthe dissociation limit of aMorse potential would one anticipate solvent cageperturbation. Thus, a stretching mode potential can be taken as a harmonic potential as an approximation to the molecular spectroscopic behavior over the vlbronic levels involved in the Franck-Condon overlaps (Eqns. 1,2). Figure 6 for a stretching mode plane of the potential hypersurface of a polyatomic molecule for the first excited state S, depicts schematically a typical harmonicpotential. Torsional modes behave in quite a different manner. In the ground state S, , a torsional mode would exhibit a deep minimum at the equilibrium geometry. But in the excited state S,, a torsional mode for molecules with essential ethylenic bonds generally exhibit a maximum. This effect arises from the antibondlng character of the molecular orbital in the excited state, leading to a repulsive contribution for a coplanar geometry, relieved only by twisting with respect to the “double-bond” axis (Mulliken and Roothaan, 1947). In the diphenylethylenes, i.e., trans-stilbene (Fig. 7), the torsional potential for the S, state exhibits a small intrinsic barrier (a, Fig. 6) (Orlandi and Siebrand, 1975) against relaxation to its (A) x#2 minimum angle, arising from perturbation by the S, forbidden state which in the higher diphenylpolyenes plunges below the S, state (Cf. dlphenyloctatetraene, Fig. 5). A spectroscopic theory of the solvent cage (Dellinger and Kasha, 1975,1976) offers a model for perturbations to a molecular potential by solvent cage restriction of large amplitude motion, at critical regions of a molecular potential. Thus, as the molecular microvlscosity increases the barrier grows, b, c. In a rlgld glass solvent (or pure crystal) the barrier becomes infinite. d; thus, all torsion is inhibited, and the torsional frequency becomes very high. There are two direct consequences of the sensitivity of the torsional potential to solvent cage perturbations. Firstly, the absorption envelope (Franck-Condon envelope) for the S,, + S, curve changes dramatically upon change of solvent viscosity, whereas the fluorescence envelope is relatively unaffected (Sytnik and Rasha, 1992). It is clear that the Franck-Condon integration for the absorption (Eqn. 1) involves S, stretching and S, low-frequency torsional modes at low viscosity, whereas fluorescence Franck-Condon overlaps (Eqn. 2) involve principally S,, stretching modes, the ground state (SJ torsion being of high frequency for the equilibrium configuration. At very high viscosity (rigid glass solution, molecular crystal), the torsional modes become very high frequency modes (“froxen out”), with the result that the Franck-Condon integrations @Qn. 1.2) now become modified extremely. Thus, the absorption and guorescence spectra for the “rigldifled” torsional molecule show dramatic contrasts in Franck-Condon envelope to their solution spectra in low viscosity media (&ale and Roe, 1953).

ALEXANDER

338

Fig. 6.

SYTNIK and MICHAELKASHA

Schematic potential curves for a harmonic stretching mode and a torsional mode in the S, excited state of a polyatomic molecule. To the small intrinsic barrier to torsion (a), there appear increasing barriers (b), (c), (d) representing solvent cage perturbations.

An even more dramatic change in torsional molecules subjected to extreme solvent cage perturbations is the change in fluorescence quantum yield (and lifetime). There is averitablef activation in the solvent-cage perturbation range. At low viscosity, the St excited molecule relaxes very rapidly to the(t) n/2 minima after slight Boltzmann activation to surmount the small barrier, a (Fig. 6). Consequently, the S, + S,, fluorescence is of very small quantum yield, and arises from the small fraction of the molecules in the zero-point level of the St state. As the torsional barrier increases by solvent cage perturbation, the fluorescence yield increases strongly, until in a rigid glass solution(or the molecular crystal) the trans-stilbene molecule becomes intensely fluorescent, with a quantum yield approaching unity.

\\/\/ P cs 4 -s /

'H

--

-

_y=c----

0

/

Fig. 7.

L

0

cJ/

--y

HX \ \ \

Runs-stilbene molecular skeleton with ethylenic torsional axis y.

Spectroscopic

339

criteria

The fluorescence activation of torsionally-capable molecules upon solvent cage inhibition of torsion is a universal phenomenon. Many dyes and other molecules exhibit strong fluorescence only in rigid glass matrices, in the crystalline state, upon surface adsorption, inclusion in a polymer matrix, and in any similar molecular environment in which solvent cage perturbation inhibits torsional motion. In contrast, molecular systems having no free torsional modea do not show such extreme variation of properties. For example, the absorption spectra and fluorescence spectra of a planar aromatic molecule like anthracene exhibit no Franck-Condon envelope changes with viscosity change, and even though there is spectral sharpening at low temperatures in rigid glass media, there is no essential change in Franck-Condon envelope. The quantum yield of fluorescence in fluid solutions at 298 K is moderate for such a molecule, and increases somewhat at 77 K, owing to diminution of collisional quenching effects, but such moderate quantitative changes cannot be compared with the dramatic all-or-nothing relative fluorescence activation effect of solvent-cage perturbation of torsionally active molecules. The absorption and fluorescence spectra of fruns-stilbene are illustrated in Fig. 8 in methylmethacrylate rigid polymermatrix. Asexpecmd, the absorption profile is like that for many viscous solvents, and the Franck - Condon envelopes for absorption and fluorescence represent the normally good “mirror - image symmetry.” The quantum yield of fluorescence is very high, in contrast to the extremely low quantum yield in non-

0.3

q

100

2

0.2

50

0.1

3 .r(

! 2

I

I

0.0

I

300

0

350

Wavelength

Fig. 8.

400

[nm]

Absorption (1) and fluorescence (2) spectra of trans-stilbene in methylmethacrylate rigid polymer at 298 K.

viscous solvents. Thus, such a torsionally-capable molecule in the polymer matrix would constitute an efficient plastic scintillator of short lifetime, if all other factors were favorable. However, the overlap of absorption and fluorescence, and the fact that this is a UV-range fiuorescer am both unfavorable characteristics. It is necessary to seek wavelength shifting phenomena to negate both of these difficullies.

A widely-occurring phenomenon which offers solvent-cage perturbation control of torsional potentials, and at the same time a large wavelength shift form the primary excitation region is the twisting-intramolecular-charge-tram&r (TICI’) case (Grabowski, et al, 1979), and its assodated potential function (Heldt, Gormin and Kasha, 1989; Kasha, el al, 1992). The twirling-intramol&c~-~~ge-tranrfe~ potential is illustrated schematically in Fig. 9. This diagram shows clearly that the lowest absorption band is capable of exciting two thtorescena~, the normal fluorescence band (S, + Se) adjacent to the lowest absorption band, and a second longer-wavelength fluorescence, S&X”) -, $(FC), in favorable cases. The wavelength-shifted fluorescence, which can occur far from the primary absorption, can be of high quantum yield, and has the double advantage of not only being strongly wavelength-shifted toward the red, or beyond, but dso having no companion absorption band, obviously negating any self-absorption loss of efficiency.

ALBXANDBRSYTNIK and MICHAEL KASHA

340

Fig. 9.

Schematic torsional potential curves with a non-adiabatic ctossing of charge-transfer S&I’) and locally excited S&,x*) states.

Fig. 10 offers a mechanistic interpretation of the origins of the TICT potential. Amoleculecapable of this phenomena muat posaeas (a) a torsionally-capable structure, and(b) an upper charge-transfer

s;ICT),I” Fig. 10.

TWISTINO

CT

9, h, qnin

Mechanism of twisting-intramolecular-charge-transfer.

(CT) state, i.e., a capacity for intramolecular charge-tram&r. For the TICT torsional potential to be activated, a strong dielectric environment must be provided, so that the TICI’ potential minimum S,(CI’) is lowered below the Sr potential minimum.

The coplanar structure (Fig. 10) in the excited state Sr (x,x*) would arise from the normal x--x* exdtation of aromatic molecules, and exhibit a potential minimum parallel to the S, state potential minimum in the twisting coordinate. The two halves (moieties) of the molecule could represent two equivalent parts, or two inequivalent moieties, e.g. a dimethylamine group aa one part, and a phenyl ring as the second. In fact, 4-dimethylaminobenxonitri~e, 4-&H&r -N - CJf, - CN, is the very molecule which led to the observation of dual fluorescence (Lippert, wider and Bous, 1962) and recognition of the TItT state phenomenon (Grabowski et al., 1979). As the TICT-capable molecule moieties undergo torsion, the presence of a higher lying chargetransfer state S&T) makes its presence felt as it plunges in energy upon torsion, leading to the quantum-mechanical “non-crossing” interaction as a resonance splitting between the S&I’) and Sr (x,x*) states at the degenerate configuration (- 40’ in the dimethylaminobenxene case). The consequence of this interaction is that new effective potentials arise, the Sr (x#) state going over adiabatically to the Sr(CI’) state. Thus, the S, excited molecule undergoes a Suddenpo&riratitm (Salem, 1979), and relaxes to the charge-transfer S,(CT) minimum, giving tiae to a second fluorescence for the molecule. &cause of the charge-transfer character of the S&T’) state, its potential minimum on the energy scale is very sensitive to the dielectric constant of the solvent environment. The TICT phenomenon is now recognized to be a widely occurring observation (Rettig, 1986). The TICI’ potentials include two regions at which solvent-cage perturbations can be expected. These are regions in the potential at which W/c@ - 0,i.e., maxima in the potentials at which the

Spectroscopic

341

criteria

conditions for the Born-Oppenheimer spectroscopic model for solvent-cage petturbations apply (Kasha and Dellinger, 19X,1976): (a) near the top of the !$(Cl”) barrier and (b) at the maximum of the S, (x,x*) - S&I’) potential. Thus, solvent-cage perturbations would be expected to prevent torsion near those regions. Therefore, like the knple torsionalpotendal case, twisting can be blocked by solvent-cage perturbations. In the former case, j7uorescetuwactivation occurs in the solvent-cage. In the TICf case, the second jluorescence, ie., thatporn the S&I’), or lXT, state nwkl be quenche4 with the S,(n,n*) now becoming the unique emitter (Kasha, Kubicki, and Rawls, 1990). The arresting of torsion in a TICT-state case is well illustrated by the molecule 2-hydroxy-& pyronoquinone (HPQ), (or Zhydroxytexanone), Fig. 11.

Fig. 11.

Chemical structure of 2-hydroxy-4-pyronoquinone.

This molecule is expected to exhibit both excited-state-inuamolecuklr-proton-nans (ESIPT) fluorescence (see next section) and twisting-intramolecular-charge ban&r (TICI‘) fluorescence., whereas the 2-methoxy-4-pyronoquinone should exhibit only TICT fluorescence Sr(CT) + S, These fluorescences are expected in the infrared and thus far have not been observed. The absorption spectrum of HPQ is illustrated in Fig. 12 at 298K in ethanol solvent. Neither HPQ or its methoxy-derivative have significant fluorescence at 298 K, but in ethanol glass at 77K S,

400

500

600

700

Wavelength [nm] Fig. 12

Absorption at 298 Kin ethanol (1) and fluorescence (2) in ethanol glass at 77 K of 2-hydroxy-4-pyronoquinone.

(x,x*) 4 Se strong fluorescence is observed. Thus, the TICI’ phenomenonis blocked in the rigid matrix. I&e the simple torsional potential case, the normal fluorescence is activated. This could be used as aplasticscintillator, but the usual negative aspects dted for the stilbene case still prevail. The TICP phenomenon offers the first major wavelength-shift observation, but could it be utilixed as a scintillator in a plastic matrix? As an example, the benxanilide molecule offers a good TICl’

342

ALEXANDER

SYTNIK and MICHAELKASHA

fluorescence (Heldt, Gormin, and Kasha 1989). This normal molecule fluorescence is very weak (%._ 320 run), whereas the TICI’ 5uorescence is strong and exhibits a Stokes-like shift of 12,000 cm-’ (X, 520 run). This TICI’ fluorescence is frozen out in a rigid glass matrix. There is a unique possibility however, of encapsulating a molecule such as benzanilide in an clathrate-like complex which allows internal motions to occur even if the clathrate cage is included in a solid matrix. Thus, the TICT phenomenon may still be observable if a suitable clathrate can be found, permitting both TICT fluorescence and polymer inclusion. Research on the aspect is expected to be explored fully. PROTON TRANSFER AS A WAVELENGTH-SHIFT PHENOMENON The prototype wavelength-shift phenomenon in the excited-state-intramolecular-proton_aansr (ESIPT) case. This is a widely studied phenomenon, and has been the subject of a number of comprehensive surveys (ICIijpfCzr,1977; Kasha, 1986; Barbara and Tromsdorff, 1989; Barbara, Nicol, and El-Sayed, 1991). In this phenomenon, a proton involved in an internal hydrogen-bond flips in position to its neighboring heteroatom if the excited state electron rearrangement induces the 5ip. Such a condition arises in many molecular electronic systems. The much-studied 3-hydroxyflavone cast (3.HF) (Sengupta and Kasha, 1979; McMorrow and Kasha, 1984) has been the most directly applied to the radiation-detection scintillator problem. Figure 13 shows the normal molecule and fautomermolecule structures involved in the proton transfer.

O---H Fig. 13.

Proton-transfer tautomerism in 3-hydroxy5avone (3-HF).

Fig. 14 graphs schematically the proton transfer potentials, including the triplet state potentials which play a role in parasitic processes reducing the &sired S; -+ Sb proton-transfer (tautomer) fluorescence. TheexcitationsequenceisS, - S,normal moleculeabsorption(generally aIc;x* excitation)which commonly occurs in the W, then a very rapid S, -> S; proton transfer, on the picosecond time scale, followed by the S; - $, proton transfer (tautomer) 5uorescence. The wavelength shifts observed t?z proton transfer 5uorescence correspond to &equency red-shifts of 6,000 to 12,000 cm”, which generally places the proton-transfer tautomer fluorescence in the green to red spectral region. For example, in the case of 3.HF. the first absorption bond has J._ - 355nm and a proton

NORhAL

Fig. 14.

TAUThER

Schematic diagram of proton-transfer potentials showing ground state and lowest triplet and singlet excited state potentials.

Spectroscopic

343

criteria

transfer tautomer fluorescence at h, - 525nm, corresponding to a shift of - 9,000 cm-‘. Thus, a UV absorber is made to emit fluorescence in the green region (Cf. Fig. 15).

Fig. 15.

Near UV-absorption (1) and proton-transfer fluorescence (2) of f-hydroxyflavone in methylcyclohexane at 298 K

Table 1 gives a representative sampling of proton-transfer molecules with tautomer fluorescence in the visible region, the selection covering the green to red spectral range. Table 2 gives the chemical structums of the molecules of Table 1. All of these belong to the same Class 2, OT intrinsic intramolecular proton transfer case. It is possible to distinguish five classes of intramolecular proton transfer (Rasha, 1986; Heldt, Gormin, and Rasha, 1989) based on moleoular structural and environmental requirements. However, only the intrinsic intramolecular easewould seem to be suitable for plastic scintillator applications.

Table 1.

Proton-transfer fluorescence wavelength shifts.

References

Absorption

Proton-Transfer Fhtorescenee

au

&la

Molecule

HBO HBT 3-HF BPOH HBDMI BBHQ HNT (a) (h) (c) (d) (e) (f)

at b a, b, e d, e f 8 h, i j

330 nm 350 355 328 364 417 400

Williams and Hellcr, (1970) Cohenand Plavian (1%7) Chou, Studerand Msttinez (1991) Seaguptaad Rasha (1979) MeMonuw and Kasha(1984) Bulsks, Grabowslraand Grahowski(1986)

500 nm 517 525 571 588 625 650 (g) (h) (i) (j)

Frequency Shift As, 10,300 cm-r 9,200 9,100 13,000 10,500 8,000 9,600

Mordxiaski,Lipkowski.Orxanowsksand Tauer (1990) Mordxinskiand Ktihaie (19%) Mordxiaski,Grahowska,Rtthnle aad Kr6wxyaski(1983) Jang and Kelley (1985)

The radiation stability of the molecules listed in Table 1 has not been tested, and the quantum yields 6 of many of the examples given in the low range, frequently 6 being < 0.1. However, the case of 3-hydroxyflavone will illustrate the variations possible. A study of related moleoulea Parthenopoulos, McMonow and Rasha, 1991) indicates the great variation in proton-transfer 6 uoreseenee effldenoy with variations in molecular structure. Thus, the quantum yield of fhtoredeenoe depends sensitively on substitution, as does ?k_ for the fluorescence as ahown in Table 3.

344

ALEXANDER SYTNIK~~~MICHAELKASHA

Table 2.

Chemical structures of proton-transfer fluorescers.

HBO

2-(2’-Hydroxyphenyl)-benaxole

HBT

2-(2’-Hydroxyphenyl)-benthiaxole

3-HF

3-Hydroxyflavone

BPOH

2,2’-Bipyridyl-3-ol

HBDMI

2-(2’-Hydroxy-5’-methylphenyl)-3,3-dimethylindole

BBHQ

2,S-Bis(2benxoxaxolyl)-hydroquinone

HNT

2-Hydroxy-4,5-napthotropone

Table 3.

Proton-transfer fluorescence of 3-hydroxychromones (in methylcyclohexane at 298 K).

Molecule

Fluorescence wavelength LX

Quantum yield 6

Zmethyl-3-hydroxychromone

488 nm

0.29

J-hydroxyflavone (2-phenyl-3-hydroxychromone)

523

0.36

5-hydroxy-4’-vinylflavone

535

0.72

The reasons for the variations are complex, but the participation of the triplet state potential (Fig. 14) r/trough the enhanced intersystem crossing in the tautomer electronic states (Gormin, Heldt, and Kasha, 1990) is an indication of one of the principle parasitic or degradative pathways which can lead to diminished quantum yield of S; + $&proton-transfer fluorescence. If a particular molecule has favorable fluorescence wavelength characteristics and radiation stability characteristics, research on substitution effects on electronic state energetics can serve to optin@ quantum yield. Analogously, radiation hardness canbe modified by chemical replacement within a structure, e.g., the pyrane oxygen -O-within the ring is the focus of photo-oxygenation instability, which can be greatly reduced by replacement of the ring -O- atom.

Spectroscopic

345

criteria

There ana two complementsry procedures which can serve to understand and modify the excitation characteristics of a particular molecular electronic system for optimizing proton-transfer fluorescence. Semi-imperical quantum chemical calculations can serve as a guide to electronic state energetics in the normal and tautomer molecules. Experimentally, we have extensive experience in probing excited state dynamics in the tautomer electronic state manifold (Fig. 14, !& + S;, S; + $,, T, + T, ) by picosecond transient absorption spectroscopy. So the subtleties of excitation dynamics in those systems can be explomd, and the highly desired optimixaton of the $ -, $ proton-transfer fluorescence (Cf. Fig. 14) achieved. Solvent cage effects have been adduced as being of prospective importance in proton-transfer dynamics (Taylor, El-Bayoumi, and Kasha, 1969; Dellinger and Kasha, 1976). In the first report on 3-hydroxyflavone proton-transfer fluorescence it was suggested that torsion of the side phenyl (B-ring) with respect to the benxopyrone(A-ring) would affect the electron-density at the carbonyl proton acceptor site, and thus introduce torsional sensitivity to microviscosity (molecular friction). Recently a demonstration has been given of the effect of viscosity on proton-transfer efficiency (Mordxinski, Lipkowski, Orxanowska and Tauer, 1990), three fold increase in quantum yield of proton-transfer fluorescence being registered for HBDMI (Table 1) and a two-fold increase for HBO, at room temperature in the solvent variation isopentane to decalin (viscosities 0.22 to 2.42 CP respectively). This effect seems to be the result of a solvent cage barrier to a torsional motion which would lead to radiationless internal conversion S; - Sb. Thus, in the plastic scintillator, solvent cage effects can lead to improved efficiency, if the barrier introduced does not hinder the # , S, + S, proton-transfer fluourescence.

RED-REGION FLUORESCENCE PROBES The triple requirement of wavelength-shifting, short mean lifetime, and radiation hardness for high quantum yield plastic scintillators (fabricated as optical fibers or as thin sheets) has been approached in the present paper, yellow to red region probes being favored. Wavelength-shifting is well provided for by the ESIPT or proton-transfer fluorescence phenomenon, as illlustrated by the examples of Table 1. A good example is that provided by 2-hydroxy4,5-napthotropone (HNT), which exhibits a proton-transfer fluorescence (Fig. 16) in the red region (Jang and Kelley, 1985). This is a rather exotic molecule and whether it would be

s,

t ABSORPTION

WAVELENGTH. nm

Fig. 16.

Absorption (1) and proton-transfer fluorescence (2) of 2-hydroxy4,5-naphthotropone in cyclohexane at 298 K.

useful ss a plastic scintillator requires further research. It is reported that the quantum yield of the proton-transfer red fluorescence is low. A full-scale spectroscopicstudy, including piuasecond transient speetrosoopy, should reveal the source(s) of the parasitic interferences. The most useful spsdnrcopic tool is the study of variations of behavior with chemical substitution changes. The molecuie %hydroxytropone has no proton-transfer wavelength shift, since the two end tautomers am equivalent: only a very small spectral splitting is observable owing to the quantum tunneling

R#: 4lr1/2-Y

346

ALEXANDER

SYTNIK ZWIMICHAELKASHA

The electronic assymmetry introduced by the 4,fGnaphtho ring substitution is the origin of the tremendous wavelength shift observed (Table 1; Fig. 16). The molecule 2-hydroxy-4,5benzotropone could be a variant which could offer a more favorable quantum yield. Would the red-shift be as great? Semi-empirical quantum chemical calculations could indicate whether the molecule is worth making. A second kind of chemical substitution could be the introduction of electron-withdrawing, or electron donating groups onto the aromatic side ring. In fact, the spectroscopic effects of the asymmetry of the tropone ring could also be directly tested by, i.e., disubstitution of electron donating or withdrawing groups directly on the main ring. FJinally, if all other factors are favorable, the radiation hardness must be determined. Various substitution groups could be probed for their contribution to this side effect as a crucial step in ultimate utility. The use of a solid polymer matrix generally yields an increase in fluorescence quantum yield for many fluorescent molecules. This effect is a consequence of the limiting of dynamical quenching effecta, most prominently brought about by dissolved oxygen as a fluorescence quencher. However, as our presentation on solvent-cage perturbation indicates, internal motion restriction in a solid matrix can diminish the r8le of radiationless transitions which lead to fluorescence quantum yield loss. Figure 17 shows the absorption and proton-transfer fluorescence emission spectrum of 3-hydroxy-4’-vinylflavone in methylmethacrylate solid film. The quantum yield of

Wavelength Fig. 17.

[nm]

Absorption (1) and proton-transfer fluorescence (2) of 3-hydroxy-4’~vinylflavone in methylmethacrylate film at 298 IL

proton-transfer fluorescence of this molecule is at least double that of 3-HF (Table 3), and in the solid polymatrix is still higher. This would make an ideal scintillator material; the radiation stability has not been tested.

Violanthrone is a dye in the polycyclic quinone class (Fig. 18). The polycyclic quinones form a class of “vat dyes,” the name originating from the use of the dyes in a reducing bath as a leuco (colorless) form; the impregnated cloth is thenphoto-&f&din strong light and remarkably stable colors result.

Fig. 18.

Chemical structure of violanthrone.

These dyes are stable against further photo-oxidation. It occurred to us (Rasha, Kubicki and Rawls, 1990) that the very great photo-oxidation stability of the polycyclicquinonevat dyes would offer a parallel radiation stability. Thus, although, e.g., violanthrone does not involve wavelength-shifting, it does have UV electronic excitation bands, which by internal conversion result in a lowest S, (x,x*) - S, fluorescence which lies in the red region. pigum 3 shows the lowest S, - S, absorption and fluorescence of violanthrone in fluid solution, and Fig. 19 the corresponding spectra in methylmethacrylate solid polymer film. There is considerable overlap

Spectroscopic

100

,I,

Wavelength Fig. 19.

341

criteria

[nm]

Violanthrone visible absorption (1) and red fluorescence (2) in polymethylmethacrylate solid film at 298 K.

evident between the onset of absorption and the onset of fluorescence. However, the absorption ccef5cient is very high (molar E = 77,000 in o-dichlorobenxene), so that the polymer film as prepared is so dilute in violanthrone as to be almost colorless, yet a brilliant red fluorescence is observed for the solid methylmethacrylate film. The mean lifetime calculated by Eqn. 5 as an approximation is 3.6 nsec for the o-dichlorobenxene solution. The measured lifetime for the methylmethacrylate film is 3.94 (Tate, 1992) nsec. Aside from any error in the calculation introduced by using the approximate relations, the quantum yield of fluorescence in the polymer is expected to be higher owing to the diminution of dynamical quenching, so the lifetime should be longer. The quantum yield of violanthrone fluorescence in the rigid polymer matrix probably approaches the limiting value of 1. It is possible that the violanthrone plastic scintillator would function exceedingly well from all points of view: high quantum efficiency, a reasonably short mean lifetime, high radiation stability, a red-region fluorescence, and the applicability to optical fiber techniques.

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ALEXANDERSYTNIK and MICHAEL KASHA Heldt, J., D. Gormin and M. Kasha (1989). A comparative picosecond spectroscopic study of the competitive triple fluorescence of aminosalicylates and benzanilides. Chem Physics, 136, 321-334. Hudson, B. S. and B. E. Kohler (1972). A low-lying weak transition in the polyene a,w-diphenyloctatetraene. Chem. Phys.Lett., 14,299-3&k Husdon, B. S. and B. E. Kohler (1973). Polyene spectroscopy: The lowest energy excited singlet state of diphenyloctatetraene and other linear polyenes. J. Chetn.Phys., 59,4984-5002. Jaffe, H. H. and M. Orchin (1962). Theory and Applicationsof UltravioletSpectroscopy,John Wiley and Sons, New York. Jang, D.-J. and D. F. Kelley (1985). Time-resolved and steady-state fluorescence studies of the excited-state intramolecular proton transfer and relaxation of 2-hydroxy-4,5naphthotropone. J. Phys. Chem., 89,209-211. Kasha, M. and R. V. Nauman (1949). The mestastability of the lowest excited singlet level of naphthalene. J. Chem. Phys., 17,516-520. Kasha, M. (1950). Characterization of electronic transitions in complex molecules. Faraday Sot. DiscussionNo. 9.14-19. Kasha, M. (1960). Paths of molecular excitation. In: Bioenergetics (L. G. Augenstine, Ed.), RadiationResearch, Supplement 2, pp. 243-275. Kasha, M. (1961). The nature and significance of n + rc* transitions. In: Light andLifr!(W. D. McElroy and B. Glass, Eds.) Johns Hopkins University Press, Baltimore, MD, pp. 31-64. Kasha, M. and H. R. Rawls (1968a). Correlation of orbital classification of molecular electronic transitions with transition mechanism: The aromatic amines. Photo&m PhotobioL,7, 561-569. Kasha, M. (1968b). Theory of molecular luminescence. In: Proc. InternationalConference on Luminescence, Vol. I (G. Szigeti, Ed.), Akademial Klado, Budapest, pp. 166-182; also ln Annual Science Conference Proceedings, Vol.2 (A. Gelbart, Ed.), Yeshiva University, New York, pp. l-20. Kasha, M. (1986). Proton-transfer spectroscopy. Perturbation of the tautomerlzation potential. J. Chem. Sot., Faraday Trans. 2,82,2379-2392. Kasha, M., A. Kubickl, H. R. Rawls (1990). Molecular electronic criteria for selection of radiation-hard scintillators. In: Proc. of Workshopon Radiation-Hardnessof Plastic Scintilkztors(K Johnson, Ed.), March 19-21, 1990, Tallahassee, Florida, pp. 49-59. Kasha, M., D. Parthenopoulos and B. Dellinger (1992). Challenges to computational quantum chemistry from contemporary advances in polyatomic molecular electronic spectroscopy (1992). Int. J. QuantumChem (in press). Klepffer, W. (1977). Intramolecular proton transfer in electronically excited molecules. In: Advances in Photochemistry,10 (J. N. Pitts, Jr., G. S. Hammond and K. Gollnlck, Bds.). John Wiley and Sons, New York, pp. 311-358. Levshin, W. L. (1931). Das Gesetz der Spiegelkorrespondenz der Absorption+und Fluoreszenzspektren. Z. Phys., 72,368-381. Levshln, W. L. (1935). Correspondence between absorption and fluorescence spectra of diluted solutions of dyes. Acta Physchim USSR,2,221. Lewis, G. N. and M. Kasha (1944). Phosphorescence and the triplet state. J. Am. Chem. Sot., 66,2100-2116. Lewis, G. N. and M. Kasha (1945). Phosphorescence in fluid media and the reverse process of singlet-triplet absorption. J. Am. Chem. Sot., 67,994-1003. Llppert, E., W. LQder and H. Boos (1962). Fluoreszenzapektrum und Franck-Condon-Prlnzip in L&ungen aromatischer Verbindungen. In: Advun.Mol. Spectrmc, Vol. 1 (A. Manglni, Ed.). Pergamon Press, New York, pp. 443-457. McMorrow, D. and M. Kasha (1984). Intramolecular excited-state proton transfer in fhydroxyflavone. Hydrogen-bonding solvent perturbations. J. Phys. Chem., 88, 22352243. Mordzlnskl, A., A. Grabowska, W. Kiihnle and A. Kr6wczynski (1983). Intramolecular single and double proton transfer in benzoxazole derivatives. Chem Phys. Lett., 202.291-296. Mordzinskl, A. and W. Kilhnle (1986). Kinetics of excited-state proton transfer in “double” benzoxazoles: 2,S-Bis(2-benzoxazolyl)-4-methoxyphenol. J. Phys. Chem., %I,14551458. Mordzinskl, A., J. Lipkowskl, G. Otzanowska and E. Tauer (1990). On the viscosity effects in proton transfer systems. Structure and photophysica of 2-(2’~hydroxy-5’-methylphenyl)3,3-dimethylindole, Chem. Phys., Z40,167-176.

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Sengupta, P. K, and M. Kasha (1979). Excited state proton-transfer spectroscopy of Zhydroxyflavone and quercetin. Chem. Phys. Mt., &?,382-385. Shulten, K and M. &plus (1972). On the origin of a low-lying forbidden transition in polyenes and related molecules. Chem Phys. Lett., 14,305309. Striclder, S. J. and R. A. Berg(1962). Relationship between absorption intensity and fiuorescence lifetime of molecules. J. Chem Phys., 37,814-822. Svirschevsky, S. (1992). The relation between the life-time of excited states and the intensity of molecular spectral bands. Institute of Molecular Biophysics, Florida State University, Tallahassee (to be submitted). Sytnik, A. I. and M. Kasha (1992). Viscosity effects on the Franck-Condon envelope of the S, + St absorption spectrum of stilbene. Institute of Molecular Biophysics, Florida State University, Tallahassee (to be published). Tam, K. (1992). Streak camera measurement of mean fluorescence lifetimes. Department of Chemistry, Florida State University, Tallahassee (work in progress). Taylor, C. A., M. A. El-Bayoumi and M. Kasha (1969). Excited-state two-proton tautomerism in hydrogen-bonded N-heterocyclic base pairs. Proc. Nat. Acad Sri, USA, 63,253-260. Williams, D. L and A. Heller (1970). Intramolecular proton transfer reactions in excited fluorescent compounds. J. Phys. Chem., 74.4473-4480.

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