7281
J. Phys. Chem. 1995,99, 7281-7284
Triplet State Potentials in the Competitive Excitation Mechanisms of Intramolecular Proton Transfer? Michael Kasha,” Jozef Heldt,* and David Gormin institute of Molecular Biophysics and Department of Chemistry, Florida State University, Tallahassee, Florida 32306-3015 Received: August 17, 1994; In Final Form: December I , 1994@
A phenomenological analysis is given of the three cases of lowest triplet state potentials relative to ground singlet and lowest excited singlet state potentials in the presence of intramolecular proton transfer. In each case, singular excitation phenomena are observed respectively: enhanced normal tautomer molecule phosphorescence (TI SO),dual phosphorescence, and unique proton-transfer (FT) tautomer phosphorescence (T1’ SO’). The application of these model cases to proton-transfer examples involving radiation-detector scintillators, biomolecule fluorescence probes, and four-level lasers is discussed. The development of the potential curve interaction models permits the optimization of the desirable large wavelength shift and maximization of quantum yield of the PT fluorescence. The competition of normal tautomer and PT tautomer phosphorescences with the FT fluorescence is discussed as a limiting factor.
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Researches on the excitation behavior of molecules exhibiting excited state intramolecular proton transfer (ESIPT)have been greatly broadened in recent years1,*with many new molecular systems investigated, involving numerous novel and highly detailed spectroscopic and dynamics researches. A determining factor in the efficiency of the fluorescence which can be observed from the tautomer produced by the ESIPT excitation is the role played by the lowest triplet state potential between the normal tautomer and the proton-transfer tautomer species. Nevertheless, very few cases have yet been reported in which direct involvement of the lowest triplet state potential has been revealed. In the present paper, we analyze the cases representing the three spectroscopic schemes which are possible and discuss the conditions for and consequences of the occurrence of each. The four reported cases of tautomer triplet state emissions and their subtleties will be analyzed on the basis of the three cases.
Case A. Enhancement of TI ESIPT
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SOPhosphorescence via
Case A is considered to correspond to the state energy ordering
diagrammed schematically in Figure 1. For this case we assume that the energy difference TI’ - T1 is too great for significant Boltzmann excitation of the TI’ state from TI to occur, even at 298 K. This is the case suggested to be present for the aminosalicylatesstudied by Gormin et permitting a T1 state excitation enhancement mechanism via an ESIPT pathway. The reported comparative spectroscopy showed an apparently intensified T1 SO phosphorescence relative to the S1 SO fluorescence, for the two species in which ESIPT was possible.
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f This paper is dedicated to the celebration of the magnificent scientific research career of Professor Mostafa El-Sayed, who has made keynote contributions to the study of triplet states of molecules and many other areas. Institute of Experimental Physics, University of Gdansk, Gdansk, Poland. Abstract published in Advance ACS Abstracts, May 1, 1995. @
0022-365419512099-7281$09.0010
NO~MAL
TAU~OMER
Q (PT)+-
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Figure 1. Schematic proton-transfer potentials for case A, with TI SO phosphorescence enhancement via a back-proton transfer in the
lowest triplet state potential from the proton-transfer tautomer to the normal molecule tautomer.
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The qualitative spectroscopic results (Figure 3, ref 3) indicated a strong enhancement of the T1 SOemission where ESIF’T was present. The semiquantitative data (Table 11, ref 3) on excitation and exit slits, gain factor, together with the observed relative intensities indicated a considerable enhancement factor as well. However, a quantitive quantum yield ratio determination was needed to offer full confiiation. The spectroscopic data could have been interpreted to indicate merely that the fluorescence was quenched relative to the quantum yield of phosphorescence. The data on experimental parameters (slit width, gain setting) were too approximate to permit a calculation o f absolute quantum yield. A new study on the same set of four aminosalicylates by Gormin et al.4 yields quantitative confirmation of the T1 triplet state excitation enhancement suggested by the earlier work.3 Therefore, the scheme for case A (Figure 1) must be valid for the aminosalicylates described: excited state intramolecular proton transfer can facilitate intersystem crossing (at SI’ -> TI’)and subsequent enhancement of T1 state excitation (over intrinsic normal tautomer intersystem crossing) via a TI’ -> T1 potential pathway for “back-proton transfer” for case A.
0 1995 American Chemical Society
7282 J. Phys. Chem., Vol. 99, No. 19, I995
Kasha et al.
Case A has several novel features which deserve mention. First of all, if it should prove to be a common case, this could explain why to date almost no cases of the ESIPT tautomer TI’ state emissions have been observed. In fact, only two cases of phosphorescence from a stable TI’ state (Le., where the absolute energy ordering is TI’ < TI) have been reported, to be discussed under case C. The pattern of T1 vs TI’ state energy ordering distinguishes the three cases we are exploring. In earlier work5s6 we considered that the SI =- SI’ state ordering pattern would be mimicked by the state ordering pattern TI > TI’ on the basis of comparable lowest excited singlet-triplet splitting arising from analogous orbital configurations. However, in case A the lowest triplet state ordering pattern T I TI’ follows the ground state energy ordering pattern SO < SO’. This of course requires that the normal tautomer S I - T ~split to be considerably greater than the proton-transfer tautomer &’-TI’ split. For a n,n*configuration, the greater n-electron delocalization expected for a highly aromatic proton-transfer tautomer compared with the comparatively more restricted electron delocalization in the normal tautomer should lead to smaller electron repulsion and exchange interaction, hence smaller S1’-T1’ configurational splitting for the proton-transfer tautomer. In contrast to the discussion just given, it had been suggested7 that the SO < SO’ pattern should be paralleled by a T1 < TI’ pattern on the basis of similar proton-ionization p r s observed* for SO and T1 states, compared with the very contrasting pK value for the corresponding SI state for the molecules then studied. However, it would seem a very peculiar aspect of the quantum chemistry to find electron density maps of the SOand TI states to be analogous (at least at the critical heteroatom yielding analogous proton dissociation), whereas that for the S1 state, the configurational companion of TI, to be so contrasting as to yield a pK differing by 6 units. Although a similar proton acidity for SO and T1 states could arise from electron densities which are locally comparable by chance, nevertheless the absolute energy pattern of the respective triplet states, TI, TI’, may only appear to mimic the pattern of the corresponding ground states, SO, SO’. As we have stated, the energy ordering pattern must have its origin in more fundamental quantum chemical bases.
Case B. Dual Phosphorescence (TI via ESIPT
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SOand TI’
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So’)
Case B we picture to be the one with the state energy ordering
for which the corresponding potentials are diagrammed in Figure 2. This is the case discovered by Mordzinsky et for 2-(2’hydroxypheny1)benzoxazole (HBO). For this molecule at 298 K both transient absorptions T1 T, and TI’ T,’ could be observed,l0 as well as dual phosphorescence” TI SOand TI’ SO’. The nearly temperature-independent ratio of the two phosphorescences was attributed to the approximately isoenergetic T1 and TI’ states. This near equivalence was recognized to be fortuitous by the authors,” who noted that substitution effects could upset the delicate balance (see case C, below). The temperature dependence of the phosphorescences was studied over the range 140-90 K, in the degassed liquid solvent 3-methylpentane. However, the situation was dramatically altered when the same solution was lowered to 85 K, at which temperature the 3-methylpentane is a rigid glass: only TI SOemission is ~
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~
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NORMAL
TAUTOMER
Q (PT) Figure 2. Schematic proton-transfer potentials for case B, wherein the chance approximate degeneracy of the TIand TI’ states leads to temperature-independentdual phosphorescence TI SOand TI’ So’.
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NORMAL
TAUTOMER
Q (PT) Figure 3. Schematic proton-transfer potentials for case C, leading to a normal intersystem crossing pathway to TI’population, with
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So’ proton-transfer tautomer phosphorescence, consequent TI’ endangered by possible TI’ -> SO’radiationless competition.
observedg%’for the glass solution. This conflicting behavior could be understood if in the rigid glass at 77 K the state ordering becomes T1 < TI’ (thus case A) instead of the T1 x TI’ (case B) energy equivalence at 298 K in fluid solution. These observations could be interpreted most easily if it is realized that electronic states may be energetically displaced with a change in medium environment by differential intermolecular forces and thus exhibit a change in state ordering. The severe volume contraction upon glass formation suggests such a differential change in intermolecular perturbation. A classic example is the triplet state ordering observed by Kellogg12for anthracene in liquid solution at 298 K as SO < TI < T2 .c SI, contrasting with that observed for pure crystal 298 K with SO < T1 < S1 < T2. As a consequence of this change in state ordering, the quantum yield of fluorescence of anthracene crystal is @F = 0.99, whereas in solution the yield is strongly diminished.
Case C. Intersystem Crossing after ESIPT with Unique TI’ So’ Phosphorescence
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Case C would be that for which the state energy ordering is (So < S)’,
< (T,’ < T,) < (SI’ < S,)
for which the corresponding potentials are diagrammed schematically in Figure 3. This case is obviously one which requires a small split, and a comparable or larger S1’-T1’ split. It could be expected that this case would be the most typical of the proton-transfer spectroscopy. However, of the numerous
J. Phys. Chem., Vol. 99, No. 19, 1995 7283
Triplet State Potentials Studied by ESIPT proton-transfer researches reported,’p2J3phosphorescence of the corresponding tautomer has been observed only rarely. It is obvious that as the proton-transfer tautomer fluorescence approaches the red limit of the spectrum, the TI’ SO’ phosphorescence will appear in the near infrared and must be specifically searched for in that region. A greater restriction on the observability of TI’ SO‘ emissions will be the strong tendency to undergo radiationless system crossing instead of appearing as a radiative process, as the T1’-S( band gap decreases, and the density of SO’ vibronic states at the TI’ zero point level increases; it may be necessary generally to perdeuterate an aromatic molecule to reduce this tendency. One example of an observed TI’ So’ phosphorescence is that for 2-(2’-hydroxyphenyl)benzothiazole (HBT) reported at 650 nm I , (onset at 590 nm) by Chou et al.;14in this molecule the ring 0-atom of HBO is replaced by an S-atom. The very low quantum yield and the 1.95-ms half-life observed at 77 K in (very dry) methylcyclohexane (MCH) glass suggests a strongly competing TI’ *> SO’ radiationless process. In “unpurified but anhydrous MCH’ the TI SOphosphorescence at 77 K competes and is dominent in ethanol glass at 77 K. These latter are simply chemical competition interferences to case C behavior, in which intramolecular H-bonding is interrupted, precluding ESIPT. Another and more subtle example is the case uncovered by Grabowska et aZ.15 for 2,2’-bipyridyl-3,3’-diol(here BPDO). The observation of the TI’ So’ phosphorescence at 790 nm (1260 cm-I) (298 K) was obviously difficult, as the caption to their Figure 4 indicates: “Difference between the near-IR steadystate emission spectra of the degassed and air-saturated bromobenzene solution.” To observe this weak emission, the authors had to use the solvent-heavy atom perturbation technique16 as well as singlet-oxygen sensitization to first enhance the emission (only the 0,O band is shown), and then to demonstrate its TI’ SO’ character by the lAg 3Xg- oxygensensitized emission” at 1268 nm. Grabowska et al. thus have found a second confirmed case C behavior since only TI’ SO’ emission was observed by direct steady state excitation, albeit under the difficulties exemplified by their Figure 4. They observed also a T I - SOnormal tautomer phosphorescence (Imm= 478 nm) for BPDO (their Figure 2) but only by indirect sensitization via a sensitizing molecule (toluene triplet) primary excitation (90 K, acetonitrile solvent; glass?). Unlike the subtleties of the TI‘ SO’ phosphorescence emission pre~ented,’~ the normal tautomer T I - SOemission was observed easily by the intermolecular triplet-sensitization described. However, this observation presents an additional puzzle: if case C is at hand (Figure 3), it would have been expected that the triplet potential for this case would rapidly funnel excitation to the TI’ minimum as compared with the low rate constant for the T I - SOemission. Above 140 K no T I - SOemission was observed, suggesting a triplet potential barrier for T1 *> TI’. Barriers for proton transfer in excited states are rare, this case offering an interesting example for further study. Proton-transfer fluorescence has become important in many special applications, in part because the large wavelength shifts from the normal tautomer absorption region (commonly in the UV) have permitted green-to-red-region fluorescence to be observed. Thus, ESIPT has become important as a source of fluorescence probes for radiation scintillators,l3 immunology, l8 and in the four-level proton-transfer laser. The presence of triplet state excitation pathways serves to diminish the quantum yield of the desired proton-transfer tautomer SI’ So’ fluorescence. Werner has investigated21the quenching of
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triplet state emission via intramolecular H-bonding in some benzotriazole cases; the estimated decay times of
