Volume 185, number 3,4 CHEMICAL PHYSICS LETTERS I8 October I99 I

Triplet states of molecules undergoing internal double-proton transfer in the S, state: 2,2’-bipyridyl-diol and its 5,5’-dimethylated derivative *

Anna Grabowska, Pawel Borowicz Instifute ofPhysical Chemistry, Polish Academy oj”Sciences. M. Kasprzaka 4415.2, 01-224 Warsaw, Poland

Daniel 0. M&ire and Silvia E. Braslavsky Max-Planck-lnsfitul ftir Strahlenchemie, Stijstrasse 34-36, W-4330 Miilheim am Ruhr, Germany

Received 26 June 199 I

2,2’-bipyridyl-3,3’-diol and its dimethyl derivative undergo an ultrafast double-proton transfer upon excitation to Sr state, and no direct population of T, state is observed. Thus, the primary (dienol) triplet (T, ) was populated. both by sensrtization from the toluene triplet and by enhancement of the forbidden T, +-So absorption. The low-energy phosphorescence of the phototauto- merit (diketo) triplet was detected by irradiation in bromobenzene and was quenched by Oz(%;) to yield O,( ‘A& The triplet state is located at 21300+ 300 and I2700+ 100 cm-’ for the primary and phototautomerrc form, respectively, in good agreement with the values predicted by the INDO/S method.

1. Introduction

The proton-transfer (PT) reactivity of aromatic molecules having internal hydrogen bonds is almost unknown on the level of their triplet states. The vast majority of the data published so far concerns pho- totautomerization reactions upon excitation to S, states [ 11, A series of excellent papers by the GBt- tingen group reporting on the proton-transfer reac- tivity of hydroxy-phenylbenzoxazoles excited to T, state is in this context a rare exception [2].

The reason for this is easy to explain: intramolec- ular proton-transfer reactions are usually so fast that there is no kinetic chance for population of the trip- let state from the primary excited structure. The phototautomeric triplet is also difftcult to detect in a standard experiment due to its very low energy, al- though its lifetime is in most cases reasonable.

In the present paper, we report on the observa- tions of both triplet states, primary and phototau-

* Dedicated to Professor Kurt Schaffner on the occasron of his 60th bnthday.

tomeric, of two molecules recently proved to undergo a transfer of two protons in the S, state [3]. These molecules are 2,2’-bipyridyl-3,3’-diol (hereafter BP(OH),) and its derivative with methyl groups in positions 5,5’ (hereafter Me2BP(OH),).

Both systems were described as undergoing very fast proton transfer in an efficient reaction in the S, state, as well as revealing lasing properties [4]. In spite of several attempts, we failed to detect the emission from any of the triplet states. The indirect manifestations of their contributions in overall pho- tophysics were the transient absorption of BP ( OH)z arising most probably from the phototautomeric

206 0009-2614/91/$ 03.50 0 1991 Elsevier Science Publishers B.V. All rights reserved.

Volume 185, number 3,4 CHEMICAL PHYSICS LETTERS 18 October 1991

‘i

Fig. 1. Experimental energy levels of BP(OH)? and toluene.

triplet state, the delayed fluorescence, and the de- tection of “slow heat” in photoacoustic experiments

]51. The main aim of the present work was to identify

both triplet states in question: primary dienol, and phototautomeric, most probably diketo (see section 4), with the application of tools suitable to handle such a system: oxygen perturbation enhancing the forbidden T, tS,-, absorption, triplet-triplet energy transfer, and the identification of heavy-atom-in- duced near-IR phosphorescence by the quenching by O? and the detection of the O,(‘$) emission. Our only guides before performing the experiments were the energy levels of both triplet states of BP(OH)2 calculated by the lNDO/S method [4] (fig. 1 and table 1 below).

2. Experimental

Chemiculs: BP(OH)* and Me,BP(OH): were synthesized as described in refs. [ 6,7]; toluene “for spectroscopy” was additionally distilled before use: xanthone spectrally pure was kindly given to us by Dr. B. Nickel (Gottingen).

Solt.ents: butyronitrile (Merck) was purified as recommended [ 81; benzene “for fluorescence spec- troscopy” was distilled just before use; cyclohexane (Fluka) “for UV spectroscopy” and bromobenzene (Merck) “Uvasol” were used without purification. All solvents were carefully checked for the absorp- tion and phosphorescence prior to experiment.

Methods: Absorption spectra were measured with a Specord M 40 Zeiss (Jena) spectrophotometer. The determination of the oxygen-perturbed absorption was performed in a quartz optical cell 10 cm long, situated inside the stainless-steel cell with thick su- prasil windows. The needle valve was open and the cuvette was shaken for about 15 min with oxygen under SO-140 atm (8.1 x 106-1.42~ IO' Pa).

Phosphorescence spectra sensitized by toluene or xanthone were measured with a Jasny spectrofluori- meter [ 91 with two rotating discs out of phase and a photon-counting system.

Near-IR phosphorescence of the ketotautomeric triplet state was measured as described in ref. [ lo]; basically, the set consisted of a 2.5 kW Xe lamp as excitation source and Ge diode to observe far-red and near-IR emissions. Lock-in amplification was used to improve the S/N ratio. A 360 nm cut-off filter was employed between the lamp and the cell; the intense near-IR component of the lamp was removed with 3 cm water iilter and two pieces of an infrared block- ing filter (Schott KG-5). In order to avoid the de- tection of higher orders of diffraction, for 760

nm<& ~900 nm, a 630 nm cut-off filter was em- ployed between the monochromator and the Ge de- tector; for 870 nm &,< 1350 nm, it was replaced by an 850 nm cut-off filter.

The bromobenzene solutions were degassed by five freeze-pump-thaw cycles at < 10-l Pa.

3. Results

The oxygen-perturbation technique of T,+S, ab- sorption developed by Evans [ 111 and widely used for the location of the lowest triplet states of many aromatic and heteroaromatic systems was applied here for BP(OH)2. The results are shown in fig. 2.

In both solvents (benzene and cyclohexane), the concentration of BP( OH), was as high as possible, 3x 10e3 M in cyclohexane and 8 x 10e2 M in ben-

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Volume 185, number 3,4 CHEMICAL PHYSICS LETTERS I8 October 199 1

-1 cm

Fig. 2. Singlet-triplet spin-forbidden absorption (curve I ) In benzene under 140 atm of oxygen at room temperature, and toluene- sensitized phosphorescence of BP(OH), (curve 2) in acetonitrile at 90 K (concentration of BP(OH)>: 8x 10m2 M for absorption, 8 x 10e3 M for phosphorescence). ( * ) absorption of oxygen dimole. Iph is the phosphorescence intensity in arbitrary units.

zene. It was proved that the spin-forbidden absorp- tion increased with the concentration of chromo- phore and with OZ pressure.

Under the highest oxygen pressure in the control- ling experiment, the well-known absorption of oxy- gen dimole, [ 0212, was detected at 20900? 100 cm-‘, corresponding to the transition: (‘A;) (‘C,‘)+(3C;) (3E;). Thisabsorption had only a small influence on the experiments with the sample, as shown in fig. 2.

Fig. 2 also contains the sensitized phosphores- cence spectrum obtained at low temperature in the presence of toluene as triplet energy donor.

From the intersection point of the room-temper- ature absorption and low-temperature emission T, c-) S,, spectra, the (0,O) energy of the phosphorescent state can be read out as 2 1300? 300 cm-‘.

The phosphorescence emission is indeed a prod- uct of sensitization, as shown by its anisotropy, mea- sured along with that of the donor phosphorescence. In fig. 3, the expected effect of complete depolari- zation of the acceptor emission is clearly seen. This emission was undetectable at temperatures higher than 120 K even in carefully deaerated solutions.

The sensitization of BP(OH)2 phosphorescence by xanthone gave essentially the same result al- though the contribution of the remaining phospho- rescence of the donor was much more difficult to subtract.

208

The near-IR emission spectra for BP(OH), and Me,BP(OH), were measured in bromobenzene, in order to increase the intersystem-crossing by exter- nal heavy-atom effect. They are presented in fig. 4 as the difference between the emissions from a de- gassed and an air-saturated solution.

In both difference spectra, the prominent bands at 790 and 785 nm (12660 and 12740 cm-‘), ascribed to (0, 0) bands of T; +SO transitions for BP( OH)? and Me?BP(OH),, respectively, are clearly seen.

The lower energy band at 1270 nm (7874 cm-‘) is the fingerprint frequency for the O,( ‘Ap) phosphorescence.

The quenching of the lowest energy flank of the emission spectrum in the degased solution by 0,(3C;), concomitant with the opposite sign of the emission of O?( ‘As), demonstrates that the triplet states of the studied molecules (3X ) are quenched by 0, ( 3C; ) following the usual process,

3X+02(3C,)+ ‘X+Oz( ‘4) . (1)

4. Discussion

The T-T transfer process is here a safe technique excluding the error caused by coexcitation of donor and acceptor: neither BP(OH)Z nor its methylated derivative phosphorescence when excited directly in

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CHEMICAL PHYSICS LETTERS 18October 1991

Fig. 3. Corrected, normalized phosphorescence spectra of toluene and BP(OH), in acetonitrile at 90 K and their anisotropy, R (3,,, = 37000 cm --I$ concentration of BP(OH)*: 8x 10-j M). f,,, - the same as in fig. 2.

the absence of sensitizer or a heavy atom. Phosphorescence only appears for direct excita-

tion in bromobenzene by the effect of the external heavy atom from the solvent (fig. 4). The next ques- tion is why upon sensitization, the phosphorescence observed is emitted from the primary excited form, i.e. why is the triplet energy not immediately trapped on the ketotautomeric triplet state situated by some 10000 cm-’ below the enol triplet? This is not a triv- ial question since in the fluorescent state, phototau- tomerization is an extremely fast, totally irreversible process, and even at liquid-helium temperature no trace of primary fluorescence is observed. The an- swer may be suggested by analogy with the best- known example of PT reactivity at the triplet level, namely the case of the 2-(2’-hydroxyphenyl)-ben- zoxazole molecule (HBO) [2]. Authors of the pa- pers describing the enol-keto tautomerization of this molecule in the T, state have shown that, in contrast to S,, and S, states, in the phosphorescent state the reaction is slowed down by an energy barrier. The tentative hypothesis explaining this was that the T, state is described by a non-polar canonical structure and the reaction is rather a hydrogen-atom shift than a proton-transfer process; such a process requires creation of a new single bond [ 21. Assuming this hy- pothesis, we can explain our low-temperature data as an effect of an intrinsic barrier on the reaction path on the triplet level, and the disappearance of pri- mary sensitized phosphorescence at temperatures

higher than 120 K as an effect of the thermally ac- tivated PT reaction leading to a phototautomeric triplet. In contrast to the case of HBO, no repopu-

800 900 1000 1100 1200 1300

h [nml

Fig. 4. Difference between the near-la steady-state emission spectra of the degassed and air-saturated bromobenzene solu- tions of (a) BP(OH)2 (absorbance at 380 nm: A= 1.38) and (b) Me>BP(OH), (absorbance at 380 nm: A= 1.16). Between the monochromator and the Fe detector, two different cut-off filters were employed: a 630 nm and an 850 nm for thehigh-energy (-----) and low-energy (-) spectral regions, respectively.

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Volume 185, number 3,4 CHEMICAL PHYSICS LETTERS 18 October I99 I

Table I Experimental and INDO/S calculated energies ofprimary (T,) and phototautomeric (T’,) triplet states o~BP(OH)~ and Me,BP(OH),. INDO/S calculations of electron excess densities on nitrogen (pN) and oxygen (po) atoms m So, T, and S, slates

Molecule Energies Electron densities

experimental calculated nitrogen atom oxygen atom

(0,O) (cm-‘) (cm-‘) (_PN) (-PO)

Tl T T, T; so T, S, &I T, S,

BP(OH)z 21300+300 12660 k 200 23000 13400 0.36 0.46 0.50 0.30 0.23 0.24 Me2BP(OH)> - 12740+200 23600 13700 0.36 0.47 0.50 0.30 0.23 0.24

lation of the primary triplet state is possible because of very deep stabilization of the phototautomeric (T’, ) state with respect to the primary (T, ) level. The driving force for the PT reaction calculated by INDO/S method is fairly strong, as represented by the electron-density increase on the nitrogen atoms accompanied by the opposite trend on the proton- donating oxygen atoms. Electron-density values along with experimental energies are shown in table 1.

The next problem is the nature of the phototau- tomeric triplet state. Is it the product of double or single proton transfer? It was shown in our studies [ 31 that at least in the S, state, both molecules undergo the double-proton transfer retaining their symmetry. The resulting tautomeric forms cannot be represented by nonionic canonical structures. The zwitterionic forms used to describe the tautomer, to- gether with the nonionic quinoid monoketo-struc- ture, a product of single-proton transfer, are shown in scheme 1. In principle, there is no reason to expect the same type of reactivity in two different electronic states of

the same molecule. In the present case, the photo- tautomer could bear the diketo-character in the S, state and monoketo-character in the T’, phosphores- cent level. This seems, however, unlikely since the energy of the T’, state calculated for the diketo-form agrees very well with the experimental value. Hence, most probably also in the triplet state, the phototau- tomer has a “diketo”-character.

In conclusion, we believe that we have delivered a new example of the internal proton-transfer reac- tivity at the level of the phosphorescent state with a direct observation of the phototautomeric triplet emission, opposed to the generalization published recently [ 121.

Acknowledgement

Our thanks are due to Dr. Andrzej Mordziriski for valuable discussion. DOM is in part recipient of a CONICET (Argentina) fellowship.

References

[ 11 Chem. Phys. 133, Nr. 2 (1989); Special Issue on Spectroscopy and dynamics of elementary proton transfer in polyatomic systems.

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M.F. Rodriguez-Prieto, B. Nickel, K.H. Grellmann and A. Mordziliski, Chem. Phys. Letters 146 (1988) 387; K.H. Grellmann, A. Mordzlliski and A. Heinrich, Chem. Phys. 136 (1989) 201.

I3 ] W. Liptay, R. Wortmann, P. Borowicz and A. Grabowska, Tautomerization Reaction in S, Fluorescent States of Bipyridyl-dials: A direct Confirmation of Intramolecular Double-Proton Transfer by the Excited-State Dipole Moment Determination, in preparation.

Volume 185, number 3,4 CHEMlCAL PHYSICS LETTERS I8 October 199 1

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