Role of rotamerisation and excited state intramolecular proton transfer in the photophysics of 2-(2º-hydroxyphenyl)benzoxazole, 2-(2º-hydroxyphenyl)benzimidazole and 2-(2º-hydroxyphenyl)benzothiazole : a theoretical study Pradipta Purkayastha and Nitin Chattopadhyay* Department of Chemistry, Jadavpur University, Calcutta - 700 032, India Received 19th October 1999, Accepted 22nd November 1999

Semiempirical (AM1-SCI) calculations have been performed to rationalise the experimental Ðndings in relation to the photophysics of 2-([email protected])benzoxazole (HBO), 2-([email protected])benzimidazole (HBI) and 2-([email protected])benzothiazole (HBT). The calculations reveal that, while for HBO and HBI, two rotameric isomers are present in the ground state, there is only one stable species in the S state of HBT. 0 Excited state intramolecular proton transfer (ESIPT) reaction is, however, operative in the lowest excited singlet (S ) and triplet (T ) states for all the three molecular systems ; resulting altogether three Ñuorescence 1 1 bands for HBO and HBI and two for HBT. The excitation, Ñuorescence and phosphorescence bands have been assigned theoretically. The calculated results agree well with the existing experimental reports. The potential energy surfaces (PES) have been generated for the intramolecular proton transfer (IPT) reactions. The PES reÑect that although the IPT process is not favourable in the ground state, the ESIPT process is feasible, both thermodynamically as well as kinetically, for all the three molecular systems in the S as well as T states. 1 1

Introduction The photophysical properties, including the prototropic behaviour of the electronically excited molecules are often found to change markedly from those observed in their ground states.1h4 The di†erence in the electronic distribution in the two states is principally responsible for the observed di†erences in the properties. The excited state intramolecular proton transfer (ESIPT) phenomenon has provoked massive interest to the photophysicists and photochemists because of the huge potential in it. Studies range from the choice of a variety of homo- and heterocyclic aromatic molecular systems to a wide variety of pure and mixed homogeneous and microheterogeneous solvents.5h15 The transfer of the proton in the excited state is evidenced by a large Stokes shift in the emission spectra of the azoles. The change in the electronic distribution e†ecting the transfer has been demonstrated by the studies of pH changes in the excited states.16h20 The ESIPT process is extremely fast occurring within the subpicosecond time scale which falls within the range of the period of low frequency vibrations.21 It is believed that the FranckÈCondon excited state of the molecule has a close proximity to the intersection with the potential energy surface (PES) of the phototautomer and, hence, on excitation, the molecule passes to the tautomeric species almost instantaneously and then relaxes vibrationally.22h26 The phenomenon of proton transfer in the excited states has been established Ðrmly by the studies made on the thermochromic and photochromic properties of anils of o-hydroxybenzaldehydes, all having the following basic structure.27h32 The barriers to both the normal* ] tautomer* and tauto-

mer ] normal processes are very low ; greater than the energies of the lowest frequency vibrations but similar to, or smaller than those of the stretching modes of the concerned compound skeleton. It has, thus, been stated that the proton transfer comes about through the vibrational relaxation rather than thermal activation.33 After analysing the molecular geometry as a function of the reaction coordinate it was inferred that, for these molecules, the proton transfer process takes place in the molecular plane. Rios and Rios conceived the ESIPT process as the following : the Ðrst phase of the process is the mutual approach of the oxygen and the respective heteroatoms in the said compounds.33 The OÈH distance or the heteroatomÈhydrogen distance practically remains constant during the process. The chemical process occurs as soon as the H atom comes within the chemical interaction range of the other electronegative atom. Proton transfer is completed by the separation of the heavy atoms.33 ESIPT process, in the three studied compounds, namely, HBO, HBI and HBT follows a strict conformational requirement. Logically there are two possible rotamers (I and II) for these molecules in the ground state (Scheme 1). Electronic excitation of the species may lead to ESIPT product from only one of the two rotameric forms. Taking these into consideration, one expects, in general, three emission bands from these molecular species in selected solvents. There are a good number of experimental reports available on the photophysics of these systems in di†erent environments.6,13,21h50 Woolfe et al. established the existence of three Ñuorescence bands for HBO depending on the solvent.6 Mordzinski and Grabowska showed that in the S state ESIPT occurs through a low acti1 vation barrier.34 The existence of the tautomeric form has also been conÐrmed in the lowest triplet state (T ).35h37 In HBI, 1 there exist two rotameric species in the ground state which has been revealed from the di†erent excitation spectra.38 Selective excitation of one of the two forms produces the normal emission while the other, after excitation, undergoes Phys. Chem. Chem. Phys., 2000, 2, 203È210

This journal is ( The Owner Societies 2000

203

Quantum chemical calculations

Scheme 1 The normal (I), rotamer (II) and tautomeric (III) species of HBO, HBI and HBT.

ultrafast ESIPT to form the tautomer with Stokes-shifted emission. The complex photophysics of HBT has been reÑected from several works.39h44 The existence of the keto form of HBT in its S state has been established from the transient infrared 1 absorption signals corresponding to NH and CO stretching vibrations.43 Ding et al. have shown that for HBT the very fast ESIPT is followed by the vibrational relaxation of the molecule.44 The existence of di†erent species in the ground and excited states of the concerned molecules a†ect their absorption and emission spectra di†erently in solvents di†ering in dielectric property and hydrogen bonding ability.10 From a comparison of the spectroscopy of HBO and HBT, Nakagaki et al. ascribed the di†erence in the triplet yields of the two molecular systems primarily to the hydrogen bonded structures.45 The main triplet species of HBT in non-polar and alcoholic solvents were ascribed to an intramolecularly proton transferred tautomeric form and an intermolecularly hydrogen bonded form respectively.46 The triplet species of HBO, however, contains the tautomeric form in both non-polar and alcoholic solvents. When HBT and HBO are intermolecularly hydrogen bonded in alcoholic solvents, HBT is more susceptible to rotation than HBO.46 The oxazole (HBO) has a higher quantum efficiency and greater UV stability than its thiazole counterpart. Its Ñuorescence emission is more structured than that of the thiazole. HBI is a relatively weak emitter.47 In the case of HBT the Ñuorescence lifetime generally decreases with increased polarity of the solvents.48 Although there are some theoretical works relating to the photophysics of some of these classes of compounds, they are really scattered in the sense that their interest was centred on some aspects, particularly to the rotational conformations etc., of the processes.6,10,21,38,46,49 To the best of our knowledge, there is no theoretical report describing the ESIPT trajectory and hence its thermodynamic and/or kinetic aspects. In the present paper, our comprehensive e†ort has endeavoured to rationalise the overall Ñuorometric experimental observations through the semi-empirical AM1-SCI calculations. We have calculated the excitation, Ñuorescence and phosphorescence spectral position from the di†erent energy states of the molecular systems in some selected solvents and compared these with the available literature data. The potential energy surfaces (PES) for the intramolecular proton transfer (IPT) reaction has been simulated in di†erent electronic states (S , S 0 1 and T ) of all the three molecular systems which corroborate 1 the occurrence of excited state intramolecular proton transfer (ESIPT) processes for all of them. 204

Phys. Chem. Chem. Phys., 2000, 2, 203È210

Although ab initio calculations involving extended basis sets with extensive conÐguration interaction (CI) have been successful in explaining structures, energetics and reactivities of small molecules in di†erent electronic states, such reports are still limited in number for large molecular systems. However, semi-empirical molecular orbital methods have already established their wide utilities in this respect. The methods provide acceptable approximations to give results which are quite close to the experimental Ðndings.49h52 Recently, we have established the reliability of the AM1-SCI method through our calculations of the energies of di†erent electronic states of two Ñuorophores and thus, realisation of the variation of the non-radiative deactivation of the Ñuorophores via internal conversion (IC) and intersystem crossing (ISC) in di†erent solvents.52 For the present semi-empirical calculations, we have used the commercial package, HYPERCHEM 5.01.53 The geometries of the molecules have been optimised in the ground state using the AM1 method. For the excited states, we have adopted the AM1-SCI, whereby we have considered all the conÐgurations (around 70 conÐgurations) within an energy window of 12 eV from the ground state, for the single electronic transitions only. The calculations yielded the energy (E ) and dipole moment (k ) in the ground state and the trang g sition energies (*E ) to di†erent excited electronic states. i?j *E corresponds to the excitation of an electron from the i?j orbital / (occupied in the ground state) to the orbital / i j (unoccupied in the ground state). The total energy of the excited state (E ) was then calculated as E \ E ] *E . The j j g i?j CI wavefunctions were used to generate orbitals and oneelectron density matrices, which were used, in turn, to calculate the dipole moments of the excited states of the molecular systems. To Ðnd the relative stability of the di†erent rotational conformers (rotamers), we have preset the torsional angle (7È8È 10È11) between the hydroxyphenyl plane and the heterocyclic plane to di†erent values and then fully optimised all other geometrical parameters. With these optimised structures, we have then performed the SCI within the aforesaid energy window to get the energies and dipole moments corresponding to ground as well as di†erent excited electronic states (S , 0 S and T ). 1 1 For the intramolecular proton transfer (IPT) reaction, we have considered the distance between the dissociable hydrogen of the hydroxy group and the nitrogen atom (to which the hydrogen gets attached when tautomer is formed) involved in the process (R ) as the reaction coordinate. For the gener7h17 ation of the PES for the IPT process, we have optimised the geometries with various preset values of the reaction coordinates. As before, AM1-SCI was applied to get the energies and dipole moments of the species on the trajectory of the reaction in di†erent electronic states. The enthalpy of reaction (*H) and activation energy (E ) for the IPT reaction of the Ñuoroact phores have been determined from the PES in the corresponding electronic states. The solvent stabilisation of di†erent states has been calculated from the solvation energies based on OnsagerÏs theory.54 Assuming for the solute molecule, having a dipole moment k i in the ith electronic state, to be fully solvated, the solvation energy is given by 2k2(e [ 1) *E \ i r solv a3(2e ] 1) r where e is the bulk relative permittivity of the solvent and a r the cavity radius. We have taken the maximum molecular length for the optimised geometry as the cavity diameter for the molecular systems (10.92, 10.94 and 11.14 AŽ for HBO, HBI and HBT respectively). It is pertinent to mention here that

speciÐc solvent interactions like hydrogen bonding etc. have not been considered for the present work.

Results and discussion Although, the crystallographic data is available for HBT,55 they are not readily available56 for the other two molecular systems (HBO, HBI). To verify the reliability of our calculations we have extended it to another similar molecule, namely, 2-(3-methoxy-2-hydroxyphenyl)benzimidazole

Scheme 2

Structure of MHBI.

(MHBI) for which the crystal structure is available.57 Table 1 compares the calculated ground state optimised geometric parameters with the crystallographic data for both HBT and MHBI molecular systems. A good correspondence between the calculated parameters and the crystallographic data establishes the suitability and reliability of our calculations. Table 2 presents the calculated geometric parameters for the di†erent forms of HBO, HBI and HBT in di†erent electronic states. The table reveals that for all the three molecular species in the isolated condition, the normal form (I) is the stablest one in the ground state. The energies of the di†erent species have also been calculated under solvent stabilised conditions. The relative stabilities of the di†erent forms, however, do not seem to be remarkable in the presence of common solvents. Fig. 1, 2 and 3 reÑect the simulated energy proÐles for the rotational motion of the hydroxyphenyl moiety relative to the heterocyclic ring. Fig. 1 and 2 reveal the existence of the

Table 1 Comparison of the calculated ground state optimised geometric parameters of HBT and MHBI with the crystallographic data [bonds (AŽ ) and angles (¡)] HBTa

MHBIb

Molecular parameters

Calculated

O ÈC 16 11 N ÈC N7ÈC8 7 6 S ÈC S9ÈC8 9 5

1.366 1.327 1.400 1.751 1.687

Cryst. datac

Molecular parameters

O ÈC 16 11 N ÈC N7ÈC8 7 6 N ÈC N9ÈC8 9 5 O ÈC O19ÈC12 19 20 C ÈN ÈC 110.3 110.8 C ÈN ÈC C8ÈS 7ÈC 6 91.0 88.6 C8ÈN7ÈC6 8 9 5 8 9 5 C ÈO ÈC 12 19 20 a For the structure of HBT see Scheme 1 (form I). b For the structure of MHBI see Scheme 2. c From ref. 55 d 1.305 1.280 1.404 1.749 1.757

Calculated

Cryst. datad

1.366 1.353 1.407 1.417 1.395 1.380 1.423 105.7 106.7 115.9

1.353 1.325 1.391 1.371 1.376 1.377 1.419 106.1 107.6 117.1

From ref. 54.

Table 2 Equilibrium parameters of di†erent photoisomers of HBO, HBI and HBT in di†erent electronic states. Energy (E) and dipole moment (k) are expressed in eV and debye units respectively. R represents the interatomic distance (AŽ ) between the two atoms referred to by the 7h17 numbers (see Scheme 1). T is the torsional angle (degrees) developed by the atoms referred to by the numbers 7h8h10h11 Molecule

Parameters

Normal (I)

Rotamer (II)

Tautomer (III)

HBO

E(S ) k(S 0) 0 R T 7h17 7h8h10h11 E(S ) k(S 1) 1 T 7h8h10h11 E(T ) k(T 1) T 1 7h8h10h11 E(S ) k(S 0) R 0 T 7h17 7h8h10h11 E(S ) k(S 1) T 1 7h8h10h11 E(T ) k(T 1) T 1 7h8h10h11 E(S ) 0 k(S ) 0 R T 7h17 7h8h10h11 E(S ) 1 k(S ) 1 T 7h8h10h11 E(T ) k(T 1) 1 T 7h8h10h11

[126.4425 1.77 2.17 0 [122.9232 1.15 0 [123.9292 1.84 0 [129.5468 3.36 2.29 40 [126.0484 2.65 0 [126.9800 3.07 0 [125.8044 2.11 2.16 0 [122.4303 2.00 0 [123.3868 1.80 0

[126.4369 0.80 3.70 150 [122.8528 1.33 180 [123.8853 1.19 180 [129.5230 1.67 3.67 140 [125.9378 1.97 160 [126.8966 1.19 160 È È È È È È È È È È

[126.0482 4.26 0.996 0 [123.0303 3.08 0 [124.4109 2.22 0 [129.1756 5.49 0.998 0 [126.2733 4.00 0 [127.4739 3.43 0 [125.4224 4.03 1.00 0 [122.7835 0.85 0 [124.0442 2.72 0

HBI

HBT

Phys. Chem. Chem. Phys., 2000, 2, 203È210

205

Fig. 1 Plot of total molecular energy as a function of torsional angle (7È8È10È11) in S , S and T states of HBO (i, isolated ; s, solvated in 0 1 1 ethanol).

normal (I) and rotameric (II) forms of HBO and HBI in the ground state which is corroborated from the experimental observations.6,21,38 Our calculations establish the existence of two rotameric forms (I and II) of HBO in the ground state which has also been established by Nagaoka et al.46 as well as Das et al.21 However, there remains slight discrepancy in the reported ground state geometries of the stable rotational isomers. While we calculate T for I and II of HBO 7h8h10h11

Fig. 2 Plot of total molecular energy as a function of torsional angle (7È8È10È11) in S , S and T states of HBI (i, isolated ; s, solvated in 0 1 1 ethanol).

Fig. 3 Plot of total molecular energy as a function of torsional angle (7È8È10È11) in S , S and T states of HBT (i, isolated ; s, solvated in 0 1 1 ethanol).

as 0 and 150¡, Nagaoka et al. found the corresponding angles to be 0 and 180¡ and the Das group found the same to be 30 and 180¡ respectively. The corresponding angles for the similar ground state rotameric species (I and II) of HBI are calculated to be 40 and 140¡. The values match with those calculated by Das et al.21 Fig. 3, however, indicates that there is only one stable form, the normal one (I) for HBT, in the ground state. Although there is a shallow well shaped portion on the PES corresponding to the rotameric form (II), there is practically no energy barrier for the transition I % II that can incur stability to the species II for HBT. Our AM1 results agree well with the ab initio calculations of Nagaoka et al. where they reported that the rotamer of HBT is unstable.46 The theoretical interpretation matches exactly with the experimental observations where we do not Ðnd any evidence in favour of the presence of the rotamer (II) for HBT although similar isomers exist for HBO and HBI systems. The low electronegativity and the bulkier size of the sulfur atom may be responsible for the instability of the rotameric species (II) in HBT.46 For all the three molecular systems we Ðnd a change in the molecular geometry in the excited electronic states. The energy of activation for the conversion of I to II is found to be 0.101 and 0.045 eV for HBO and HBI respectively in the isolated conditions. These values come out to be 0.113 and 0.054 eV in ethanol solution. The barrier height for the reverse

Table 3 Calculated S , S and T energies (eV) of the normal (I), rotamer (II) and tautomer (III) of HBO in isolated and solvated 0 1radius, a1\ 5.46 AŽ ) conditions (OnsagerÏs cavity Medium/ solvent

Relative permittivity (e ) r

Vacuum

È

Cyclohexane

2.0

p-Dioxane

2.2

Ethanol

24.3

Acetonitrile

38

Water

80

206

Phys. Chem. Chem. Phys., 2000, 2, 203È210

Species

E(S ) 0

E(S ) 1

E(T ) 1

I II III I II III I II III I II III I II III I II III

[126.4425 [126.4369 [126.0482 [126.4474 [126.4379 [126.0766 [126.4478 [126.4380 [126.0793 [126.4538 [126.4392 [126.1140 [126.4540 [126.4393 [126.1155 [126.4543 [126.4393 [126.1169

[122.9232 [122.8528 [123.0303 [122.9253 [122.8553 [123.0450 [122.9255 [122.8553 [123.0450 [122.9280 [122.8586 [123.0645 [122.9281 [122.8588 [123.0653 [122.9282 [122.8589 [123.0660

[123.9292 [123.8853 [124.4109 [123.9345 [123.8871 [124.4185 [123.9350 [123.8876 [124.4193 [123.9415 [123.8895 [124.4287 [123.9418 [123.8896 [124.4291 [123.9421 [123.8897 [124.4295

Table 4 Calculated S , S and T energies (eV) of the normal (I), rotamer (II) and tautomer (III) of HBI in isolated and solvated conditions 0 1 1 (OnsagerÏs cavity radius, a \ 5.47 AŽ ) Medium/ solvent

Relative permittivity (e ) r

Vacuum

È

Cyclohexane

2.0

p-Dioxane

2.2

Ethanol

24.3

Acetonitrile

38

Water

80

Species

E(S ) 0

E(S ) 1

E(T ) 1

I II III I II III I II III I II III I II III I II III

[129.5468 [129.5230 [129.1756 [129.5641 [129.5273 [129.2220 [129.5661 [129.5278 [129.2271 [129.5875 [129.5330 [129.2845 [129.5885 [129.5333 [129.2870 [129.5896 [129.5336 [129.2901

[126.0484 [125.9378 [126.2733 [126.0592 [125.9438 [126.2980 [126.0604 [125.9445 [126.3007 [126.0738 [125.9518 [126.3312 [126.0744 [125.9522 [126.3325 [126.0751 [125.9525 [126.3341

[126.9800 [126.8966 [127.4739 [126.9944 [126.8988 [127.4919 [126.9960 [126.8991 [127.4939 [127.0139 [126.9018 [127.5163 [127.0147 [126.9019 [127.5173 [127.0156 [126.9020 [127.5185

transformation for the said molecules are 0.096 and 0.021 eV in the isolated condition and 0.099 and 0.005 eV in ethanol solution respectively. The low energy barriers suggest that the two rotameric forms I and II are in equilibrium. There is practically no energy barrier for II ] I transformation in the case of HBT resulting in the nonexistence of form II of this system.

Thus, from the calculations, we support the existence of the normal isomer (I) and the rotamer (II) for HBO and HBI in the ground state and only the normal (I) species for HBT in the same state. These can now account for an assignment of the excitation, Ñuorescence and phosphorescence spectra of the concerned photospecies in di†erent chosen solvents,

Table 5 Calculated S , S and T energies (eV) of the normal (I) and tautomer (III) of HBT in isolated and solvated conditions (OnsagerÏs 1 cavity radius, a \ 5.570AŽ ) 1 Medium/ solvent

Relative permittivity (e ) r

Vacuum

È

Cyclohexane

2.0

p-Dioxane

2.2

Ethanol

24.3

Acetonitrile

38

Water

80

Species

E(S ) 0

E(S ) 1

E(T ) 1

I III I III I III I III I III I III

[125.8044 [125.4224 [125.8110 [125.4462 [125.8116 [125.4486 [125.8196 [125.4777 [125.8199 [125.4790 [125.8203 [125.4802

[122.4303 [122.7835 [122.4362 [122.7845 [122.4368 [122.7846 [122.4440 [122.7859 [122.4443 [122.7860 [122.4446 [122.7860

[123.3868 [124.0442 [123.3916 [124.0551 [123.3920 [124.0561 [123.3978 [124.0694 [123.3981 [124.0699 [123.3983 [124.0705

Table 6 Assignment of excitation, Ñuorescence and phosphorescence spectra of HBO in di†erent solvents in terms of calculated energies (eV). Numbers within parentheses refer to the references corresponding to the experimental data (n.a. indicates non-availability of data) Excitation

Fluorescence

Phosphorescence

Solvent

Species

Calc.

Expt. (ref.)

Calc.

Expt. (ref.)

Calc.

Expt. (ref.)

Cyclohexane

I II III I II III I II III I II III I II III

3.52 3.59 È 3.52 3.59 È 3.53 3.59 È 3.53 3.59 È 3.53 3.59 È

3.71(6) 3.88(6) È n.a. n.a. È 3.77(6) 3.94(6) È 3.77(6) 3.94(6) È 3.77(13) 3.94(13) È

3.52 3.58 3.00 3.52 3.58 2.98 3.51 3.58 2.98 3.51 3.58 2.98 3.51 3.58 2.98

n.a. 3.40(6) 2.60(6) n.a. n.a. n.a. n.a. 3.40(6) 2.64(6) n.a. 3.45(6) 2.62(6) n.a. 3.45(13) 2.59(13)

2.51 2.55 1.63 2.51 2.55 1.63 2.50 2.55 1.62 2.50 2.55 1.62 2.50 2.55 1.62

n.a. n.a. n.a. n.a. n.a. n.a. 2.30(12) 2.80(12) n.a. n.a. n.a. n.a. n.a. n.a. n.a.

p-Dioxane Ethanol Acetonitrile Water

Phys. Chem. Chem. Phys., 2000, 2, 203È210

207

Table 7 Assignment of excitation, Ñuorescence and phosphorescence spectra of HBI in di†erent solvents in terms of calculated energies (eV). Numbers within parentheses refer to the references corresponding to the experimental data (n.a. indicates non-availability of data) Excitation

Fluorescence

Phosphorescence

Solvent

Species

Calc.

Expt. (ref.)

Calc.

Expt. (ref.)

Calc.

Expt. (ref.)

Cyclohexane

I II III I II III I II III I II III I II III

3.52 3.59 È 3.52 3.59 È 3.54 3.60 È 3.54 3.60 È 3.54 3.60 È

3.70(50) 3.88(50) È n.a. n.a. È 3.75(38) 3.80(38) È n.a. n.a. È n.a. n.a. È

3.49 3.58 2.88 3.49 3.58 2.87 3.47 3.57 2.84 3.47 3.57 2.84 3.47 3.57 2.84

n.a. n.a. n.a. n.a. 3.55(21) 2.64(21) n.a. 3.60(38) 3.10(38) n.a. n.a. n.a. n.a. 3.55(21) 2.86(21)

2.55 2.49 1.68 2.55 2.49 1.68 2.53 2.47 1.66 2.53 2.47 1.66 2.53 2.47 1.66

n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a.

p-Dioxane Ethanol Acetonitrile Water

proper consideration being paid to the solvation of the states involved in the photoprocesses. Tables 3, 4 and 5 represent the calculated energy values of S , S and T states of the di†er0 1 1 ent photoisomers (I, II and III) of HBO, HBI and HBT respectively, in isolated and solvated conditions. Since, in pure and homogeneous solvents the solvation dynamics requires a faster timescale than the Ñuorescence lifetime,58 the probe molecule is solvated before it Ñuoresces. The Ñuorescence has, thus, been correlated with the transition from the solvated S 1 state to the corresponding FranckÈCondon S state. The exci0 tation spectra have, similarly, been correlated with the transition between the solvated S state and the FranckÈCondon 0

S state. We have also calculated the transition energies from 1 the solvated T state to the corresponding FranckÈCondon S 1 0 state and, crudely, assigned the phosphorescence spectra of the compounds to it, although the phosphorescence emission is obviously modiÐed in the solid matrix. The assignments for the excitation, Ñuorescence and phosphorescence spectra of the three Ñuorophores are presented in Tables 6, 7 and 8. The potential energy surfaces (PES) for the intramolecular proton transfer (IPT) process of the probes have been generated in S , S and T states considering R (distance 0 1 1 7h17 between the N and H ) as the reaction coordinate. Figs. 4, 5 7 17 and 6 represent the simulated PES for the IPT process of the Ñuorophores HBO, HBI and HBT respectively in the three aforesaid states in isolated condition as well as in ethanolic

Fig. 4 Simulated PES for IPT process of HBO in S , S and T 0 1 1 states (i, isolated ; s, solvated in ethanol).

Fig. 5 Simulated PES for IPT process of HBI in S , S and T 0 1 1 states (i, isolated ; s, solvated in ethanol).

Table 8 Assignment of excitation, Ñuorescence and phosphorescence spectra of HBT in di†erent solvents in terms of calculated energies (eV). Numbers within parentheses refer to the references corresponding to the experimental data (n.a. indicates non-availability of data) Excitation

Fluorescence

Phosphorescence

Solvent

Species

Calc.

Expt. (ref.)

Calc.

Expt. (ref.)

Calc.

Expt. (ref.)

Cyclohexane

I III I III I III I III I III

3.38 È 3.38 È 3.39 È 3.39 È 3.39 È

n.a. È n.a. È 3.77(42) È n.a. È n.a. È

3.37 2.64 3.37 2.64 3.36 2.64 3.36 2.64 3.36 2.64

n.a. n.a. n.a. n.a. 3.36(42) 2.70(42) n.a. n.a. n.a. n.a.

2.41 1.37 2.41 1.37 2.41 1.35 2.41 1.35 2.41 1.35

n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a.

p-Dioxane Ethanol Acetonitrile Water

208

Phys. Chem. Chem. Phys., 2000, 2, 203È210

Table 9 Calculated activation energies (E in eV) and reaction enthalpies (*H in eV) for the intramolecular proton transfer reaction of HBO, act HBI and HBT in S , S and T states 0 1 1 HBO

HBI

Medium/solvent

State

E act

*H

E

Vacuum

S 0 S 1 T 1 S S0 1 T S1 0 S T1 1 S 0 S 1 T S1 0 S 1 T 1 S S0 1 T 1

1.251 0.892 0.975 1.239 0.892 0.972 1.238 0.892 0.972 1.223 0.893 0.968 1.223 0.893 0.968 1.222 0.893 0.967

]0.394 [0.088 [0.375 ]0.371 [0.102 [0.385 ]0.369 [0.103 [0.386 ]0.340 [0.121 [0.399 ]0.339 [0.121 [0.400 ]0.338 [0.122 [0.400

1.075 0.781 0.829 1.050 0.777 0.820 1.047 0.777 0.819 1.015 0.772 0.807 1.014 0.772 0.807 1.013 0.772 0.807

Cyclohexane p-Dioxane Ethanol Acetonitrile Water

HBT

act

*H

E act

*H

]0.335 [0.236 [0.448 ]0.306 [0.250 [0.460 ]0.302 [0.252 [0.462 ]0.266 [0.269 [0.477 ]0.265 [0.270 [0.477 ]0.263 [0.270 [0.478

1.187 0.722 0.806 1.170 0.722 0.806 1.168 0.722 0.806 1.146 0.723 0.805 1.145 0.723 0.805 1.144 0.723 0.805

]0.380 [0.338 [0.627 ]0.362 [0.333 [0.639 ]0.361 [0.332 [0.641 ]0.339 [0.326 [0.656 ]0.338 [0.325 [0.656 ]0.337 [0.325 [0.657

(1) HBO and HBI have two rotameric forms (I and II) in the ground state while HBT exists only in the normal form (I) under similar condition. This corroborates the observation of three Ñuorescence bands from HBO and HBI and only two emissions from HBT. (2) For all the three molecular systems, the intramolecular proton transfer (IPT) reaction is unfavourable in the ground state both from the thermodynamic as well as kinetic points of view. However, both factors favour the excited state intramolecular proton transfer (ESIPT) process in the lowest excited singlet and triplet states.

Acknowledgements

Fig. 6 Simulated PES for IPT process of HBT in S , S and T 0 1 1 states (i, isolated ; s, solvated in ethanol).

solution. The general observations that come out from the Ðgures can be summed up as follows. For all the three Ñuorophores tautomer formation (through IPT) in the ground state is an endothermic process. The ESIPT reaction is, however, exothermic in the S as well as in the T states. Thus, the IPT 1 1 process, for the three studied molecular systems, is thermodynamically unfavourable in the S state but favourable in either 0 of the S or T states. The activation energy for the IPT reac1 1 tion is found to be appreciably large, for all the three molecules in the S state compared to the corresponding values in 0 the S and T states. An appreciable lowering of the activation 1 1 barrier (kinetic parameter) in the lowest excited singlet and triplet electronic states indicates that the IPT process is favoured in the excited states compared to the ground state. Table 9 presents the kinetic (E ) and thermodynamic paramact eters (*H) for the IPT process of the three molecular systems in the three electronic states.

Financial assistance from the Council of ScientiÐc and Industrial Research, Govt. of India, is gratefully acknowledged. Thanks are due to Dr. S. C. Bera for discussions.

References 1 2 3 4 5 6 7 8 9 10 11

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Paper a908359f

Role of rotamerisation and excited state intramolecular proton transfer ...

Excited state intramolecular proton transfer (ESIPT) reaction is, however, operative in the lowest .... Although, the crystallographic data is available for HBT,55.

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