- Email: [email protected]
Intramolecular charge transfer in jet-cooled aromatic amines
Volume 145, number 4
CHEMICAL PHYSICS LETTERS
8 April 1988
INTRAMOLECULAR CHARGE TRANSFER IN JET-COOLED AROMATIC AMINES Julie AUGUST, T. Frank PALMER, John P. SIMONS Department of Chemistry, University ofNottingham. Nottingham NG7 2RD. UK
Christophe JOUVET Laboratoire de Photophysique Mol.&laire du CNRS, Universit.4Paris&d,
Orsay Cedex, France
and Wolfgang RETTIG Iwan N. Stranski Institute for Physicaland Theoretical Chemistry, Technical University ofBerlin, D-l 000 Berlin 12, Germany
Received 16 November 1987; in final form 1February 1988
LIF excitation spectra have been recorded for a family of molecules whose dual fluorescence in solution has been ascribed to twisted intramolecular charge transfer (TICT) state formation, promoted through torsional motion in the initially photoexcited state. To eliminate the influence of solvation, the LIF spectra have been recorded under jet-cooled isolated-molecule conditions: the molecules include para-substituted nitrile or ester derivatives of dialkylamino benzene compounds. Evidence both for torsional motion and TICT state formation has been gained for the ester derivatives where access into the TICT state correlates with the onset of a quasi-continuous component in the LIF system and the appearance of a red-shifted fluorescence emission.
1. Introduction The dual luminescence exhibited by simple aromatic molecules in solution such as 4-N,N-dimethylaminobenzonitrile (DMABN) and related compounds in which a large degree of charge separation is associated with a twisted (or small overlap) arrangement of the two chromophores, has been the subject of increasing activity in recent years [ 11. The “normal” B band fluorescence emission can be assigned to an S,-So transition from the “locally excited” B* state, but the parallel appearance of a redshifted “A band” fluorescence has led to the notion of a second, twisted intramolecular charge transfer (TICT ) state [ l-3 1. TICT state formation is particularly favoured in molecules which have nonplanar ground-state conformations [ 4,5 1. Most investigations have been carried out in the solution phase where specific solute-solvent interactions and the microscopic hindrance of intramolecular mobility due to the solvent cage lead to severe
complications. The solvent environment can also have a profound effect on the ordering of excited-state energy levels, and whilst TICT state emission occurs in strongly polar solvents for the dialkylamino benzonitriles, the corresponding alkoxycarbonyl derivatives show anomalous “A” band fluorescence in both polar and non-polar solvents [ 6-8 1. TICT state emission in the gas phase has been observed in derivatives of DMABN [ 9, lo] which are twisted about the central aromatic carbon-nitrogen bond, and in pcyanophenylpyrrole [ 1 ] but the thermal congestion of energy levels at the ambient or elevated temperatures lead to broad featureless emission spectra which greatly restrict spectroscopic interpretation. The cooling and consequent spectral simplification introduced by free-jet expansion offers an alternative strategy for assessing the geometrical changes which may follow electronic excitation; recently attempts have been made to gain evidence for TICT state formation, through laser-induced fluorescence (LIF) spectroscopy of solvent van
0 009-2614/88/I 03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division )
273
Volume 145, number 4
CHEMICAL PHYSICS LETTERS
der Waals complexes of DMABN [ 1l- 13 ] and 9,9’bianthryl [ 14,15 1. The present investigation focuses on the LIF excitation spectroscopy of a range of isolated jet-coold dialkylaminobenzoic acid nitriles and esters where strong perturbations introduced by the energetically close lying TICT state could be expected. The esters are especially favourable candidates for population of the TICT state even under “jet” conditions, since the ‘Lb- and ‘La-type states are respectively raised and lowered energetically with respect to the nitriles [ 7,16,17 ] and it is the ‘L, state which correlates directly with the twisted TICT state [18] (see fig. 1).
8 April 1988
aci,
1
2. Experimental Amino- and dialkylamino-benzoic acid nitriles and esters, were vaporised in a small oven at temperatures between 110 and lSO”C, mixed with helium carrier gas at stagnation pressures of 2 to 3 atm and expanded into a vacuum chamber through an 800 pm diameter orifice pulsed nozzle valve (General Valve Corporation) [ 19,201. Replacement of the seals of the valve by a polyfluorocarbon “0” ring (“Kalrez”, DuPont Ltd.) and polyamide fibre plunger (“Vespel SPl” Du Pont Ltd.) and the solenoid windings by copper wire (0.14 mm diameter coated with a temperature-resistant lacquer (“Lewcos” F.D. Sims Ltd.), allowed prolonged operation at elevated temperatures. Chromel-alumel thermocouples were located close to the nozzle orifice and oven to provide continuous monitoring of the temperatures. Independent heating of both the pulsed nozzle valve and the oven allowed the temperature of the orifice to be maintained a few degrees above the temperature of the evaporating samples. The vacuum chamber was evacuated with a vapour booster pump (Edwards 9B3) backed by a rotary pump (Edwards’ED 500) which maintained a background pressure z 0.1 Pa. A delayed pulse generator was used to trigger both the pulsed valve and the N,-laser-pumped dye laser (Molectron), allowing synchronisation of the pulsed supersonic jet and the intersecting laser beam a few mm downstream from the nozzle. The pulsed valve had a characteristic open-shut time of x 1 ms and was operated at 20 Hz. LIF spectra were generated by simultaneously 214
(‘8
m Sl~}Density
Local excitation e%Lm
ofstates
TKT
Twist angle (0)
Fig. 1. (a) Potential energy surfaces representing sections along the C-N torsional coordinate in (i ) alkylamino benzonitrile and (ii) alkylamino bezoic ester derivatives in solution at 300K.(b) Schematic representation of vertical transitions leading to “normal” (B band) and TICT (A band) fluorescence in isolated jet-
cooled alkylaminobenzoic acid esters.
recording the incident laser intensity and that of the total fluorescence emitted along a perpendicular axis using a photomultiplier/boxcar (Brookdeal 464, 465) combination. The structures of the amino- and dialkylamino-
Volume 145, number 4
CN ABN
COOMe DMABME
CN DMABN
CooEt DMABEE
CHEMICAL PHYSICS LETTERS
CN PYRBN
CODE1 PYRBEE
8 April 1988
CN 0
PIPBN
200
400
Wavenumber
(a?) /cm-’
Wavenumber
cav)I cm.’
600
COOEt 2.5.PYRBEE
Fig. 2. Aminobenzonitrile, dialkylamino- and cyclic amino-benzoic acid nitriles and esters.
Fig. 3. LIF excitation spectrum ofjet-cooted PYRBN vU= 32030 cm-‘.
benzoic acid nitriles and esters investigated are shown in fig. 2. The cyclic aminobenzonitriles and esters, PYRBN, PIPBN and PYRBEE were synthesized via nucleophilic substitution of the 4-fluorobenzoic acid nitrile or ester, according to the general procedure given by Suhr [ 2 1] and purified by recrystallization and sublimation. The details of preparation and identification, as well as the photophysical properties in solution, have been described extensively elsewhere [7,8].
ment to a torsional mode and suggests that the potentials along the twist coordinate are somewhat different in the ground and excited states. Comparison with the jet-cooled spectra of aniline 14
t
I
3. Results LIF excitation spectra of PYRBN, PYRBEE, PIPBN, DMABEE and DMABME were recorded in the wavelength range 312.7-296.0 nm; typical spectra are shown in figs. 3-5.
Wavenumber
tap)I cm-l
3.1. PYRBN The long wavelength region of the LIF excitation spectrum is dominated by an intense band observed at 32027 cm-‘, see fig. 3a which has been assigned to the O-O transition, consistent with a planar equilibrium geometry in both its ground and vertically excited “B*” states. However, the appearance of a long progression of weak features to shorter wavelengths with intervals w 70 cm-’ encourages assign-
700
RGfl
900
1wO
Wavenumber (J?) /cm”
Fig. 4. LIF excitation u,_,=32800 cm-‘.
spectrum
of jet-cooled
PYRBEE
275
Volume145, number4
8 April 1988
CHEMICALPHYSICSLETTERS
(a) x ‘-’ z 9 9
v = 33095 cm-’
Continuum
Jet off
I
bl#LA
II* 0
100
I
x
I
10
(b)
I 200
300
400
500
600
Wavenumber (6V) /cm-’
Fig. 5. LIF excitation spectrum of jet-cooled DMABME, (a) total fluorescence, (b) fluorescence filtered through UV absorbingglass (A,m.,,b395 nm).
[ 22 1, benzonitrile [ 23 ] and aminobenzonitrile (ABN) [ 241 suggests that the first prominent vi-
496 and 528 cm-’ can be assigned (492 cm-’ in aniline) corresponding to the benzene 6a mode and a >C-CN bend (527 cm-’ in ABN and 5 17 cm-’
bronic features at
respectively to a ring deformation mode
in benzonitrile).
Vibrational
progressions were re-
corded to energies x 1600
cm- ’beyond the band origin and no broadening or spectral congestion was observed (fig. 3b). 3.2. PYRBEE
The LIF excitation spectrum of PYRBEE shown in fig. 4, displays striking differences to that of PYRBN. The band origin, assigned to the feature at 32800 cm-‘, is split into a doublet and it is shifted by 773 cm-’ from the band origin in PYRBN. The doublet structure may be due to level splittings associated with a double minimum torsional potential or perhaps the freezing of alternative conformers. Torsional motions are probably associated with the low frequency progressions based on the doublet band origin. At higher energies the LIF excitation spectrum becomes increasingly broadened and congested (see fig. 4b), particularly when compared to PYRBN.
276
The insertion of a long wavelength band pass filter (G 395 nm) in front of the detection photomultiplier revealed the presence of a weak red-shifted emission associated with an underlying quasi-continuum in the excitation spectrum, beginning at energies > 700 cm-’ above the band origin. 3.3. DMABiUE and DMABEE The fluorescence excitation spectra for DMABME presented in fig. 5 and DMABEE have several similarities. Both spectra show sets of low frequency vibrations in the 33090 to 33250 cm-’ region but the O-O transitions are not evident. Well resolved vibrational progressions were generally short (covering < 130 cm- ’) with only weak partially resolved bands at higher energies and were superimposed on an underlying continuum which extends over most of the spectral range. In the low frequency region, the LIF excitation spectra of both compounds display patterns of vibronic spacings and intensities that closely resemble those of the nitrile, DMABN. However, no evidence could be found of any quasi-continuum absorption in DMABN, or, in agreement with other authors [ 121, any evidence for dual luminescence in DMABN when excited as an isolated molecule under
Volume 145. number 4
CHEMICAL PHYSICS LETTERS
“jet-cooled” conditions. In contrast, measurement of the intensity of fluorescence from the two esters DMABME and DMABEE, transmitted through a long wavelength band pass filter (13 395 nm) (fig. 5b) relative to that recorded in the absence of a filter (fig. 5a), showed its quasi-continuum to be associated with a strongly red-shifted emission. The fluorescence associated with the structured features was completely filtered out. 3.4. PIPBN This molecule does not appear to contain any resolvable vibronic features in its LIF excitation spectrum, which remains weak and unstructured over the entire sampled wavelength region, 310 to 290 nm.
4. Discussion 4.1. The intramolecular
twist angle (q$)
Relaxation into the TICT state in solutions becomes increasingly favoured as the ground state equilibrium twist angle (&) increases [4,5,8] and rough estimates of @=can be made if it is assumed that the extinction coefficient (E) of the IL,-type band follows the cosine square law [ 25,261. For the nearcoplanar molecule PYRBN, ( &) exceeds 3 X 1O4dm3 mol-’ cm-‘, whereas for PIPBN, the low extinction coefficient
suggests a twisted equilibrium conformation [ 61. Recent estimates based upon analysis of ionisation energy data from photoelectron spectroscopy [ 51 show that the mean angle of twist of the dialkylamino groups with respect to the aromatic plane increases in the order PYRBN ( 9, = 0 ) ;5 DMABN < PIPBN. The intense O-O transitions experimentally observed in the jet-cooled LIF excitation spectra of PYRBN and PYRBEE show that in these molecules planarity is still preserved in the vertically excited state. In contrast, the failure to observe any resolved vibronic structure in the LIF excitation spectrum of PIPBN is consistent with the already large equilibrium ground state twist angle estimated for this compound ( qSc z 30-40” ) [ 5,8 1. Vertical excitation must
8 April 1988
transfer the molecule close to the critical twist angle & at which the ‘I_,,(vertical S, state) and ‘L, (vertical SZstate) potential energy surfaces intersect (see fig. 1). Indeed, in view of the absence of any resolved structure in PIPBN, the vertical excitation may even lead directly into the TICT state, i.e. & < &. The low frequency vibronic structures observed in the jet-cooled LIF excitation spectra of DMABN, DMABME and DMABEE, together with the Franck-Condon forbidden appearance of the bands and the absence of unique O-O transitions indicate increasing changes in the torsion angle following photon absorption. 4.2. Substituent effects There is a striking contrast between the jet-cooled LIF excitation spectra of the dialkylaminobenzoic esters and the nitrile derivatives, DMABN and PYRBN. The esters (DMABME and DMABEE) produced a red-shifted fluorescence emission with a quasi-continuous excitation spectrum which underlies well resolved vibronic bands. The red-shifted fluorescence and its associated continuum are both absent in the nitrile counterparts (PYRBN and DMABN). The difference is unlikely to be due to the presence of uncooled molecules in the jet since the excitation spectra recorded at the stagnation pressures ranging from 1 to 3 atm were virtually identical in every case. The ratio between “hot” and “cold” molecules in the jet should be constant; thus the ratio between the intensities of the continuum and the vibronic bands would be expected to depend only on the type of molecule. This latter ratio is estimated to be 2 10% for DMABME but < 1% for DMABN and PYRBN. It can be concluded that the appearance/ non-appearance of the continuum reflects differences between the topology of the excited state hypersurface for the esters and the nitriles. There is considerable evidence from solution studies (fluorescence kinetics and polarisation) showing that the potential energy profiles along the twist coordinate are indeed very different for the nitrile and the ester derivatives [ 7,27,28] (see fig. 1). The pathway to the TICT state in the nitrile compounds involves an avoided crossing: access to the TICT forming state ‘L,(S,) is gained by crossing from the initially populated ‘L,,( S, ) state at a conical intersection situated 277
Volume 145, number 4
CHEMICAL PHYSICS LETTERS
at the critical twist angle. This requires coupling with a non-totally symmetric vibration in the isolated molecule or solvent relaxation in solution. For the ester derivatives, the ordering of the excited states is reversed ( ‘L, below ‘Lb), and direct access to the TICT state is possible following vertical excitation into the ‘L,(S) state (fig. lb). Unlike the nitriles no vibronic coupling is necessary, although an energy barrier may still be present. 4.3. The potential energy surfaces Observation of the onset of a quasi-continuum in the LIF excitation spectra of the ester derivatives, PYRBEE, DMABME and DMABEE can be rationalised if the TICT state has an energy minimum substantially below the minimum of the initially excited ‘L,( S, ) state. This would explain the appearance of low frequency vibronic structure near the band origin of the jet-cooled LIF excitation spectra, the broadening of the vibronic band structure at higher energies and the onset of an underlying quasicontinuum, In DMABN, the barrier to TICT state formation lies at much higher energy; indeed formation of the TICT state in hexane is calculated to be endothermic [ 17 1. It lies higher in energy since the CN group is a less strong acceptor than the ester group. Additionally, in the nitriles, transfer into the TICT state requires passage through the conical intersection (see fig. la). In solution, this results in low fluorescence quantum yields for emission from the TICT state as well as low TICT state formation rates [ 5,271. Finally, a quasi-continuum component in the LIF excitation spectra of the nitriles is not expected to be observed under jet-cooled conditions if #=-A is large. In PIPBN, the equilibrium twist angle 9, in the ground electronic state could be great enough for & - & = 0; in this situation vertical photon absorption will carry the molecule directly into the quasicontinuum region close to the conical intersection. Strikingly the jet-cooled LIF excitation spectrum displayed no resolved structure at all, paralleling the behaviour recently observed in the hindered nitrile, 4-dimethylamino-3,5dimethylbenzonitrile (TMABN) [ 291. In solution, however, PIPBN does show both normal (B*) and TICT (A*) fluores278
8 April 1988
cence bands [ 4,7 1, though the rate of TICT state formation (monitored by the decay of the B* fluorescence) ,‘is accelerated in comparison with the other nitriles, especially PYRBN. The appearance of the normal (B*) fluorescence in solution and the absence of structured emission in the jet suggests that the solvent environment profoundly changes the hypersurface topology, by altering the barrier height and/or its position, so moving & to larger twist angles. There is evidence from picosecond kinetic studies, that the barrier height is strongly solvent polarity dependent [ 301.
5, Conclusion
The preliminary results presented here for jetcooled isolated molecules in the gas phase provide excellent support for the dynamics of TICT formation based upon experiments conducted on the same molecules in solution [ 4,5,17,18,28 1. Formation of the TICT state is favoured when a strong acceptor (e.g. an ester group) replaces a less strong one (e.g. the nitrile group). Emission from the TICT state is readily observed when the molecular equilibrium ground-state geometry already corresponds to a partially twisted configuration about the central >C-N axis. For the strongly twisted nitrile derivative PIPBN, it is only possible to detect the red-shifted TICT fluorescence and its associated quasi-continuous excitation spectrum. The main distinction between the photophysical behaviour followed in solution and under cold, isolated-molecule conditions is the unambiguous identification of direct excitation into the TICT state as well as a truly intramolecular pathway from the locally excited “B*” state, fig. lb. The contributions from both unstructured and structured components in the LIF excitation spectra of the same compound (e.g. DMABME or PYRBEE) demonstrate that local excitation (to the B* state) can be spectroscopically separated from non-local excitation (into the TICT state). This is not the case in solution, where excitation spectra for the B* and TICT (A*) bands coincide in solvents of medium polarity [ 3 11. A Iinal point concerns the continuous nature of the direct TICT fluorescence excitation spectra recorded in isolated molecules. The TICT state lies some 5000
Volume 145, number 4
CHEMICAL PHYSICS LETTERS
cm- ’or more below the locally excited state ( 1) and, as a referee has pointed out, at excess vibrational energies of > 5000 cm-’ the vibrational manifold is expected to be effectively a continuum.
Acknowledgement CJ, JA and WR are grateful to CNRS, SERC and the Deutsche Forschungsgemeinschaft respectively for financial support.
References [ 1 ] W. Rettig, Angew. Chem. 98 (1986) 969; Angew. Chem. Intern. Ed. Engl. 25 (1986) 971. [2] Z.R. Grabowski, K. Rotkiewicz, A. Siemiarczuk, D.J. Cowley and W. Baumann, Nouv. J. Chim. 3 ( 1979) 443. [ 31 Z.R. Grabowski and J. Dobkowski, Pure Appl. Chem. 55 (1983) 245. [4] W. Rettig, J. Phys. Chem. 86 (1982) 170. [ 51 W. Rettig and R. Gleiter, J. Phys. Chem. 89 ( 1985) 4676. [6] G. Wermuth, W. Rettig and E. Lippett, Ber. Bunsenges. Physik. Chem. 85 (1981) 64. [ 71 W. Rettig and G. Wermuth, J. Photochem. 28 ( 1985) 35 1. [8] W. Rettig, J. Luminescence 26 (1981) 21. [9]P.C.M. Weisenbom, C.A.G.O. Varma, M.P. de Haas and J.M. Warman, Chem. Phys. Letters 129 (1986) 562. [ lo] K. Rotkiewicz and W. Rubaszewska, J. Luminescence 27 (1982) 221. [ 111T. Kobayashi, M. Futakami and 0. Kajimoto, Chem. Phys. Letters 130 (1986) 63. [ 121E.M. Gibson, A.C. Jones and D. Phillips, Chem. Phys. Letters 135 (1987) 454.
8 April 1988
[ 131 P. Rettschnik, private communication.
[ 141 K. Yamasaki, K. Arita, 0. Kajimoto and K. Hara, Chem. Phys. Letters 123 (1986) 277.
[ 15 ] 0. Kajimoto, K. Yamasaki, K. Arita and K. Hara, Chem. Phys. Letters 125 ( 1986) 184.
[ 161 G. Wermuth and W. Rettig, J. Phys. Chem. 88 ( 1984) 2729. [ 171 G. Wermuth, Z. Naturlorsch. A 38 (1983) 641. [ 181 W. Rettig and V. BonaEiC-Koutecky, Chem. Phys. Letters 62 (1979) 115.
[ 191 A.T. Amos, S.M. Cohen, J.C. Kettley, T.F. Palmerand J.P. Simons, in: Structure and dynamics of weakly bound molecular complexes, NATO ARW Maratea, 1986,ed. A. Weber (Reidel, Dordrecht, 1987). [20] J.C. Kettley, J.W. Oram, T.F. Palmer, J.P. Simons and A.T. Amos, Chem. Phys. Letters 140 (1987) 286. [ 211 H. Suhr, Liebigs Ann. Chem. 687 ( 1965) 182. [ 221 N. Mikami, A. Hiraya, I. Fujiwara and M. Ito, Chem. Phys. Letters 73 (1980) 531. [ 231 T. Kobayashi and 0. Kajimoto, J. Chem. Phys. 86 ( 1987) 1118. [ 241 J. August, T.F. Palmer, J.P. Simons, C. Jouvet and W. Rettig, to be published. [25] B.M. Wepster, Rec. Trav. Chim. 76 (1957) 335,357. [26] E.A. Braude and J. Sondheimer, J. Chem. Sot. (1955) 3754. [27] W. Rettig, G. Wermuth and E. Lippert, Ber. Bunsenges. Physik. Chem. 83 (1979) 692. [28] W. Rettig, M. Vogel, E. Lippert and H. Otto, Chem. Phys. 103 (1986) 381. [ 291 T. Kobayashi, M. Futakami and 0. Kajimoto, Chem. Phys. Letters 141 (1987) 450. [ 301 J. Hicks, M. Vandersall, Z. Babarogic and K.B. Eisenthal, Chem. Phys. Letters 116 ( 1985 ) 18. [ 3 1 ] E. Lippert, W. Luder and H. Boos, Advances in Molecular Spectroscopy. Proceedings of the 4th International Meeting, 1959 ( 1962) p. 443.
279
CHEMICAL PHYSICS LETTERS
8 April 1988
INTRAMOLECULAR CHARGE TRANSFER IN JET-COOLED AROMATIC AMINES Julie AUGUST, T. Frank PALMER, John P. SIMONS Department of Chemistry, University ofNottingham. Nottingham NG7 2RD. UK
Christophe JOUVET Laboratoire de Photophysique Mol.&laire du CNRS, Universit.4Paris&d,
Orsay Cedex, France
and Wolfgang RETTIG Iwan N. Stranski Institute for Physicaland Theoretical Chemistry, Technical University ofBerlin, D-l 000 Berlin 12, Germany
Received 16 November 1987; in final form 1February 1988
LIF excitation spectra have been recorded for a family of molecules whose dual fluorescence in solution has been ascribed to twisted intramolecular charge transfer (TICT) state formation, promoted through torsional motion in the initially photoexcited state. To eliminate the influence of solvation, the LIF spectra have been recorded under jet-cooled isolated-molecule conditions: the molecules include para-substituted nitrile or ester derivatives of dialkylamino benzene compounds. Evidence both for torsional motion and TICT state formation has been gained for the ester derivatives where access into the TICT state correlates with the onset of a quasi-continuous component in the LIF system and the appearance of a red-shifted fluorescence emission.
1. Introduction The dual luminescence exhibited by simple aromatic molecules in solution such as 4-N,N-dimethylaminobenzonitrile (DMABN) and related compounds in which a large degree of charge separation is associated with a twisted (or small overlap) arrangement of the two chromophores, has been the subject of increasing activity in recent years [ 11. The “normal” B band fluorescence emission can be assigned to an S,-So transition from the “locally excited” B* state, but the parallel appearance of a redshifted “A band” fluorescence has led to the notion of a second, twisted intramolecular charge transfer (TICT ) state [ l-3 1. TICT state formation is particularly favoured in molecules which have nonplanar ground-state conformations [ 4,5 1. Most investigations have been carried out in the solution phase where specific solute-solvent interactions and the microscopic hindrance of intramolecular mobility due to the solvent cage lead to severe
complications. The solvent environment can also have a profound effect on the ordering of excited-state energy levels, and whilst TICT state emission occurs in strongly polar solvents for the dialkylamino benzonitriles, the corresponding alkoxycarbonyl derivatives show anomalous “A” band fluorescence in both polar and non-polar solvents [ 6-8 1. TICT state emission in the gas phase has been observed in derivatives of DMABN [ 9, lo] which are twisted about the central aromatic carbon-nitrogen bond, and in pcyanophenylpyrrole [ 1 ] but the thermal congestion of energy levels at the ambient or elevated temperatures lead to broad featureless emission spectra which greatly restrict spectroscopic interpretation. The cooling and consequent spectral simplification introduced by free-jet expansion offers an alternative strategy for assessing the geometrical changes which may follow electronic excitation; recently attempts have been made to gain evidence for TICT state formation, through laser-induced fluorescence (LIF) spectroscopy of solvent van
0 009-2614/88/I 03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division )
273
Volume 145, number 4
CHEMICAL PHYSICS LETTERS
der Waals complexes of DMABN [ 1l- 13 ] and 9,9’bianthryl [ 14,15 1. The present investigation focuses on the LIF excitation spectroscopy of a range of isolated jet-coold dialkylaminobenzoic acid nitriles and esters where strong perturbations introduced by the energetically close lying TICT state could be expected. The esters are especially favourable candidates for population of the TICT state even under “jet” conditions, since the ‘Lb- and ‘La-type states are respectively raised and lowered energetically with respect to the nitriles [ 7,16,17 ] and it is the ‘L, state which correlates directly with the twisted TICT state [18] (see fig. 1).
8 April 1988
aci,
1
2. Experimental Amino- and dialkylamino-benzoic acid nitriles and esters, were vaporised in a small oven at temperatures between 110 and lSO”C, mixed with helium carrier gas at stagnation pressures of 2 to 3 atm and expanded into a vacuum chamber through an 800 pm diameter orifice pulsed nozzle valve (General Valve Corporation) [ 19,201. Replacement of the seals of the valve by a polyfluorocarbon “0” ring (“Kalrez”, DuPont Ltd.) and polyamide fibre plunger (“Vespel SPl” Du Pont Ltd.) and the solenoid windings by copper wire (0.14 mm diameter coated with a temperature-resistant lacquer (“Lewcos” F.D. Sims Ltd.), allowed prolonged operation at elevated temperatures. Chromel-alumel thermocouples were located close to the nozzle orifice and oven to provide continuous monitoring of the temperatures. Independent heating of both the pulsed nozzle valve and the oven allowed the temperature of the orifice to be maintained a few degrees above the temperature of the evaporating samples. The vacuum chamber was evacuated with a vapour booster pump (Edwards 9B3) backed by a rotary pump (Edwards’ED 500) which maintained a background pressure z 0.1 Pa. A delayed pulse generator was used to trigger both the pulsed valve and the N,-laser-pumped dye laser (Molectron), allowing synchronisation of the pulsed supersonic jet and the intersecting laser beam a few mm downstream from the nozzle. The pulsed valve had a characteristic open-shut time of x 1 ms and was operated at 20 Hz. LIF spectra were generated by simultaneously 214
(‘8
m Sl~}Density
Local excitation e%Lm
ofstates
TKT
Twist angle (0)
Fig. 1. (a) Potential energy surfaces representing sections along the C-N torsional coordinate in (i ) alkylamino benzonitrile and (ii) alkylamino bezoic ester derivatives in solution at 300K.(b) Schematic representation of vertical transitions leading to “normal” (B band) and TICT (A band) fluorescence in isolated jet-
cooled alkylaminobenzoic acid esters.
recording the incident laser intensity and that of the total fluorescence emitted along a perpendicular axis using a photomultiplier/boxcar (Brookdeal 464, 465) combination. The structures of the amino- and dialkylamino-
Volume 145, number 4
CN ABN
COOMe DMABME
CN DMABN
CooEt DMABEE
CHEMICAL PHYSICS LETTERS
CN PYRBN
CODE1 PYRBEE
8 April 1988
CN 0
PIPBN
200
400
Wavenumber
(a?) /cm-’
Wavenumber
cav)I cm.’
600
COOEt 2.5.PYRBEE
Fig. 2. Aminobenzonitrile, dialkylamino- and cyclic amino-benzoic acid nitriles and esters.
Fig. 3. LIF excitation spectrum ofjet-cooted PYRBN vU= 32030 cm-‘.
benzoic acid nitriles and esters investigated are shown in fig. 2. The cyclic aminobenzonitriles and esters, PYRBN, PIPBN and PYRBEE were synthesized via nucleophilic substitution of the 4-fluorobenzoic acid nitrile or ester, according to the general procedure given by Suhr [ 2 1] and purified by recrystallization and sublimation. The details of preparation and identification, as well as the photophysical properties in solution, have been described extensively elsewhere [7,8].
ment to a torsional mode and suggests that the potentials along the twist coordinate are somewhat different in the ground and excited states. Comparison with the jet-cooled spectra of aniline 14
t
I
3. Results LIF excitation spectra of PYRBN, PYRBEE, PIPBN, DMABEE and DMABME were recorded in the wavelength range 312.7-296.0 nm; typical spectra are shown in figs. 3-5.
Wavenumber
tap)I cm-l
3.1. PYRBN The long wavelength region of the LIF excitation spectrum is dominated by an intense band observed at 32027 cm-‘, see fig. 3a which has been assigned to the O-O transition, consistent with a planar equilibrium geometry in both its ground and vertically excited “B*” states. However, the appearance of a long progression of weak features to shorter wavelengths with intervals w 70 cm-’ encourages assign-
700
RGfl
900
1wO
Wavenumber (J?) /cm”
Fig. 4. LIF excitation u,_,=32800 cm-‘.
spectrum
of jet-cooled
PYRBEE
275
Volume145, number4
8 April 1988
CHEMICALPHYSICSLETTERS
(a) x ‘-’ z 9 9
v = 33095 cm-’
Continuum
Jet off
I
bl#LA
II* 0
100
I
x
I
10
(b)
I 200
300
400
500
600
Wavenumber (6V) /cm-’
Fig. 5. LIF excitation spectrum of jet-cooled DMABME, (a) total fluorescence, (b) fluorescence filtered through UV absorbingglass (A,m.,,b395 nm).
[ 22 1, benzonitrile [ 23 ] and aminobenzonitrile (ABN) [ 241 suggests that the first prominent vi-
496 and 528 cm-’ can be assigned (492 cm-’ in aniline) corresponding to the benzene 6a mode and a >C-CN bend (527 cm-’ in ABN and 5 17 cm-’
bronic features at
respectively to a ring deformation mode
in benzonitrile).
Vibrational
progressions were re-
corded to energies x 1600
cm- ’beyond the band origin and no broadening or spectral congestion was observed (fig. 3b). 3.2. PYRBEE
The LIF excitation spectrum of PYRBEE shown in fig. 4, displays striking differences to that of PYRBN. The band origin, assigned to the feature at 32800 cm-‘, is split into a doublet and it is shifted by 773 cm-’ from the band origin in PYRBN. The doublet structure may be due to level splittings associated with a double minimum torsional potential or perhaps the freezing of alternative conformers. Torsional motions are probably associated with the low frequency progressions based on the doublet band origin. At higher energies the LIF excitation spectrum becomes increasingly broadened and congested (see fig. 4b), particularly when compared to PYRBN.
276
The insertion of a long wavelength band pass filter (G 395 nm) in front of the detection photomultiplier revealed the presence of a weak red-shifted emission associated with an underlying quasi-continuum in the excitation spectrum, beginning at energies > 700 cm-’ above the band origin. 3.3. DMABiUE and DMABEE The fluorescence excitation spectra for DMABME presented in fig. 5 and DMABEE have several similarities. Both spectra show sets of low frequency vibrations in the 33090 to 33250 cm-’ region but the O-O transitions are not evident. Well resolved vibrational progressions were generally short (covering < 130 cm- ’) with only weak partially resolved bands at higher energies and were superimposed on an underlying continuum which extends over most of the spectral range. In the low frequency region, the LIF excitation spectra of both compounds display patterns of vibronic spacings and intensities that closely resemble those of the nitrile, DMABN. However, no evidence could be found of any quasi-continuum absorption in DMABN, or, in agreement with other authors [ 121, any evidence for dual luminescence in DMABN when excited as an isolated molecule under
Volume 145. number 4
CHEMICAL PHYSICS LETTERS
“jet-cooled” conditions. In contrast, measurement of the intensity of fluorescence from the two esters DMABME and DMABEE, transmitted through a long wavelength band pass filter (13 395 nm) (fig. 5b) relative to that recorded in the absence of a filter (fig. 5a), showed its quasi-continuum to be associated with a strongly red-shifted emission. The fluorescence associated with the structured features was completely filtered out. 3.4. PIPBN This molecule does not appear to contain any resolvable vibronic features in its LIF excitation spectrum, which remains weak and unstructured over the entire sampled wavelength region, 310 to 290 nm.
4. Discussion 4.1. The intramolecular
twist angle (q$)
Relaxation into the TICT state in solutions becomes increasingly favoured as the ground state equilibrium twist angle (&) increases [4,5,8] and rough estimates of @=can be made if it is assumed that the extinction coefficient (E) of the IL,-type band follows the cosine square law [ 25,261. For the nearcoplanar molecule PYRBN, ( &) exceeds 3 X 1O4dm3 mol-’ cm-‘, whereas for PIPBN, the low extinction coefficient
suggests a twisted equilibrium conformation [ 61. Recent estimates based upon analysis of ionisation energy data from photoelectron spectroscopy [ 51 show that the mean angle of twist of the dialkylamino groups with respect to the aromatic plane increases in the order PYRBN ( 9, = 0 ) ;5 DMABN < PIPBN. The intense O-O transitions experimentally observed in the jet-cooled LIF excitation spectra of PYRBN and PYRBEE show that in these molecules planarity is still preserved in the vertically excited state. In contrast, the failure to observe any resolved vibronic structure in the LIF excitation spectrum of PIPBN is consistent with the already large equilibrium ground state twist angle estimated for this compound ( qSc z 30-40” ) [ 5,8 1. Vertical excitation must
8 April 1988
transfer the molecule close to the critical twist angle & at which the ‘I_,,(vertical S, state) and ‘L, (vertical SZstate) potential energy surfaces intersect (see fig. 1). Indeed, in view of the absence of any resolved structure in PIPBN, the vertical excitation may even lead directly into the TICT state, i.e. & < &. The low frequency vibronic structures observed in the jet-cooled LIF excitation spectra of DMABN, DMABME and DMABEE, together with the Franck-Condon forbidden appearance of the bands and the absence of unique O-O transitions indicate increasing changes in the torsion angle following photon absorption. 4.2. Substituent effects There is a striking contrast between the jet-cooled LIF excitation spectra of the dialkylaminobenzoic esters and the nitrile derivatives, DMABN and PYRBN. The esters (DMABME and DMABEE) produced a red-shifted fluorescence emission with a quasi-continuous excitation spectrum which underlies well resolved vibronic bands. The red-shifted fluorescence and its associated continuum are both absent in the nitrile counterparts (PYRBN and DMABN). The difference is unlikely to be due to the presence of uncooled molecules in the jet since the excitation spectra recorded at the stagnation pressures ranging from 1 to 3 atm were virtually identical in every case. The ratio between “hot” and “cold” molecules in the jet should be constant; thus the ratio between the intensities of the continuum and the vibronic bands would be expected to depend only on the type of molecule. This latter ratio is estimated to be 2 10% for DMABME but < 1% for DMABN and PYRBN. It can be concluded that the appearance/ non-appearance of the continuum reflects differences between the topology of the excited state hypersurface for the esters and the nitriles. There is considerable evidence from solution studies (fluorescence kinetics and polarisation) showing that the potential energy profiles along the twist coordinate are indeed very different for the nitrile and the ester derivatives [ 7,27,28] (see fig. 1). The pathway to the TICT state in the nitrile compounds involves an avoided crossing: access to the TICT forming state ‘L,(S,) is gained by crossing from the initially populated ‘L,,( S, ) state at a conical intersection situated 277
Volume 145, number 4
CHEMICAL PHYSICS LETTERS
at the critical twist angle. This requires coupling with a non-totally symmetric vibration in the isolated molecule or solvent relaxation in solution. For the ester derivatives, the ordering of the excited states is reversed ( ‘L, below ‘Lb), and direct access to the TICT state is possible following vertical excitation into the ‘L,(S) state (fig. lb). Unlike the nitriles no vibronic coupling is necessary, although an energy barrier may still be present. 4.3. The potential energy surfaces Observation of the onset of a quasi-continuum in the LIF excitation spectra of the ester derivatives, PYRBEE, DMABME and DMABEE can be rationalised if the TICT state has an energy minimum substantially below the minimum of the initially excited ‘L,( S, ) state. This would explain the appearance of low frequency vibronic structure near the band origin of the jet-cooled LIF excitation spectra, the broadening of the vibronic band structure at higher energies and the onset of an underlying quasicontinuum, In DMABN, the barrier to TICT state formation lies at much higher energy; indeed formation of the TICT state in hexane is calculated to be endothermic [ 17 1. It lies higher in energy since the CN group is a less strong acceptor than the ester group. Additionally, in the nitriles, transfer into the TICT state requires passage through the conical intersection (see fig. la). In solution, this results in low fluorescence quantum yields for emission from the TICT state as well as low TICT state formation rates [ 5,271. Finally, a quasi-continuum component in the LIF excitation spectra of the nitriles is not expected to be observed under jet-cooled conditions if #=-A is large. In PIPBN, the equilibrium twist angle 9, in the ground electronic state could be great enough for & - & = 0; in this situation vertical photon absorption will carry the molecule directly into the quasicontinuum region close to the conical intersection. Strikingly the jet-cooled LIF excitation spectrum displayed no resolved structure at all, paralleling the behaviour recently observed in the hindered nitrile, 4-dimethylamino-3,5dimethylbenzonitrile (TMABN) [ 291. In solution, however, PIPBN does show both normal (B*) and TICT (A*) fluores278
8 April 1988
cence bands [ 4,7 1, though the rate of TICT state formation (monitored by the decay of the B* fluorescence) ,‘is accelerated in comparison with the other nitriles, especially PYRBN. The appearance of the normal (B*) fluorescence in solution and the absence of structured emission in the jet suggests that the solvent environment profoundly changes the hypersurface topology, by altering the barrier height and/or its position, so moving & to larger twist angles. There is evidence from picosecond kinetic studies, that the barrier height is strongly solvent polarity dependent [ 301.
5, Conclusion
The preliminary results presented here for jetcooled isolated molecules in the gas phase provide excellent support for the dynamics of TICT formation based upon experiments conducted on the same molecules in solution [ 4,5,17,18,28 1. Formation of the TICT state is favoured when a strong acceptor (e.g. an ester group) replaces a less strong one (e.g. the nitrile group). Emission from the TICT state is readily observed when the molecular equilibrium ground-state geometry already corresponds to a partially twisted configuration about the central >C-N axis. For the strongly twisted nitrile derivative PIPBN, it is only possible to detect the red-shifted TICT fluorescence and its associated quasi-continuous excitation spectrum. The main distinction between the photophysical behaviour followed in solution and under cold, isolated-molecule conditions is the unambiguous identification of direct excitation into the TICT state as well as a truly intramolecular pathway from the locally excited “B*” state, fig. lb. The contributions from both unstructured and structured components in the LIF excitation spectra of the same compound (e.g. DMABME or PYRBEE) demonstrate that local excitation (to the B* state) can be spectroscopically separated from non-local excitation (into the TICT state). This is not the case in solution, where excitation spectra for the B* and TICT (A*) bands coincide in solvents of medium polarity [ 3 11. A Iinal point concerns the continuous nature of the direct TICT fluorescence excitation spectra recorded in isolated molecules. The TICT state lies some 5000
Volume 145, number 4
CHEMICAL PHYSICS LETTERS
cm- ’or more below the locally excited state ( 1) and, as a referee has pointed out, at excess vibrational energies of > 5000 cm-’ the vibrational manifold is expected to be effectively a continuum.
Acknowledgement CJ, JA and WR are grateful to CNRS, SERC and the Deutsche Forschungsgemeinschaft respectively for financial support.
References [ 1 ] W. Rettig, Angew. Chem. 98 (1986) 969; Angew. Chem. Intern. Ed. Engl. 25 (1986) 971. [2] Z.R. Grabowski, K. Rotkiewicz, A. Siemiarczuk, D.J. Cowley and W. Baumann, Nouv. J. Chim. 3 ( 1979) 443. [ 31 Z.R. Grabowski and J. Dobkowski, Pure Appl. Chem. 55 (1983) 245. [4] W. Rettig, J. Phys. Chem. 86 (1982) 170. [ 51 W. Rettig and R. Gleiter, J. Phys. Chem. 89 ( 1985) 4676. [6] G. Wermuth, W. Rettig and E. Lippett, Ber. Bunsenges. Physik. Chem. 85 (1981) 64. [ 71 W. Rettig and G. Wermuth, J. Photochem. 28 ( 1985) 35 1. [8] W. Rettig, J. Luminescence 26 (1981) 21. [9]P.C.M. Weisenbom, C.A.G.O. Varma, M.P. de Haas and J.M. Warman, Chem. Phys. Letters 129 (1986) 562. [ lo] K. Rotkiewicz and W. Rubaszewska, J. Luminescence 27 (1982) 221. [ 111T. Kobayashi, M. Futakami and 0. Kajimoto, Chem. Phys. Letters 130 (1986) 63. [ 121E.M. Gibson, A.C. Jones and D. Phillips, Chem. Phys. Letters 135 (1987) 454.
8 April 1988
[ 131 P. Rettschnik, private communication.
[ 141 K. Yamasaki, K. Arita, 0. Kajimoto and K. Hara, Chem. Phys. Letters 123 (1986) 277.
[ 15 ] 0. Kajimoto, K. Yamasaki, K. Arita and K. Hara, Chem. Phys. Letters 125 ( 1986) 184.
[ 161 G. Wermuth and W. Rettig, J. Phys. Chem. 88 ( 1984) 2729. [ 171 G. Wermuth, Z. Naturlorsch. A 38 (1983) 641. [ 181 W. Rettig and V. BonaEiC-Koutecky, Chem. Phys. Letters 62 (1979) 115.
[ 191 A.T. Amos, S.M. Cohen, J.C. Kettley, T.F. Palmerand J.P. Simons, in: Structure and dynamics of weakly bound molecular complexes, NATO ARW Maratea, 1986,ed. A. Weber (Reidel, Dordrecht, 1987). [20] J.C. Kettley, J.W. Oram, T.F. Palmer, J.P. Simons and A.T. Amos, Chem. Phys. Letters 140 (1987) 286. [ 211 H. Suhr, Liebigs Ann. Chem. 687 ( 1965) 182. [ 221 N. Mikami, A. Hiraya, I. Fujiwara and M. Ito, Chem. Phys. Letters 73 (1980) 531. [ 231 T. Kobayashi and 0. Kajimoto, J. Chem. Phys. 86 ( 1987) 1118. [ 241 J. August, T.F. Palmer, J.P. Simons, C. Jouvet and W. Rettig, to be published. [25] B.M. Wepster, Rec. Trav. Chim. 76 (1957) 335,357. [26] E.A. Braude and J. Sondheimer, J. Chem. Sot. (1955) 3754. [27] W. Rettig, G. Wermuth and E. Lippert, Ber. Bunsenges. Physik. Chem. 83 (1979) 692. [28] W. Rettig, M. Vogel, E. Lippert and H. Otto, Chem. Phys. 103 (1986) 381. [ 291 T. Kobayashi, M. Futakami and 0. Kajimoto, Chem. Phys. Letters 141 (1987) 450. [ 301 J. Hicks, M. Vandersall, Z. Babarogic and K.B. Eisenthal, Chem. Phys. Letters 116 ( 1985 ) 18. [ 3 1 ] E. Lippert, W. Luder and H. Boos, Advances in Molecular Spectroscopy. Proceedings of the 4th International Meeting, 1959 ( 1962) p. 443.
279