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The emitting state of the imino nitroxide radical
Chemical Physics Letters 405 (2005) 153–158 www.elsevier.com/locate/cplett
The emitting state of the imino nitroxide radical Re´mi Beaulac a, Dominique Luneau b, Christian Reber
a,*
a
b
De´partement de Chimie, Universite´ de Montre´al, C.P. 6128, Succ. Centre-ville, Montre´al, Que´., Canada, H3C3J7 Cristallographie et Inge´nierie Mole´culaire, Laboratoire des Multimate´riaux et Interfaces (UMR 5615), Universite´ Claude-Bernard Lyon 1, 43 Avenue du 11 Novembre 1918, 69622 Villeurbanne Cedex, France Received 15 October 2004; in final form 3 February 2005
Abstract The solid-state low-temperature luminescence spectrum of an uncoordinated imino nitroxide radical, 2-(2-imidazolyl)-4,4,5,5tetramethylimidazoline-1-oxyl, is reported. The luminescence maximum is at 16 300 cm1 and the band has a width at half height of 3140 cm1 with a prominent vibronic progression interval of 1150 cm1. Corresponding spectroscopic results are shown for the nitronyl nitroxide analog, where a lower-energy luminescence maximum at 14 700 cm1 and a narrower band with a width at half height of 1900 cm1 are observed. These differences are analyzed in terms of the different delocalized p-systems of the radicals. Ó 2005 Elsevier B.V. All rights reserved.
1. Introduction Nitroxide radicals are important building blocks for materials with novel optical and magnetic properties [1–4]. These stable and chemically versatile molecules have been used to design molecule-based magnets, either as organic materials [5–7] or as ligands combined with metal ions [8,9]. Their electronic ground state has been characterized extensively by many different physical methods,[1] but their excited electronic states have received much less attention [2,3,10]. The lowest-energy intense absorption band of imino nitroxides has been reported higher in energy than the corresponding band of their nitronyl nitroxide analogs [3,9]. To the best of our knowledge, no luminescence spectrum has been reported for imino nitroxide radicals, in contrast to nitronyl nitroxides and their transition metal and lanthanide complexes, where distinct luminescence bands were observed in the near-infrared region [2,3,9,11,12]. The traditional resonance structures shown in Fig. 1 are symmetric for nitronyl nitroxide radicals, but asym*
Corresponding author. Fax: +1 514 343 7586. E-mail addresses: [email protected] (D. Luneau), [email protected] (C. Reber). 0009-2614/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.02.024
metric for imino nitroxide radicals, qualitatively illustrating their different electronic structures. Fig. 1 also shows the calculated singly occupied molecular orbital (SOMO) for both radicals. The SOMO electron density has p-antibonding character perpendicular to the plane of the molecule and is located predominantly on the NCNO fragment of the imino nitroxide radical in Fig. 1a and along the ONCNO fragment of the nitronyl nitroxide radical in Fig. 1b. In the following, we present the first luminescence spectrum for an imino nitroxide radical, determine its emitting state characteristics and compare them to the corresponding nitronyl nitroxide radical.
2. Experimental 2.1. Syntheses 2-(2-Imidazolyl)-4,4,5,5-tetramethylimidazoline-1-oxyl3-oxide (abbreviated in the following as NITImH) and 2-(2-imidazolyl)-4,4,5,5-tetramethylimidazoline-1-oxyl (abbreviated in the following as IMImH) were prepared according to literature methods and characterized as described previously [7,13–15].
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R. Beaulac et al. / Chemical Physics Letters 405 (2005) 153–158
Fig. 1. Schematic representations of resonance structures for: (a) imino nitroxide (IMImH); (b) nitronyl nitroxide (NITImH) radicals. Calculated SOMO orbitals (a spin) are shown for both radicals.
2.2. Spectroscopic instrumentation Luminescence spectra were measured using a Renishaw System 3000 micro-Raman spectrometer. The 514.5 nm line of an Ar+ ion laser was used to excite the samples. Special care was taken to reduce the laser power to avoid photodegradation of the compounds. Spectra were corrected to take into account the change in sensitivity of the detector in the near-infrared region [16]. Cryogenic measurements were made using a continuous flow cryostat designed for the Raman microscope (Janis Research Supertran ST-500). Absorption spectra were measured with a Varian Cary 5E spectrometer. Calculations were made with the GAUSSIAN 98 package [17]. Semi-empirical calculations were made at the PM3 level; the PBE1PBE functional and the 6-31+G(3d) basis set were used for density-functional calculations.
3. Spectroscopic results Luminescence spectra of the IMImH and NITImH radicals are shown in Fig. 2. The differences between the spectra of these two similar molecules are striking. The spectrum of IMImH in Fig. 2a has its onset at 18 400 cm1 and shows resolved structure with a highest-energy maximum at 17 450 cm1 and its most intense peak lower in energy by 1150 cm1 at 16 300 cm1. The band width at half height is 3140 cm1. In contrast, the
NITImH radical in Fig. 2b shows a spectrum with an onset at 15 400 cm1, lower in energy by 3000 cm1 than observed for IMImH in Fig. 2a, and its highest energy maximum, which is also the most intense peak, at 14 640 cm1. A second resolved maximum occurs at 13 525 cm1, lower in energy by 1140 cm1 and a shoulder is observed at 13 905 cm1, lower in energy by 735 cm1 than the most intense maximum. The energy difference between resolved maxima is therefore very similar for both radicals. The width at half height of the NITImH spectrum is 1900 cm1, lower by 40% than for IMImH. Luminescence spectra of other nitronyl nitroxide radicals show similar characteristics, as illustrated by the well resolved spectrum of a nitronyl nitroxide radical with a benzimidazole substituent. This spectrum shows an overall bandwidth at half height of approximately 1100 cm1, a short vibronic progression with an interval of 1450 cm1 [12] and an intensity distribution very similar to NITImH. Table 1 summarizes the spectroscopic properties determined from the spectra in Figs. 2 and 3. The solution absorption spectra in Fig. 3 show band onsets at approximately 13 000 and 16 000 cm1 for NITImH and IMImH, respectively, confirming the energetic order of the lowest-energy electronic transitions established from the luminescence spectra in Fig. 2. Their band shapes are similar as they both have lowest-energy maxima that are less intense than the second maxima. However, the widths of the absorption band systems are different, 3600 and 5300 cm1 for NITImH
R. Beaulac et al. / Chemical Physics Letters 405 (2005) 153–158
Molar Absorptivity [M-1cm-1]
Luminescence Intensity
(a)
-1
3140 cm
80
155
(a)
60
40
20
0
10000
12000
14000
16000
18000
15000
20000
25000
300 Molar Absorptivity [M-1cm-1]
Luminescence Intensity
(b)
-1
1900 cm
(b)
200
100
0
10000
12000
14000
16000
15000
18000
-1
Wavenumber [cm ] Fig. 2. Solid-state luminescence spectra of (a) IMImH and (b) NITImH measured at 5 K.
Table 1 Selected peak positions and energy intervals from absorption and luminescence spectra Quantity
IMImH
NITImH
Width at half height
17 450 1150 2300 3140
14 640 735 1140 1900
Absorption Lowest energy maximum Intervals
18 800 1600
14 535 1440 1415 1400
Luminescence Highest energy maximum Intervals
20000 Wavenumber [cm-1]
25000
Fig. 3. Solution absorption spectra of (a) IMImH and (b) NITImH at room temperature in dichloromethane.
differences in the electronic structure of the emitting state that are analyzed in the following section.
4. Discussion
All values are in cm1.
and IMImH, respectively, and the spectrum of IMImH in Fig. 3a is less resolved than the spectrum of NITImH in Fig. 3b. The average interval between resolved maxima in the absorption spectrum of NITImH is approximately 1420 cm1, significantly higher than the intervals seen in the luminescence spectrum in Fig. 2b. A corresponding spacing of 1600 cm1 appears in the absorption spectrum of IMImH in Fig. 3a. The spectroscopic results in Figs. 2 and 3 show that despite similar absorption profiles, the luminescence bands indicate distinct
4.1. Vibronic structure in the luminescence spectra and emitting-state distortions The luminescence spectra in Fig. 2 show a single dominant vibronic progression. Its average intervals are 1140 and 1150 cm1 for NITImH and IMImH, respectively. The progression in both spectra is typical for nitroxide radicals and its interval is in the frequency range expected for the N–O oscillator. Raman spectra and literature analyses lead to a more detailed identification as the ONCNO stretching mode for nitronyl nitroxide radicals and as the corresponding NCNO mode for imino nitroxide radicals [18–21]. The N–O oscillators contribute strongly to both modes and the frequency of the NCNO mode in imino nitroxide radicals has been found to be higher by up to 50 cm1 than in the corresponding nitronyl nitroxide radicals [18–21]. A quantitative determination of the change in molecular structure along the normal coordinate of this mode can be made from calculated luminescence spectra with
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the offset along the normal coordinate as adjustable parameter. All calculations are based on the timedependent theory of spectroscopy, the details of which have been discussed thoroughly in the past [22–24]. The calculated spectrum is obtained by assuming harmonic potential energy surfaces for both the ground and excited states. The general form for the surface of a harmonic excited-state potential is given by E¼
1X 2 xk ðQk Dk Þ þ E00 ; 2 k
Parameter
IMImH
NITImH
E00 (cm1) hxground state ðcm1 Þ hxemitting state (cm1) |D| C (cm1)
18 000 1125 1550 1.55 325 (luminescence) 575 (absorption)
14 660 1200 1366 1.34 190
ð1Þ
where k identifies the (dimensionless) normal coordinate Qk with vibrational frequency xk. In situations where the spectral resolution is too low to distinguish many vibronic progressions, as is the case here, the sum in Eq. (1) retains only a single term. Dk is the offset of the potential surface with respect to the ground-state potential energy minimum along normal coordinate Qk. The offset is the only adjustable parameter not directly available from experiment. E00 is the energy of the electronic origin. The ground-state potential energy surface is given by E¼
Table 2 Parameter values used to calculate the luminescence and absorption spectra in Fig. 4
1X 2 xQ: 2 k k k
ð2Þ
(b)
Absorbance
Luminescence Intensity
(a)
The ground-state vibrational frequencies xk describing this potential energy surface are not identical to those in Eq. (1). Spectra are calculated with the split operator method [25,26] and the width of each vibronic peak is reproduced by the phenomenological damping factor C. All parameter values for the calculation of the spectra are summarized in Table 2. The calculated luminescence spectrum for IMImH is compared to the experimental spectrum in Fig. 4a. The offset |D| is determined to be 1.55 and the calculated spectrum is in good agreement with the experiment. The parameters in Table 2 were used to calculate the absorption spectrum in Fig. 4b, again compared to the experimental trace. This spectrum was obtained without adjusting parameters and quantitatively confirms that
14500
17000
(c)
10000
15000
20000
25000
(d)
Absorbance
Luminescence Intensity
12000
12500
15000 -1
Wavenumber [cm ]
15000
17500
20000 -1
Wavenumber [cm ]
Fig. 4. Experimental (solid lines) and calculated (dotted lines) luminescence and absorption spectra: (a) luminescence of IMImH; (b) absorption of IMImH; (c) luminescence of NITImH; (d) absorption of NITImH.
R. Beaulac et al. / Chemical Physics Letters 405 (2005) 153–158
the absorption and luminescence spectra are mirror images, as expected for an emitting state well removed in energy from other excited states. The situation is different for NITImH. The calculated luminescence spectrum is shown in Fig. 4c compared to the experimental spectrum. Again, all parameters used for the calculation are summarized in Table 2. The offset |D| along the normal coordinate is 1.34, smaller by 15% than for IMImH. The comparison with the experimental absorption spectrum in Fig. 4d reveals an important difference between NITImH and IMImH: the absorption spectrum calculated using the parameter values obtained from the luminescence analysis does not reproduce the experimental spectrum. The most important difference occurs for the relative intensities of the two lowest-energy maxima in the calculated spectrum in Fig. 4d. In contrast to the experimental trace, where the first peak is approximately 25% less intense than the following higher energy peak, the intensities of the first two vibronic peaks are identical in the calculated spectrum. In addition, the overall width of the calculated absorption spectrum is too narrow, a spectroscopic feature shared by other nitronyl nitroxide radicals and analyzed in terms of two overlapping transitions [12]. The higher vibronic intervals in the absorption spectra suggest stronger and therefore shorter bonds in the emitting state than in the ground state, leading to negative values for D in both IMImH and NITImH. 4.2. Comparison of the lowest energy excited states for NITImH and IMImH The spectroscopic results and their analysis in the preceding section show that nitronyl nitroxides have a lower luminescence energy and smaller absolute value of the offset |D| along the relevant normal coordinate in the emitting state than their imino analogs. Electronic structure calculations can be used to qualitatively rationalize these findings. The SOMO orbitals shown in Fig. 1 are similar for both radicals: the unpaired electron density is p-antibonding and involves mainly the atoms defining the single normal coordinate for which the offset |D| was determined. For NITImH, this normal coordinate involves primarily the symmetric stretching vibration of the N–O oscillators, with smaller contributions from the central NCN fragment. In close analogy, the single N–O oscillator also provides the dominant contribution to the coordinate for the IMImH radical. Calculated atomic displacements for this mode are indicated by dotted arrows on the schematic structures in Fig. 5. A qualitative understanding of the difference between IMImH and NITImH radicals can be obtained from the p-electron system perpendicular to the plane of the molecule on the ONCNO and the NCNO fragments for the NITImH and IMImH radicals, respectively. This is appro-
157
∆E
N
C N H
O
O
N H
H
H
O H
IMH
H H
N
N
O
N
H
H
N
C
N
O
O H
NITH
Fig. 5. Calculated out-of-plane p-molecular orbitals and energy levels for simplified imino nitroxide (IMH) and nitronyl nitroxide (NITH) radicals. Solid horizontal lines denote energies for the a spin system, dotted horizontal lines those for the b spin system. The calculated atomic motions defining the normal coordinate dominating the vibronic structure are illustrated by the dotted arrows on the schematic structures.
priate as the calculated molecular orbitals close in energy to SOMO and therefore involved in the electron configurations of the lowest excited electronic states have predominant p-character localized mainly on these fragments. For a five-atom system with one out-of-plane p-orbital for each atom, as is the case for the ONCNO fragment of NITImH in Fig. 1b, five levels are expected, with the unpaired electron occupying the fourth level. In contrast, the four-atom NCNO fragment for IMImH leads to only four levels, with the unpaired electron occupying the third-lowest level. Fig. 5 summarizes DFT calculations for simplified nitronyl and imino nitroxide radicals, where the imidazole substituent is replaced by a single H atom. The key aspect of this model are the out-of-plane p-energy levels for both the a and b spin systems, shown with their amplitudes for the a molecular orbitals on each of the atoms for the NCNO and ONCNO fragments,
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respectively. The p system in IMH is shorter than in NITH and larger energy differences are therefore obtained between the molecular orbital energy levels for IMImH in the left-hand column of Fig. 5. The most important energy differences DE to be compared are those between the highest occupied and lowest unoccupied orbitals for the a and b spin systems, the former denoted by the vertical arrows in Fig. 5. The differences are significantly larger for the imino nitroxide model system, a key reason for the higher-energy luminescence transition observed for IMImH compared to NITImH. The larger absolute value of |D| in IMImH is another consequence of the less extended p system, where removal or addition of p electron density from SOMO leads to a larger decrease in p-antibonding character for the imino nitroxide model radical, in particular on the N–O fragment. The calculations in Fig. 5 also qualitatively explain the larger vibronic spacing in the absorption spectra, corresponding to a higher vibrational frequency in the emitting state indicative of a negative sign for the offset D, due to the less antibonding N–O fragment in the lowest empty orbital compared to the highest occupied level. This qualitative picture rationalizes the most important differences between the luminescence spectra in Fig. 2, illustrating the important role of the outof-plane p-system. A more advanced quantitative analysis of the spectroscopic properties of the nitroxide family would have to include the in-plane electronic system, the influence of the imidazole or other substituent groups, simplified as a hydrogen atom for the models in Fig. 5, and effects beyond the adiabatic approximation inherent to the calculations presented here, leading to multiple coupled potential energy surfaces [27,28]. Acknowledgments Financial support from the Natural Sciences and Engineering Research Council (Canada) is gratefully acknowledged. Funding for collaborative research was provided by the Centre Jacques-Cartier (DL), the Centre de Coope´ration Interuniversitaire Franco-Que´be´coise, CCIFQ (RB) and through an invited faculty fellowship at Universite´ Claude-Bernard (CR).
References [1] C. Benelli, D. Gatteschi, Chem. Rev. 102 (2002) 2369. [2] S. Kaizaki, Bull. Chem. Soc. Jpn. 76 (2003) 673. [3] G. Bussie`re, R. Beaulac, H. Be´lisle, C. Lescop, D. Luneau, C. Reber, Top. Curr. Chem. 241 (2004) 97. [4] M.T. Lemaire, Pure Appl. Chem. 76 (2004) 277. [5] M. Kinoshita, P. Turek, M. Tamura, K. Nozawa, D. Shiomi, Y. Nakazawa, M. Ishikawa, M. Takahashi, K. Awaga, T. Inabe, Y. Maruyama, Chem. Lett. (1991) 1225. [6] A. Caneschi, F. Ferraro, D. Gatteschi, A. Le Lirzin, M.A. Novak, E. Rentschler, R. Sessoli, Adv. Mater. 7 (1995) 476. ¨ hrstro¨m, G. Bussie`re, C. [7] C. Hirel, D. Luneau, J. Pe´caut, L. O Reber, Chem. Eur. J. 8 (2002) 3157. [8] A. Caneschi, D. Gatteschi, R. Sessoli, P. Rey, Acc. Chem. Res. 22 (1989) 392. [9] C. Lescop, D. Luneau, P. Rey, G. Bussie`re, C. Reber, Inorg. Chem. 41 (2002) 5566. [10] Y. Tsukahara, H. Nakata, S. Kaizaki, Inorg. Chem. 43 (2004) 4383. [11] C. Lescop, D. Luneau, G. Bussie`re, M. Triest, C. Reber, Inorg. Chem. 39 (2000) 3740. [12] R. Beaulac, G. Bussie`re, C. Reber, C. Lescop, D. Luneau, New J. Chem. 27 (2003) 1200. [13] E.F. Ullman, J.H. Osiecki, D.G.B. Boocock, R. Darcy, J. Am. Chem. Soc. 94 (1972) 7049. [14] K. Fegy, D. Luneau, T. Ohm, C. Paulsen, P. Rey, Angew. Chem., Int. Ed. 37 (1998) 1270. [15] C. Lescop, E. Belorizky, D. Luneau, P. Rey, Inorg. Chem. 42 (2002) 3375. [16] M.J. Davis, C. Reber, Inorg. Chem. 34 (1995) 4585. [17] M.J. Frisch et al., GAUSSIAN 98, Revision A.9, Gaussian, Inc., Pittsburgh, PA, 1998. [18] G. Zoppellaro, A. Ivanova, V. Enkelmann, A. Geies, M. Baumgarten, Polyhedron 22 (2003) 2099. [19] Y. Tsukahara, T. Kamatani, A. Iino, T. Suzuki, S. Kaizaki, Inorg. Chem. (2002). [20] Y. Tsukahara, A. Iino, T. Yoshida, T. Suzuki, S. Kaizaki, J. Chem. Soc., Dalton Trans. (2002) 181. [21] L. Catala, R. Feher, D.B. Amabilino, K. Wurst, J. Veciana, Polyhedron 20 (2001) 1563. [22] J.I. Zink, Coord. Chem. Rev. 211 (2001) 69. [23] J.I. Zink, K.-S.K. Shin, Adv. Photochem. 16 (1991) 119. [24] E.J. Heller, Acc. Chem. Res. 14 (1981) 368. [25] M.D. Feit, J.A. Fleck Jr., A. Steiger, J. Comput. Phys. 47 (1982) 412. [26] C. Reber, J.I. Zink, J. Phys. Chem. 95 (1991) 9151. [27] S.E. Bailey, J.I. Zink, S.F. Nelsen, J. Am. Chem. Soc. 125 (2003) 5939. [28] J.V. Lockard, J.I. Zink, A.E. Konradsson, M.N. Weaver, S.F. Nelsen, J. Am. Chem. Soc. 125 (2003) 13471.
The emitting state of the imino nitroxide radical Re´mi Beaulac a, Dominique Luneau b, Christian Reber
a,*
a
b
De´partement de Chimie, Universite´ de Montre´al, C.P. 6128, Succ. Centre-ville, Montre´al, Que´., Canada, H3C3J7 Cristallographie et Inge´nierie Mole´culaire, Laboratoire des Multimate´riaux et Interfaces (UMR 5615), Universite´ Claude-Bernard Lyon 1, 43 Avenue du 11 Novembre 1918, 69622 Villeurbanne Cedex, France Received 15 October 2004; in final form 3 February 2005
Abstract The solid-state low-temperature luminescence spectrum of an uncoordinated imino nitroxide radical, 2-(2-imidazolyl)-4,4,5,5tetramethylimidazoline-1-oxyl, is reported. The luminescence maximum is at 16 300 cm1 and the band has a width at half height of 3140 cm1 with a prominent vibronic progression interval of 1150 cm1. Corresponding spectroscopic results are shown for the nitronyl nitroxide analog, where a lower-energy luminescence maximum at 14 700 cm1 and a narrower band with a width at half height of 1900 cm1 are observed. These differences are analyzed in terms of the different delocalized p-systems of the radicals. Ó 2005 Elsevier B.V. All rights reserved.
1. Introduction Nitroxide radicals are important building blocks for materials with novel optical and magnetic properties [1–4]. These stable and chemically versatile molecules have been used to design molecule-based magnets, either as organic materials [5–7] or as ligands combined with metal ions [8,9]. Their electronic ground state has been characterized extensively by many different physical methods,[1] but their excited electronic states have received much less attention [2,3,10]. The lowest-energy intense absorption band of imino nitroxides has been reported higher in energy than the corresponding band of their nitronyl nitroxide analogs [3,9]. To the best of our knowledge, no luminescence spectrum has been reported for imino nitroxide radicals, in contrast to nitronyl nitroxides and their transition metal and lanthanide complexes, where distinct luminescence bands were observed in the near-infrared region [2,3,9,11,12]. The traditional resonance structures shown in Fig. 1 are symmetric for nitronyl nitroxide radicals, but asym*
Corresponding author. Fax: +1 514 343 7586. E-mail addresses: [email protected] (D. Luneau), [email protected] (C. Reber). 0009-2614/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.02.024
metric for imino nitroxide radicals, qualitatively illustrating their different electronic structures. Fig. 1 also shows the calculated singly occupied molecular orbital (SOMO) for both radicals. The SOMO electron density has p-antibonding character perpendicular to the plane of the molecule and is located predominantly on the NCNO fragment of the imino nitroxide radical in Fig. 1a and along the ONCNO fragment of the nitronyl nitroxide radical in Fig. 1b. In the following, we present the first luminescence spectrum for an imino nitroxide radical, determine its emitting state characteristics and compare them to the corresponding nitronyl nitroxide radical.
2. Experimental 2.1. Syntheses 2-(2-Imidazolyl)-4,4,5,5-tetramethylimidazoline-1-oxyl3-oxide (abbreviated in the following as NITImH) and 2-(2-imidazolyl)-4,4,5,5-tetramethylimidazoline-1-oxyl (abbreviated in the following as IMImH) were prepared according to literature methods and characterized as described previously [7,13–15].
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Fig. 1. Schematic representations of resonance structures for: (a) imino nitroxide (IMImH); (b) nitronyl nitroxide (NITImH) radicals. Calculated SOMO orbitals (a spin) are shown for both radicals.
2.2. Spectroscopic instrumentation Luminescence spectra were measured using a Renishaw System 3000 micro-Raman spectrometer. The 514.5 nm line of an Ar+ ion laser was used to excite the samples. Special care was taken to reduce the laser power to avoid photodegradation of the compounds. Spectra were corrected to take into account the change in sensitivity of the detector in the near-infrared region [16]. Cryogenic measurements were made using a continuous flow cryostat designed for the Raman microscope (Janis Research Supertran ST-500). Absorption spectra were measured with a Varian Cary 5E spectrometer. Calculations were made with the GAUSSIAN 98 package [17]. Semi-empirical calculations were made at the PM3 level; the PBE1PBE functional and the 6-31+G(3d) basis set were used for density-functional calculations.
3. Spectroscopic results Luminescence spectra of the IMImH and NITImH radicals are shown in Fig. 2. The differences between the spectra of these two similar molecules are striking. The spectrum of IMImH in Fig. 2a has its onset at 18 400 cm1 and shows resolved structure with a highest-energy maximum at 17 450 cm1 and its most intense peak lower in energy by 1150 cm1 at 16 300 cm1. The band width at half height is 3140 cm1. In contrast, the
NITImH radical in Fig. 2b shows a spectrum with an onset at 15 400 cm1, lower in energy by 3000 cm1 than observed for IMImH in Fig. 2a, and its highest energy maximum, which is also the most intense peak, at 14 640 cm1. A second resolved maximum occurs at 13 525 cm1, lower in energy by 1140 cm1 and a shoulder is observed at 13 905 cm1, lower in energy by 735 cm1 than the most intense maximum. The energy difference between resolved maxima is therefore very similar for both radicals. The width at half height of the NITImH spectrum is 1900 cm1, lower by 40% than for IMImH. Luminescence spectra of other nitronyl nitroxide radicals show similar characteristics, as illustrated by the well resolved spectrum of a nitronyl nitroxide radical with a benzimidazole substituent. This spectrum shows an overall bandwidth at half height of approximately 1100 cm1, a short vibronic progression with an interval of 1450 cm1 [12] and an intensity distribution very similar to NITImH. Table 1 summarizes the spectroscopic properties determined from the spectra in Figs. 2 and 3. The solution absorption spectra in Fig. 3 show band onsets at approximately 13 000 and 16 000 cm1 for NITImH and IMImH, respectively, confirming the energetic order of the lowest-energy electronic transitions established from the luminescence spectra in Fig. 2. Their band shapes are similar as they both have lowest-energy maxima that are less intense than the second maxima. However, the widths of the absorption band systems are different, 3600 and 5300 cm1 for NITImH
R. Beaulac et al. / Chemical Physics Letters 405 (2005) 153–158
Molar Absorptivity [M-1cm-1]
Luminescence Intensity
(a)
-1
3140 cm
80
155
(a)
60
40
20
0
10000
12000
14000
16000
18000
15000
20000
25000
300 Molar Absorptivity [M-1cm-1]
Luminescence Intensity
(b)
-1
1900 cm
(b)
200
100
0
10000
12000
14000
16000
15000
18000
-1
Wavenumber [cm ] Fig. 2. Solid-state luminescence spectra of (a) IMImH and (b) NITImH measured at 5 K.
Table 1 Selected peak positions and energy intervals from absorption and luminescence spectra Quantity
IMImH
NITImH
Width at half height
17 450 1150 2300 3140
14 640 735 1140 1900
Absorption Lowest energy maximum Intervals
18 800 1600
14 535 1440 1415 1400
Luminescence Highest energy maximum Intervals
20000 Wavenumber [cm-1]
25000
Fig. 3. Solution absorption spectra of (a) IMImH and (b) NITImH at room temperature in dichloromethane.
differences in the electronic structure of the emitting state that are analyzed in the following section.
4. Discussion
All values are in cm1.
and IMImH, respectively, and the spectrum of IMImH in Fig. 3a is less resolved than the spectrum of NITImH in Fig. 3b. The average interval between resolved maxima in the absorption spectrum of NITImH is approximately 1420 cm1, significantly higher than the intervals seen in the luminescence spectrum in Fig. 2b. A corresponding spacing of 1600 cm1 appears in the absorption spectrum of IMImH in Fig. 3a. The spectroscopic results in Figs. 2 and 3 show that despite similar absorption profiles, the luminescence bands indicate distinct
4.1. Vibronic structure in the luminescence spectra and emitting-state distortions The luminescence spectra in Fig. 2 show a single dominant vibronic progression. Its average intervals are 1140 and 1150 cm1 for NITImH and IMImH, respectively. The progression in both spectra is typical for nitroxide radicals and its interval is in the frequency range expected for the N–O oscillator. Raman spectra and literature analyses lead to a more detailed identification as the ONCNO stretching mode for nitronyl nitroxide radicals and as the corresponding NCNO mode for imino nitroxide radicals [18–21]. The N–O oscillators contribute strongly to both modes and the frequency of the NCNO mode in imino nitroxide radicals has been found to be higher by up to 50 cm1 than in the corresponding nitronyl nitroxide radicals [18–21]. A quantitative determination of the change in molecular structure along the normal coordinate of this mode can be made from calculated luminescence spectra with
156
R. Beaulac et al. / Chemical Physics Letters 405 (2005) 153–158
the offset along the normal coordinate as adjustable parameter. All calculations are based on the timedependent theory of spectroscopy, the details of which have been discussed thoroughly in the past [22–24]. The calculated spectrum is obtained by assuming harmonic potential energy surfaces for both the ground and excited states. The general form for the surface of a harmonic excited-state potential is given by E¼
1X 2 xk ðQk Dk Þ þ E00 ; 2 k
Parameter
IMImH
NITImH
E00 (cm1) hxground state ðcm1 Þ hxemitting state (cm1) |D| C (cm1)
18 000 1125 1550 1.55 325 (luminescence) 575 (absorption)
14 660 1200 1366 1.34 190
ð1Þ
where k identifies the (dimensionless) normal coordinate Qk with vibrational frequency xk. In situations where the spectral resolution is too low to distinguish many vibronic progressions, as is the case here, the sum in Eq. (1) retains only a single term. Dk is the offset of the potential surface with respect to the ground-state potential energy minimum along normal coordinate Qk. The offset is the only adjustable parameter not directly available from experiment. E00 is the energy of the electronic origin. The ground-state potential energy surface is given by E¼
Table 2 Parameter values used to calculate the luminescence and absorption spectra in Fig. 4
1X 2 xQ: 2 k k k
ð2Þ
(b)
Absorbance
Luminescence Intensity
(a)
The ground-state vibrational frequencies xk describing this potential energy surface are not identical to those in Eq. (1). Spectra are calculated with the split operator method [25,26] and the width of each vibronic peak is reproduced by the phenomenological damping factor C. All parameter values for the calculation of the spectra are summarized in Table 2. The calculated luminescence spectrum for IMImH is compared to the experimental spectrum in Fig. 4a. The offset |D| is determined to be 1.55 and the calculated spectrum is in good agreement with the experiment. The parameters in Table 2 were used to calculate the absorption spectrum in Fig. 4b, again compared to the experimental trace. This spectrum was obtained without adjusting parameters and quantitatively confirms that
14500
17000
(c)
10000
15000
20000
25000
(d)
Absorbance
Luminescence Intensity
12000
12500
15000 -1
Wavenumber [cm ]
15000
17500
20000 -1
Wavenumber [cm ]
Fig. 4. Experimental (solid lines) and calculated (dotted lines) luminescence and absorption spectra: (a) luminescence of IMImH; (b) absorption of IMImH; (c) luminescence of NITImH; (d) absorption of NITImH.
R. Beaulac et al. / Chemical Physics Letters 405 (2005) 153–158
the absorption and luminescence spectra are mirror images, as expected for an emitting state well removed in energy from other excited states. The situation is different for NITImH. The calculated luminescence spectrum is shown in Fig. 4c compared to the experimental spectrum. Again, all parameters used for the calculation are summarized in Table 2. The offset |D| along the normal coordinate is 1.34, smaller by 15% than for IMImH. The comparison with the experimental absorption spectrum in Fig. 4d reveals an important difference between NITImH and IMImH: the absorption spectrum calculated using the parameter values obtained from the luminescence analysis does not reproduce the experimental spectrum. The most important difference occurs for the relative intensities of the two lowest-energy maxima in the calculated spectrum in Fig. 4d. In contrast to the experimental trace, where the first peak is approximately 25% less intense than the following higher energy peak, the intensities of the first two vibronic peaks are identical in the calculated spectrum. In addition, the overall width of the calculated absorption spectrum is too narrow, a spectroscopic feature shared by other nitronyl nitroxide radicals and analyzed in terms of two overlapping transitions [12]. The higher vibronic intervals in the absorption spectra suggest stronger and therefore shorter bonds in the emitting state than in the ground state, leading to negative values for D in both IMImH and NITImH. 4.2. Comparison of the lowest energy excited states for NITImH and IMImH The spectroscopic results and their analysis in the preceding section show that nitronyl nitroxides have a lower luminescence energy and smaller absolute value of the offset |D| along the relevant normal coordinate in the emitting state than their imino analogs. Electronic structure calculations can be used to qualitatively rationalize these findings. The SOMO orbitals shown in Fig. 1 are similar for both radicals: the unpaired electron density is p-antibonding and involves mainly the atoms defining the single normal coordinate for which the offset |D| was determined. For NITImH, this normal coordinate involves primarily the symmetric stretching vibration of the N–O oscillators, with smaller contributions from the central NCN fragment. In close analogy, the single N–O oscillator also provides the dominant contribution to the coordinate for the IMImH radical. Calculated atomic displacements for this mode are indicated by dotted arrows on the schematic structures in Fig. 5. A qualitative understanding of the difference between IMImH and NITImH radicals can be obtained from the p-electron system perpendicular to the plane of the molecule on the ONCNO and the NCNO fragments for the NITImH and IMImH radicals, respectively. This is appro-
157
∆E
N
C N H
O
O
N H
H
H
O H
IMH
H H
N
N
O
N
H
H
N
C
N
O
O H
NITH
Fig. 5. Calculated out-of-plane p-molecular orbitals and energy levels for simplified imino nitroxide (IMH) and nitronyl nitroxide (NITH) radicals. Solid horizontal lines denote energies for the a spin system, dotted horizontal lines those for the b spin system. The calculated atomic motions defining the normal coordinate dominating the vibronic structure are illustrated by the dotted arrows on the schematic structures.
priate as the calculated molecular orbitals close in energy to SOMO and therefore involved in the electron configurations of the lowest excited electronic states have predominant p-character localized mainly on these fragments. For a five-atom system with one out-of-plane p-orbital for each atom, as is the case for the ONCNO fragment of NITImH in Fig. 1b, five levels are expected, with the unpaired electron occupying the fourth level. In contrast, the four-atom NCNO fragment for IMImH leads to only four levels, with the unpaired electron occupying the third-lowest level. Fig. 5 summarizes DFT calculations for simplified nitronyl and imino nitroxide radicals, where the imidazole substituent is replaced by a single H atom. The key aspect of this model are the out-of-plane p-energy levels for both the a and b spin systems, shown with their amplitudes for the a molecular orbitals on each of the atoms for the NCNO and ONCNO fragments,
158
R. Beaulac et al. / Chemical Physics Letters 405 (2005) 153–158
respectively. The p system in IMH is shorter than in NITH and larger energy differences are therefore obtained between the molecular orbital energy levels for IMImH in the left-hand column of Fig. 5. The most important energy differences DE to be compared are those between the highest occupied and lowest unoccupied orbitals for the a and b spin systems, the former denoted by the vertical arrows in Fig. 5. The differences are significantly larger for the imino nitroxide model system, a key reason for the higher-energy luminescence transition observed for IMImH compared to NITImH. The larger absolute value of |D| in IMImH is another consequence of the less extended p system, where removal or addition of p electron density from SOMO leads to a larger decrease in p-antibonding character for the imino nitroxide model radical, in particular on the N–O fragment. The calculations in Fig. 5 also qualitatively explain the larger vibronic spacing in the absorption spectra, corresponding to a higher vibrational frequency in the emitting state indicative of a negative sign for the offset D, due to the less antibonding N–O fragment in the lowest empty orbital compared to the highest occupied level. This qualitative picture rationalizes the most important differences between the luminescence spectra in Fig. 2, illustrating the important role of the outof-plane p-system. A more advanced quantitative analysis of the spectroscopic properties of the nitroxide family would have to include the in-plane electronic system, the influence of the imidazole or other substituent groups, simplified as a hydrogen atom for the models in Fig. 5, and effects beyond the adiabatic approximation inherent to the calculations presented here, leading to multiple coupled potential energy surfaces [27,28]. Acknowledgments Financial support from the Natural Sciences and Engineering Research Council (Canada) is gratefully acknowledged. Funding for collaborative research was provided by the Centre Jacques-Cartier (DL), the Centre de Coope´ration Interuniversitaire Franco-Que´be´coise, CCIFQ (RB) and through an invited faculty fellowship at Universite´ Claude-Bernard (CR).
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