- Email: [email protected]
On the vibronic structure of the fluorescence spectrum of biphenylene
Volume 169, number 6
CHEMICAL PHYSICS LETTERS
ON THE VIBRONIC STRUCTURE OF THE FLUORESCENCE OF BIPHENYLENE
22 June 1990
SPECTRUM
Giancarlo MARCONI lstituto
FRAE-C&R, via Castagnoli I, 40126 Bologna, Italy
Received 15 February 1990; in final form 29 March 1990
The vibronic sequences shown by the SI+So spectrum of biphenylene have been calculated by means of the relevant vibronic coupling and Franck-Condon factors. The results of our calculations make it possible to identify the bzu modes and a, modes responsible for the spectrum and involved in the radiationless transition from S2 to S,.
1. Introduction Due to its peculiar photophysical properties, biphenylene has received much attention in the past. For a long time this molecule was thought to emit from electronic states higher than S,, thus contradicting Kasha’s rule [ 1,2]. This conclusion was questioned by other authors [ 3 1,who attributed such type of emission to the presence of impurities. Recently the spectroscopic and photophysical properties of biphenylene have been thoroughly reinvestigated with the purpose of clarifying the problem of the emission and of relating these peculiar properties to the electronic structure [ 4-6 1. Biphenylene belongs to the class of the antiaromatic 4n-annulenes and is characterized by a system localized in the perimeter rather than delocalized as in the aromatic compounds [7]. As a consequence, one observes a large change of electron density and bond lengths in the excited states with related long Franck-Condon progressions in the absorption and emission spectra. The latter appear also quite dissymmetric with respect to the former, indicating an interaction of geometric and vibronic factors. For the system under examination these features were revealed by detailed low-temperature absorption and emission spectra [ 5 1. The structural changes accompanying the electronic excitation in the two low-lying excited singlets have also been studied by means of semi-empirical quantum-mechanical calculations [ 41. Since it is the first singlet state of biphenylene of B1, symmetry in 0009-2614/90/$
DZh, the lowest transition is forbidden and the detected intensity results totally vibronic in nature. Therefore an elucidation of the fluorescence spectrum requires a vibronic-coupling calculation on the bzu modes responsible for coupling S1to the low-lying electronic states of B3” symmetry. Among them, the major candidate is the close-lying S2 state, as indicated by the favourable energy gap (A_K(S,-S,) 04000 cm-’ deduced from low-temperature fluorescence spectra in alkanes) and by the positive fluorescence polarization degree [ 5 1. In this paper we investigate the structure of the lowest excited singlet state of biphenylene, through a semi-empirical quantum-mechanical calculation of the relevant spectral properties. This approach differs from previous treatments based on the search for molecular parameters by means of best-fitting procedures [ 8,9]. Once we have identified the most active non-totally symmetric modes, we combine the vibronic integrals to yield a series of molecular constants pertinent to the a, modes; these results are, in turn, composed by Franck-Condon (FC) and vibronic integrals to simulate the S,-& spectrum. Identification of the most active bZu and ag modes allows also a prediction of the complicated structure in the region of SZ. In fact, on the basis of analogy with aromatic systems [ lo-121 and some experimental evidence [ 3,131, it is expected that the allowed S,, ti=O level results are embedded in the quasi-continuum of levels built on the lowest-transition false origins. Related to this feature is the im-
03.50 8 Elsevier Science Publishers B.V. (North-Holland)
617
Volume169,number6
portant behavior of biphenylene as an intermediate molecule in the radiationless transition scheme and, thus, the possibility of delayed emission from S2 [ 61.
2. Calculations and results 2.1. Vibronic coupling induced by bZumodes The analysis of the fluorescence spectrum reported in ref. [ 51 performed on the basis of the vibrational intervals, restricts to 4 modes of bzu symmetry the possibility of vibronic borrowing from higher excited states. With the labelling of ref. [ 51, these are ~~~=1638, ~~,=1267, ~,,=751 and v4, = 212 cm-‘. In DZh point symmetry there are 9 bzUmodes which in principle can couple S, (B,,) to upper states. We performed a calculation of the vibronic coefficients of the 7 modes with frequency < 3000 cm-‘, using the well-known “orbital following” method, a method that has been successfully employed to elucidate problems in absorption [ 14 1, emission [ 151, Raman [ 161 and multiphoton [ 171 spectroscopy. According to this method, the vibronic coupling induced by the ith mode between the states r and g, VY,
(1) appearing in the equation of the induced intensities as
is calculated through the variation of the CIs coefficients with the normal-coordinate displacements, keeping fixed the MOs coefficients [ 18 1. In this calculation the Vts were obtained using the CNDO/S method, with a Cl including singly and doubly excited configurations in a space of 100 configurations. The necessary Cartesian displacements were obtained performing a MIND0/3 vibrational calculation of force constants and ground-state normal coordinates.
618
22June 1990
CHEMICALPHYSICSLETTERS
2.2. Vibronic activity and FC factors of the a, modes It has been recognized by several authors [ 19,201 that a major cause of lack of mirror symmetry between absorption and emission spectra is due to the interference of FC and vibronic mechanisms for the totally symmetric modes that form the progression. Examples of analysis of this type include pyrene [ 81, anthracene [ 91, and phenanthrene [ 191. Following a formalism previously proposed [ 8,19 1, we express the transition moment as n;i(k:)=(k”‘In;i,+
$ Qkvkl~L),
(3)
where J& represents in this case the vibronically induced moment expressed by eq. (2) and k labels the totally symmetric modes, which couple the pertinent electronic states through the coupling constant V,. By approximating the vibrational wave-functions as harmonic oscillators and using the recursion formulae for the Hermite polynomials, one obtains QkIk,)=(~/2P,)*‘21~-l) + [ (u+ 1)/vk1”2
I v+ I>,
(4)
wherePk=4n2vk/h represents the oscillator constant for the kth mode in the ground state, and the expression of the moment induced by a totally symmetric mode can be written as (5) with
Fk=l+
t;kS(i3v/(2p,)“2.
Similar expressions can be obtained for overtones and binary combinations. Inspection of eq. (5) shows that the intensity carried by a vibronically active ag mode is composed of its FC factor, modulated by the vibronic coupling of the other active modes and of the intrinsic vibronic term. While the calculation of the vibronic factors V, can be carried out as described in section 2.1, the evaluation of the FC factor requires an explicit knowledge of the geometry changes occurring along the relevant coordinates
Volume 169,number 6
CHEMICALPHYSICSLETTERS
22 June 1990
Table 1 Calculated vibronic and FC terms for the a, modes appearing in the S,+S, spectrum
1462 1105 765 395
V4
n7 v9 110
0.4195 0.2090 0.0783 0.2504
0.7935 0.6042 0.5336 0.9439
1.3336 1.1128 0.9538 0.9 I82
1.2534 0.9232 0.8513 0.6819
during the transition 1g) -+ 1e). In this work, we have evaluated the surface displacements using the equation that relates them to the bond-length variation in the excited states [ 211, Ak= 1 Lkl’R;=
I
C L,‘(-C,b;), i
a)
(6)
where Ri represents a stretching internal coordinate, Lkj’ its contribution to the normal coordinate Qk, b,’ the variation of the bond order in the excited state ) e) with respect to the ground state and C, is a constant, generally equal to 0.18 A for aromatic compounds [ 211. Taking into account’ literature data concerning the potential-energy distribution and force constants for the ap modes [22], and the calculated bond-density changes in the S, state [4], we obtain the displacements reported in table 1. With the reasonable assumption that the frequencies of the modes do not change much in the excited states, the FC factors will depend only on the displacements according to the formula
iL b)
II
%=exp(
-y) y”I&
where y=Dz=A2( 2xv/fi) is the dimensionless displacement calculated by eq. (6).
0.t
OS
3. Discussion
OC
The fluorescence spectrum of biphenylene at 130 K, reported in fig. 1 shows a fairly long progression built on several false origins. In ref. [ 51 the authors, by examining the frequency intervals, determined four series of bands denoted a, p, y, 6 and proposed two possible attributions for them. The two hypotheses consider the four series as built over the false origin of the modes vJ5 and v4,, respectively, with several additions of quanta of ap modes. Two other vibronic false origins, with much lower intensity are
0.2
0.c 14
16
18
20
22
24
10e3 J/“m-’
Rg. 1.Calculated ((a) and (b), see text) and experimental ((c), ref. [ 5] ) fluorescence spectrum of biphenylene.
619
CHEMICALPHYSICSLETTERS
Volume169,number 6
detected at z 700 and 1200 cm- ’ from the probable origin of S,. From the calculated vibronic intensities reported in table 2, we observe that the activity of vji is calculated to be the largest, followed by vjg and yIe. The rough proportionality found between the sum of the main vibronic terms and the total induced intensity is consistent with the polarization data, showing that the fluorescence is essentially long-axis polarized. Like some aromatic systems [ 171, the mechanism coupling the ground state to S, cannot be neglected in this case, but the main mechanism remains that involving Sz, due to the very favorable energy gap. However, the activity of v4, appears to be so small that a vibronic sequence built on this mode appears to be improbable. Therefore we attribute the a, band in the spectrum to us5 and attempt a reconstruction of the spectrum by adding to this false origin the appropriate FC factors and vibronic terms of the ap modes. These have been restricted to the four ap modes which best fit the vibrational intervals, i.e. vq, v,, vg, and vIo. The calculated vibronic and FC terms are reported in table 1, while the calculated spectrum is displayed in fig. lb. Despite the many approximations used, the calculated parameters of table 1 appear to be able to reproduce the main features of the S,-& spectrum of biphenylene. An improved simulation of the spectrum can be obtained by using all the displacements increased by a constant factor: for example, in fig. lc we report the spectrum calculated with 1.44. In this case the simulated spectrum appears more centered on the lines with three quanta of a, modes. The justification for this correction depends on a presumably larger constant C, in eq. (6) and is consistent with an expected larger Table2 Calculatedinducedintensities(normalizedto ZV,~=1.67xIO-‘) and vibronic couplingterms (cm-‘) for the most relevantbzU modesof biphenylene
V35 46
VJ’ U38 VW u40 V‘li 620
V (‘P
I
w%-w
Jfc%-s,)
1638 1444 1267 1128 1053 751 212
I 0.12 0 0.01 0.01 0.05 0.01
398 90 -6 5 -18 -110 103
858 234 29 70 77 -294 -163
22June 1990
variation of bond lengths in the excited states of antiaromatic compounds with respect to the aromatic ones. Related to these large displacements and to the antiaromaticity of biphenylene are the long progressions observed in the spectrum. The calculated parameters indicate that vibronic activity of these modes is an important source of intensity and cannot be neglected in the attempt to reproduce the spectrum. It turns out that the vibronic term contributes about half of the total value for the M(iy) moment of v.+ The latter presents the largest activity in the spectrum, being very effective in coupling vibronically S, with upper B,, states and also quite displaced in the excited state with respect to So. The activity of the mode ~,a appears, however, underestimated, mainly because of a small vibronic term. This feature of the present calculation is not surprising, as the method is known to emphasize the activity of the C=C stretchings [ 161. Finally, on the basis of the calculated spectrum in the region of S,, a prediction on the absorption spectrum in the region of S2 can be advanced. In fact the relatively small energy gap S,-S, (AE=4000 cm-’ experimental, 4700 calculated) joined to the vibronic coupling terms of table 2, leads us to confirm the prediction of a strong intermediate coupling for biphenylene [ 61. Therefore, in analogy with other aromatic systems like naphthalene and phenanthrene, biphenylene should behave as an “intermediate molecule” in the radiationless transition language. This feature is due to the fact that the allowed SZ, u=O level is embedded in a dense manifold of levels built on the false origins of S,. As shown recently for the case of pyrene [ 121, a quantum-mechanical description based on such a model is instrumental in reproducing both the intensities of the spectrum in the crowded region of S2 and the band contours observed in a resolved spectrum detected by the jet supersonic technique. In case of intermediate-level behavior, deviations from the Lorentzian band shape are to be expected [23]. Moreover, this kind of approach is able to gain insight into the dynamical processes that follow direct excitation into Sz, and to explain the presence of fast and slow components in the decay [6]. In fact it is expected that the S2-‘SI process is not an irreversible one and that recurrence may occur on the same
Volume 169, number 6
CHEMICAL PHYSICS LETTERS
time scale as the fluorescence decay, with subsequent delayed emission.Therefore a detailed spectral study of the Sz region is needed in order to get a thorough description of the spectral and dynamical behavior of this molecule.
References [ I] J.W. Hilpem, Trans. Faraday Sot. 61 (1965) 605. [2] J.B. Birks, J.M. de C. Come and G. Walker, Phys. Letters 19 (1966) 3325. [3] R.M. Hochstrasser and R.D. McAlpine, J. Chem. Phys. 44 (1966) 3325. [4] T. Elsaesser, F. tinner, W. Kaiser, B. Dick, M. Niemeger and W. Lilttke, Chem. Phys. 126 (1988) 405. [ 51 B. Nickel and J. Hertzberg, Chem. Phys. 132 ( 1989) 2 19. [6] J. Hertzberg and B. Nickel, Chme. Phys. 132 (1989) 235. [7] R. Breslow, Accounts Chem. Res. 6 (1973) 393. [ 81 A. Bree, A. Leyderman, P.R. Salvi and C. Tahani, Chem. Phys.110(1986)211. [9] A. Bree, Chem. Phys. Letters 131 (1986) 65.
22 June 1990
[IO] J. Wessel and D.S. McClure, Mol. Cryst. Liquid Cryst. 58 (1980) 121. [ 1 l] G. Fischer, Chem. Phys. 4 (1974) 62. [ 121 N. Ohta, H. Baba and G. Marconi, Chem. Phys. Letters 133 (1987) 222. [ 1311. Zanon, J. Chem. Sot. Faraday Trans. II 69 (1973) 1164. [ 141 G. Orlandi and G. Marconi, Chem. Phys. Letters 53 ( 1978) 61. [ 151 G. Marconi and P.R. Salvi, Chem. Phys. Letters 123 (1986) 254. [ 161 G. Marconi, Chem. Phys. 57 (1981) 31 I; J. RamanSpectry 14 (1983) 28. [ 171 G. Marconi, P.R. Salvi and R. Quacquarini, Chem. Phys. Letters 107 (1984) 314; P.R. Salvi and G. Marconi, J. Chem. Phys. 84 (1986) 2546. [IS] G. Orlandi, Chem. Phys. Letters 44 (1976) 277. [ 191 D.P. Craig and G.J. Small, J. Chem. Phys. 50 ( 1969) 3827. [20] A.R. Gregory, W. Siebrand and M.Z. Zgierski, J. Chem. Phys. 64 (1976) 3145. [2 I] W.L. Pet&as, D.P. Strommen and V. Lakshaminarayaman, J. Chem. Phys. 73 (1980) 4185. [22] A. Girlando and C. Pecile, J. Chem. Phys. 69 (1973) 818. [23] C.A. Langhoff and G.W. Robinson, Chem. Phys. 16 (1974) 34.
621
CHEMICAL PHYSICS LETTERS
ON THE VIBRONIC STRUCTURE OF THE FLUORESCENCE OF BIPHENYLENE
22 June 1990
SPECTRUM
Giancarlo MARCONI lstituto
FRAE-C&R, via Castagnoli I, 40126 Bologna, Italy
Received 15 February 1990; in final form 29 March 1990
The vibronic sequences shown by the SI+So spectrum of biphenylene have been calculated by means of the relevant vibronic coupling and Franck-Condon factors. The results of our calculations make it possible to identify the bzu modes and a, modes responsible for the spectrum and involved in the radiationless transition from S2 to S,.
1. Introduction Due to its peculiar photophysical properties, biphenylene has received much attention in the past. For a long time this molecule was thought to emit from electronic states higher than S,, thus contradicting Kasha’s rule [ 1,2]. This conclusion was questioned by other authors [ 3 1,who attributed such type of emission to the presence of impurities. Recently the spectroscopic and photophysical properties of biphenylene have been thoroughly reinvestigated with the purpose of clarifying the problem of the emission and of relating these peculiar properties to the electronic structure [ 4-6 1. Biphenylene belongs to the class of the antiaromatic 4n-annulenes and is characterized by a system localized in the perimeter rather than delocalized as in the aromatic compounds [7]. As a consequence, one observes a large change of electron density and bond lengths in the excited states with related long Franck-Condon progressions in the absorption and emission spectra. The latter appear also quite dissymmetric with respect to the former, indicating an interaction of geometric and vibronic factors. For the system under examination these features were revealed by detailed low-temperature absorption and emission spectra [ 5 1. The structural changes accompanying the electronic excitation in the two low-lying excited singlets have also been studied by means of semi-empirical quantum-mechanical calculations [ 41. Since it is the first singlet state of biphenylene of B1, symmetry in 0009-2614/90/$
DZh, the lowest transition is forbidden and the detected intensity results totally vibronic in nature. Therefore an elucidation of the fluorescence spectrum requires a vibronic-coupling calculation on the bzu modes responsible for coupling S1to the low-lying electronic states of B3” symmetry. Among them, the major candidate is the close-lying S2 state, as indicated by the favourable energy gap (A_K(S,-S,) 04000 cm-’ deduced from low-temperature fluorescence spectra in alkanes) and by the positive fluorescence polarization degree [ 5 1. In this paper we investigate the structure of the lowest excited singlet state of biphenylene, through a semi-empirical quantum-mechanical calculation of the relevant spectral properties. This approach differs from previous treatments based on the search for molecular parameters by means of best-fitting procedures [ 8,9]. Once we have identified the most active non-totally symmetric modes, we combine the vibronic integrals to yield a series of molecular constants pertinent to the a, modes; these results are, in turn, composed by Franck-Condon (FC) and vibronic integrals to simulate the S,-& spectrum. Identification of the most active bZu and ag modes allows also a prediction of the complicated structure in the region of SZ. In fact, on the basis of analogy with aromatic systems [ lo-121 and some experimental evidence [ 3,131, it is expected that the allowed S,, ti=O level results are embedded in the quasi-continuum of levels built on the lowest-transition false origins. Related to this feature is the im-
03.50 8 Elsevier Science Publishers B.V. (North-Holland)
617
Volume169,number6
portant behavior of biphenylene as an intermediate molecule in the radiationless transition scheme and, thus, the possibility of delayed emission from S2 [ 61.
2. Calculations and results 2.1. Vibronic coupling induced by bZumodes The analysis of the fluorescence spectrum reported in ref. [ 51 performed on the basis of the vibrational intervals, restricts to 4 modes of bzu symmetry the possibility of vibronic borrowing from higher excited states. With the labelling of ref. [ 51, these are ~~~=1638, ~~,=1267, ~,,=751 and v4, = 212 cm-‘. In DZh point symmetry there are 9 bzUmodes which in principle can couple S, (B,,) to upper states. We performed a calculation of the vibronic coefficients of the 7 modes with frequency < 3000 cm-‘, using the well-known “orbital following” method, a method that has been successfully employed to elucidate problems in absorption [ 14 1, emission [ 151, Raman [ 161 and multiphoton [ 171 spectroscopy. According to this method, the vibronic coupling induced by the ith mode between the states r and g, VY,
(1) appearing in the equation of the induced intensities as
is calculated through the variation of the CIs coefficients with the normal-coordinate displacements, keeping fixed the MOs coefficients [ 18 1. In this calculation the Vts were obtained using the CNDO/S method, with a Cl including singly and doubly excited configurations in a space of 100 configurations. The necessary Cartesian displacements were obtained performing a MIND0/3 vibrational calculation of force constants and ground-state normal coordinates.
618
22June 1990
CHEMICALPHYSICSLETTERS
2.2. Vibronic activity and FC factors of the a, modes It has been recognized by several authors [ 19,201 that a major cause of lack of mirror symmetry between absorption and emission spectra is due to the interference of FC and vibronic mechanisms for the totally symmetric modes that form the progression. Examples of analysis of this type include pyrene [ 81, anthracene [ 91, and phenanthrene [ 191. Following a formalism previously proposed [ 8,19 1, we express the transition moment as n;i(k:)=(k”‘In;i,+
$ Qkvkl~L),
(3)
where J& represents in this case the vibronically induced moment expressed by eq. (2) and k labels the totally symmetric modes, which couple the pertinent electronic states through the coupling constant V,. By approximating the vibrational wave-functions as harmonic oscillators and using the recursion formulae for the Hermite polynomials, one obtains QkIk,)=(~/2P,)*‘21~-l) + [ (u+ 1)/vk1”2
I v+ I>,
(4)
wherePk=4n2vk/h represents the oscillator constant for the kth mode in the ground state, and the expression of the moment induced by a totally symmetric mode can be written as (5) with
Fk=l+
t;kS(i3v/(2p,)“2.
Similar expressions can be obtained for overtones and binary combinations. Inspection of eq. (5) shows that the intensity carried by a vibronically active ag mode is composed of its FC factor, modulated by the vibronic coupling of the other active modes and of the intrinsic vibronic term. While the calculation of the vibronic factors V, can be carried out as described in section 2.1, the evaluation of the FC factor requires an explicit knowledge of the geometry changes occurring along the relevant coordinates
Volume 169,number 6
CHEMICALPHYSICSLETTERS
22 June 1990
Table 1 Calculated vibronic and FC terms for the a, modes appearing in the S,+S, spectrum
1462 1105 765 395
V4
n7 v9 110
0.4195 0.2090 0.0783 0.2504
0.7935 0.6042 0.5336 0.9439
1.3336 1.1128 0.9538 0.9 I82
1.2534 0.9232 0.8513 0.6819
during the transition 1g) -+ 1e). In this work, we have evaluated the surface displacements using the equation that relates them to the bond-length variation in the excited states [ 211, Ak= 1 Lkl’R;=
I
C L,‘(-C,b;), i
a)
(6)
where Ri represents a stretching internal coordinate, Lkj’ its contribution to the normal coordinate Qk, b,’ the variation of the bond order in the excited state ) e) with respect to the ground state and C, is a constant, generally equal to 0.18 A for aromatic compounds [ 211. Taking into account’ literature data concerning the potential-energy distribution and force constants for the ap modes [22], and the calculated bond-density changes in the S, state [4], we obtain the displacements reported in table 1. With the reasonable assumption that the frequencies of the modes do not change much in the excited states, the FC factors will depend only on the displacements according to the formula
iL b)
II
%=exp(
-y) y”I&
where y=Dz=A2( 2xv/fi) is the dimensionless displacement calculated by eq. (6).
0.t
OS
3. Discussion
OC
The fluorescence spectrum of biphenylene at 130 K, reported in fig. 1 shows a fairly long progression built on several false origins. In ref. [ 51 the authors, by examining the frequency intervals, determined four series of bands denoted a, p, y, 6 and proposed two possible attributions for them. The two hypotheses consider the four series as built over the false origin of the modes vJ5 and v4,, respectively, with several additions of quanta of ap modes. Two other vibronic false origins, with much lower intensity are
0.2
0.c 14
16
18
20
22
24
10e3 J/“m-’
Rg. 1.Calculated ((a) and (b), see text) and experimental ((c), ref. [ 5] ) fluorescence spectrum of biphenylene.
619
CHEMICALPHYSICSLETTERS
Volume169,number 6
detected at z 700 and 1200 cm- ’ from the probable origin of S,. From the calculated vibronic intensities reported in table 2, we observe that the activity of vji is calculated to be the largest, followed by vjg and yIe. The rough proportionality found between the sum of the main vibronic terms and the total induced intensity is consistent with the polarization data, showing that the fluorescence is essentially long-axis polarized. Like some aromatic systems [ 171, the mechanism coupling the ground state to S, cannot be neglected in this case, but the main mechanism remains that involving Sz, due to the very favorable energy gap. However, the activity of v4, appears to be so small that a vibronic sequence built on this mode appears to be improbable. Therefore we attribute the a, band in the spectrum to us5 and attempt a reconstruction of the spectrum by adding to this false origin the appropriate FC factors and vibronic terms of the ap modes. These have been restricted to the four ap modes which best fit the vibrational intervals, i.e. vq, v,, vg, and vIo. The calculated vibronic and FC terms are reported in table 1, while the calculated spectrum is displayed in fig. lb. Despite the many approximations used, the calculated parameters of table 1 appear to be able to reproduce the main features of the S,-& spectrum of biphenylene. An improved simulation of the spectrum can be obtained by using all the displacements increased by a constant factor: for example, in fig. lc we report the spectrum calculated with 1.44. In this case the simulated spectrum appears more centered on the lines with three quanta of a, modes. The justification for this correction depends on a presumably larger constant C, in eq. (6) and is consistent with an expected larger Table2 Calculatedinducedintensities(normalizedto ZV,~=1.67xIO-‘) and vibronic couplingterms (cm-‘) for the most relevantbzU modesof biphenylene
V35 46
VJ’ U38 VW u40 V‘li 620
V (‘P
I
w%-w
Jfc%-s,)
1638 1444 1267 1128 1053 751 212
I 0.12 0 0.01 0.01 0.05 0.01
398 90 -6 5 -18 -110 103
858 234 29 70 77 -294 -163
22June 1990
variation of bond lengths in the excited states of antiaromatic compounds with respect to the aromatic ones. Related to these large displacements and to the antiaromaticity of biphenylene are the long progressions observed in the spectrum. The calculated parameters indicate that vibronic activity of these modes is an important source of intensity and cannot be neglected in the attempt to reproduce the spectrum. It turns out that the vibronic term contributes about half of the total value for the M(iy) moment of v.+ The latter presents the largest activity in the spectrum, being very effective in coupling vibronically S, with upper B,, states and also quite displaced in the excited state with respect to So. The activity of the mode ~,a appears, however, underestimated, mainly because of a small vibronic term. This feature of the present calculation is not surprising, as the method is known to emphasize the activity of the C=C stretchings [ 161. Finally, on the basis of the calculated spectrum in the region of S,, a prediction on the absorption spectrum in the region of S2 can be advanced. In fact the relatively small energy gap S,-S, (AE=4000 cm-’ experimental, 4700 calculated) joined to the vibronic coupling terms of table 2, leads us to confirm the prediction of a strong intermediate coupling for biphenylene [ 61. Therefore, in analogy with other aromatic systems like naphthalene and phenanthrene, biphenylene should behave as an “intermediate molecule” in the radiationless transition language. This feature is due to the fact that the allowed SZ, u=O level is embedded in a dense manifold of levels built on the false origins of S,. As shown recently for the case of pyrene [ 121, a quantum-mechanical description based on such a model is instrumental in reproducing both the intensities of the spectrum in the crowded region of S2 and the band contours observed in a resolved spectrum detected by the jet supersonic technique. In case of intermediate-level behavior, deviations from the Lorentzian band shape are to be expected [23]. Moreover, this kind of approach is able to gain insight into the dynamical processes that follow direct excitation into Sz, and to explain the presence of fast and slow components in the decay [6]. In fact it is expected that the S2-‘SI process is not an irreversible one and that recurrence may occur on the same
Volume 169, number 6
CHEMICAL PHYSICS LETTERS
time scale as the fluorescence decay, with subsequent delayed emission.Therefore a detailed spectral study of the Sz region is needed in order to get a thorough description of the spectral and dynamical behavior of this molecule.
References [ I] J.W. Hilpem, Trans. Faraday Sot. 61 (1965) 605. [2] J.B. Birks, J.M. de C. Come and G. Walker, Phys. Letters 19 (1966) 3325. [3] R.M. Hochstrasser and R.D. McAlpine, J. Chem. Phys. 44 (1966) 3325. [4] T. Elsaesser, F. tinner, W. Kaiser, B. Dick, M. Niemeger and W. Lilttke, Chem. Phys. 126 (1988) 405. [ 51 B. Nickel and J. Hertzberg, Chem. Phys. 132 ( 1989) 2 19. [6] J. Hertzberg and B. Nickel, Chme. Phys. 132 (1989) 235. [7] R. Breslow, Accounts Chem. Res. 6 (1973) 393. [ 81 A. Bree, A. Leyderman, P.R. Salvi and C. Tahani, Chem. Phys.110(1986)211. [9] A. Bree, Chem. Phys. Letters 131 (1986) 65.
22 June 1990
[IO] J. Wessel and D.S. McClure, Mol. Cryst. Liquid Cryst. 58 (1980) 121. [ 1 l] G. Fischer, Chem. Phys. 4 (1974) 62. [ 121 N. Ohta, H. Baba and G. Marconi, Chem. Phys. Letters 133 (1987) 222. [ 1311. Zanon, J. Chem. Sot. Faraday Trans. II 69 (1973) 1164. [ 141 G. Orlandi and G. Marconi, Chem. Phys. Letters 53 ( 1978) 61. [ 151 G. Marconi and P.R. Salvi, Chem. Phys. Letters 123 (1986) 254. [ 161 G. Marconi, Chem. Phys. 57 (1981) 31 I; J. RamanSpectry 14 (1983) 28. [ 171 G. Marconi, P.R. Salvi and R. Quacquarini, Chem. Phys. Letters 107 (1984) 314; P.R. Salvi and G. Marconi, J. Chem. Phys. 84 (1986) 2546. [IS] G. Orlandi, Chem. Phys. Letters 44 (1976) 277. [ 191 D.P. Craig and G.J. Small, J. Chem. Phys. 50 ( 1969) 3827. [20] A.R. Gregory, W. Siebrand and M.Z. Zgierski, J. Chem. Phys. 64 (1976) 3145. [2 I] W.L. Pet&as, D.P. Strommen and V. Lakshaminarayaman, J. Chem. Phys. 73 (1980) 4185. [22] A. Girlando and C. Pecile, J. Chem. Phys. 69 (1973) 818. [23] C.A. Langhoff and G.W. Robinson, Chem. Phys. 16 (1974) 34.
621