Pram[.na, Vol. 17, No. 2, August 1981, pp. 163-186, © Printed in India,
Electronic emission spectra of o- and m-fluorobenzaldehyde vapours
M K HAQUE* and S N THAKUR
Department of Physics, Banaras Hindu University, Varanasi 221 005, India
*Present address: Department of Physics, T.N.B. College, Bhagalpur 812 007, India MS received 9 December 1980; revised 8 July 1981
Abstract. The high resolution n~* electronic emission spectra of o- and m-fluoro- benzaldehyde vapours in the region 365-560 nm excited in a discharge are reported for the first time. The spectra of both the compounds consist of the A 1A"-XXA'
fluorescence as well as the "~ 8 A ' - X 1A' phosphorescence bands. In the case of o- isomer, all the eleven out-of-plane vibrations have been observed in the fluorescence and the phosphorescence, though weakly in the latter, whereas in the case of m-iso- mer, only ten have been observed in the fluorescence and nine in the phosphorescence.
It is found that the most intense bands in both the fluorescence and the phosphorescence spectra of these molecules belong to the trans-O retainer.
Keywords. Fluorescence; phosphorescence; rotamer; isomer; emission spectra.
1. Introduction
The electronic spectra of o- and m-fluorobenzaldehyde were studied in the region 365-560 nm both in absorption (Padhey and Viladker 1966; Chandra and Sharma 1967; Srivastava 1968) and in emission (Srivastava 1968), but these workers made an erroneous interpretation of the observed bands as belonging to only one electronic system. The emission bands observed under high resolution in the present investi- gation can be differentiated into two types on the basis of their-rotational contours.
As in the case of benzaldehyde (Hellas and Thakur 1973) and p-fluorobenzaldehyde (Haque and Thakur 1978), here also the sharp bands have been attributed to the a - - X (phosphorescence) system and the diffuse bands to the A - - X (fluorescence) system.
There may be two different forms of o- and m-fluorobenzaldehyde called the trans-O and the cis-O retainers, and although the problem of rotational isomerism in these molecules has been studied by a number of workers both experimentally and theoreti- cally (Miller et al 1967; Drakenberg et al 1975; Green and Harrison 1976; Crowder and Northam 1969; Wasylishen and Schaefer 1971; Beck and Tomchuk 1972; Aw et al 1972; Bruce et al 1974), the study of this phenomenon in the electronic spectra has been made for the first time by Haque and Thakur (1979). It has been found that most of the prominent vibronic bands can be explained as due to the trans-O rotamer in both the molecules.
163
164 M K Haque and S N Thakur 2. Experimental Details
The samples of o- and m-fluorobenzaldehyde used in the present investigation were obtained from Koch Light Co. (U.K.) and were used without further purification.
The experimental details for recording and measuring the emission spectra wcre the same as described earlier (Haque and Thakur 1978). The absorption spectra were photographed on a 1.5 m Baush and Lomb grating spectrograph and the intensity variations were studied in the temperature range 80°C-240°C. The experimental details are given in Haque (1979).
3. Description of observed spectra
3.1 o-Fluorobenzaldehyde
The 00° band of A1A ' ' - X I A ' fluorescence emission is the strongest band in this system and lies at 26333.3 cm -1 in good agreement with its position at 26333.1 cm -1 in the absorption spectrum. There are two main peaks in its rotational contour with a separation of about 9 cm -1 and the lower wavenumber peak is the stronger of the two (figures la and 2a). The relative separations of all the bands in the spectrum have been obtained from the stronger peak. A sequence interval of + 27 om -1 is the next most prominent feature in the neighbourhood of the 0 ° band followed by the sequence intervals of -- 10 and -- 9.7 cm -1 in order of decreasing intensities. The rotational peaks of the ~ band group have a width of about 2 om -1 (full width at half of the maximum intensity, FWHM) which is the same as in benzaldehyde (HoUas and Thakur 1973) andp-fluorobenzaldehyde (Haque and Thakur 1978). The 0g band group contains about 80 % of the total intensity of the fluorescence and the rest is spread over the remaining weak vibronic bands among which the band at 25626.1 om -1 is relatively intense. The rotational contours of the weaker bandsare not as ciearly defined as those of the 0o ° band, and the assignments have been made by considering the separation of the most intense feature of the band group in question from the 0°o band. There are two features at -- 52.9 and -- 59.4 era -1 from the 0 g band which appear very prominently in the fluorescence emission but do not appear as prominently in the vapour absorption and do not show an appreciable change in their intensities as compared to that of the 0o ° band on increasing the temperature of the absorption cell.
The 0 ° band of the ff aA" - - X 1A' system has been identified at 24603.5 om -1 in good agreement with the liquid phase absorption at 24670 om -1 (Vander Donckt and Vogels 1972)2 The rotational contour of this band also consists of two peaks with a separation of about 7 cm -1 with the lower frequency peak showing more intensity (figures lb and 2b). Positive sequence intervals of 16.9, 26.8, 33.7, and 42.5 cm -1 appear with a gradual decrease in intensity. Three negative sequence intervals of -- 25.4, -- 34.6, and -- 41.6 cm -t also appear with appreciable intensities and each can be observed upto two members. A band at 24545.9 cm -1 appears with a rota- tional contour different from that of the ~ band and stands out amongst the bands in this region.
3792.156 A 3811.892 A 4055.038 A F~
4074.789 A
(a) (b) Figure 1. The different band regions of o-fluoroben7aldchyde (a) The 0 ° band region of the ,~--7~ system, (b) The 0 ° band region of the a--X system.166 M K Haque and S N Thakur
Fe 4367, 9 0 6 / , Fe. 4387,B96 A
O, 61 ,
6,%' 61 o 61 °~351 61 °3~I
Figure l(e). The 6 0 band region of the a--X system.
o- and m-fluorobenzaldehyde emission 167
,,I-- C
E
0 IfJ LIJ E
(o) ooo , 5~',,,
i 351 .~z ~
..,02 ,.... "5
I I I I I I
30 20 t0 0 -10 - 2 0 Wovenumber (crn -! )
r -
4-.
._o
W
36~
3~
I
O°o
zs~
3~
3~
(b)
Oo o
ds O?4 0
I I I I I, I I I I i
30 20 10 0 -10 - 2 0 - 3 0 - 4 0 - 5 0 - 6 0 Wovenumber (cm" 1 )
Figure 2. Microdensitometer traces of the 0o ° band ;regions of o-fluorobenzaldehyde (a) The , ~ - X system. (b) The "a--~" system.
168 M K Haque and S N Thakur 3.2 m-fluorobenzaldehyde
--1 ip
The 0 o band of the A A -- X I A ' system has been identified at 26729.7 c m -I in good agreement with its position at 26729.9 c m -1 in the absorption spectrum (Haque 1979).
The rotational contour of this band consists of two diffuse peaks separated by 9 c m -I the higher frequency component having a very weak and broad structure as compared to the lower frequency component (figures 3a and 4a). The most intense band coming next to the 0°0 band is a sequence band at 26712.2 c m -I with a scquence interval of --17.5 c m -~. The other two sequences associated with the 0 o band involve frequency intervals of ÷ 26.9 c m -I and --24.7 c m -I. About 8 0 % of the total intensity of the fluorescence system lies in the 0OO band group and the rest 20 Yo is spread over the remaining band groups.
The 0 o band of the ~ ~A" -- ~( 1A' system lies at 24984.0 c m -I in agreement with its position at 25000 c m -I observed in the liquid phase absorption (Vander Donakt et al 1972). The rotational contour of this band consists of avery sharp peak with a very weak shoulder on the higher frequency side (figures 3b and 4b). O n the higher frequency side of the 0Oo band there are sequence intervals of -{-9.7, -[-19.6 and +28.1 c m -1 with intensities gradually decreasing in that order and each sequence band has a doublet structure. O n the lower frequency side of the 0 o band, two types of band contours can be seen, the first one with only one peak and the other with two peaks.
These bands form three main sequence intervals -- 17.8, --32.7 and -- 57.1 c m -~ of which the first sequence can be observed upto four members and the remaining two upto two members each.
In both the mole~ales the most intense band Goming next to the 00 band of the a" -- X system is the 6° band representing the first quantum of C = O stretching vibra- tion in the ground state (figures Ic and 3o). This band lies at 22884-5 c m -I in o-fluoro- benzaldehyde and at 23254.3 c m -1 in m-fluorobenzaldehyde. The 60 band is also accompanied by the same sequences as the 0~ band. The C ---- O stretching vibration in the ground state forms a strong progression which is three members long. The intensities of the members as well as the separations between them go on decreasing as we move along the progression towards higher members. The structure of the band groups in the region between the 0 ° band and 6° band is repeated in the regions between the successive members of the progression.
4. Vibrational analysis
In an earlier publication (Haque and Thakur 1979) we have shown that trans-O rotamers in both ortho and metafluorobenzaldehyde are more stable than the cis-O rotamers. W e have therefore attributed the prominent vibronic bands of the two molecules observed in the present investigation to the trans-O retainers.
4.1 o.Fluorobenzaldehyde
4.1a The ~-- X emission bands: The vibronic bands in the region of the Oo ° band of the triplet-singlet (~'-- X) emission show some variations in the separations between the sharp peaks in their rotational contours (figures lb and 2b). If there are two retainers
Fe
37,,.176
A1_
i3756io 25~ ~6o
~A 3994.117 A 4016.429 A
36~ ~,' °o ° ~'~ ~l
2 r ~ Figure 3. The different band regions of m-fluorobenzaldehyde. (a) The 0~ band region of the A- X system. ;egion of the--~X a system, (b) The Oo ° band
~°
170 M K Haque and S N Thakur
Fe 4294.127 A Fe 4 3 1 5 . 0 8 7 A
60~ J60~'
Figure 3(e). The 6] band region of the a - X system,
o. and mofluorobenzaldehyde emission 171
"iTi
C Q;
"E
C 0 WJ
E
Oo
0I
10
(o)
1 I I I
0 - 1 0 - 2 0 - 3 0 Wavenumber (cm -1)
u'l C O;
r -
.9 (/1 I,'1
'[:
LU (b)
I
2O tO
Oo
0P
3~
2~I
35~"11 i ~ 25~
35, 30, A' ~ 25', ~',
I I I I I I
0 - 1 0 - 2 0 - 3 0 - 4 0 - 5 0 Wavenumber (cm -1 )
Figure 4. Microdensitometer traces of the 0~ band regions of m-fluorobonzaldehyde.
(a) The ~ - ffsystem. Co) The ~'- ~system.
172 M K Haque and S N Thakur
giving rise to these bands, then the simplest way to identify them is to look into the region of 60 band (figure lc). In the trans-O retainer of o-fluorobenzaldehyde there will be a charge stabilization involving the slightly positively charged hydrogen atom of the aldehyde group and the electronegative fluorine and oxygen atoms (Aw et al 1972). This has been referred to as intramolecular hydrogen bonding and it may lead to a larger effective mass of the oxygen atom thereby decreasing the C = O stretching frequency of the trans-O returner as compared to the value of this vibrational fre- quency in benzaldehyde (Haque and Thakur 1979). In the case of cis-O- retainer no such intramolecular hydrogen bonding is to be expected and the C = O stretching frequency Will be the same as that in benzaldehyde. We have observed that all the sequence bands that are present in the 0 ° band region are also present near the 60 band (figures lb and lc). This similarity indicates that all the prominent bands of this electronic system may be analysed in terms of only one rotamer. For the assign- ment of the vibronic bands their rotational contours as well as their separations from the 0 ° band have been taken into consideration. This becomes necessary in view of the fact that the rotational contours show significant variations amongst relatively weak vibronio bands. In assigning low frequency intervals to the in-plane and the out-of-plane bending vibrations involving the substituents, we have used the recom- mendations of Varasanyi (1969). Thus, o-fluorobenzaldehyde has to be treated as a 1, 2 di-light substituted benzene as far as the assignments of the planar vibrations are concerned but it has to be treated as a I-light-2 heavy substituted benzene for the assi~ments of the non-planar vibrations. The assignments of the vibronio bands of this system are given in table 1 where Mulliken's (1955) notation has been used for the normal modes of vibration.
The sequence bands of the 0 ° band group have been assigned on the basis of the Deslandre's schemes (Haque 1979). It is found that the strongest sequence of + 17 em -1 results from the torsional vibration which is found to be less anharmonic in the excited state than in the ground state. The sequence interval of --25 cm -1 has been assigned as 2511, where v~ corresponds to the in-plane bending motion of the CHO group with respect to the phenyl ring. The CHO out-of-plane bending vibration (vs5) gives rise to a sequence interval of -- 35 cm -1. The 6~ band is found to be the most intense after the 0o ° band and it involves a vibrational interval of 1719 cm -1 for the C = O stretching vibration. The value of v~= o in benzaldehyde and p-fluorobenzaldehyde are 1728 and 1729 cm -1 respectively and a value of 1719 cm -~
in the trans-O retainer of o-fluorobenzaldehyde suggests a weak intramoleoular hydrogen bonding involving the aldehyde hydrogen (Aw et al 1972). The aldehyde hydrogen is slightly positively charged and it will tend to attract both the oxygen and the fluorine atoms which are slightly negatively charged. The hydrogen bonding will thus lead to a decrease in the double bond character of the C = O bond and a consequent decrease in the bond stretching force constant.
The C = O stretching vibration is found to be anharmonic and the values obtained from the members of the progression for three successive ground state vibrational intervals are 1719, 1703 and 1678 om -1 respectively.
The vibronie bands 13 o and 1410 appear strongly and have been assumed to involve the C = C stretching and the ~-CHO stretching vibrations respectively. In p-ttuoro- benzaldehyde trlplet-singlet emisslon, the former does not appear while the latter appears only weakly. Two more C = C stretching vibrations appear in the vibronic bands 72 and 10 °, each with nearly half the intensity of the 13~ band. The vibrational
o- and m-fluorobenzaldehyde emission 173 Table 1. Wavenumbers and assignments of the principal band groups in the "a - ~"
phosphorescence spectrum of o-fluorobenzaldehyde.
Wavenumber Relative Separation from
(cm -1) intensity 0o ° band (cm -1) Assignment
(1) (2) (3) (4)
P . - . 4
25074"7 1 + 471"2 34]
24954"4 2 + 350.9 350 ~
21.7 1 + 318"7 25]; 350 z
24888"5 2 + 285"0 36ol; 25[
56"5 1 + 253"0 36]
39"4 1"5 + 235"9 340 ~
24795-8 3 + 192"3 34~
79"5 3"5 + 176"0 35~
62"2 2 + 158"7 25~
45"8 2 + 142"3 35~; 36~
39"4 2 + 135.9 25~
31"2 4 + 127"7 360 ~
24646"0 3 + 42'5 36~
37"2 5 + 33"7 36]
30"3 7 + 26"8 36~
20"4 10 + 16"9 361
03"5 10 0 0o °
24578"1 9 -- 25"4 251
68"9 8 -- 34"6 351
61"9 4 -- 41"6 341
56'5 4"5 -- 47"0 25~
24545.9 6 -- 57.6 0~ (cis-O)
05"5 3 -- 98'0 36 t,
24493"2 2"5 -- 110"3 36x °
19'1 2 -- 184'4 250
01"9 4 -- 201"6 251 ,
24391"8 3 -- 211"7 35~.
77"7 2 - - 225"8 36 o
(Contd.)
174 M K Haque and S N Thakur
T a b l e 1 (Contd.)
(1) (2) (3) (4)
54"2 4
25"2 2"5
24286"5 2"5
61 "8 3
38"2 2'5
24176"9 3"5
47"1 3'5
24097" 1 4
77"5 1
64"5 1
57"4 7
49'4 2
34"9 8
0 9 ' 9 3
23976"4 1'5
23895"3 1'5
39.2 2
03"3 3
23760"3 7
56'2 3
23648"4 1
09"5 0"5
23599'5 0'5
77"3 0.5
04'5 1'5
23453" 1 2
20'3 1
23372'9 5"5
26.7 6
23294"3 3"5
-- 249"3 - - 278.3 - - 317.0 - - 341'7 - 365.3 - - 426.6
-- 456.4 - - 506.4 - - 526.0
-- 539.0
-- 546"1
-- 564"1
-- 568"6
- 593"6 - - 627.1 - 708.2 - - 764.3
-- 800.2
-- 843.2 - - 847"3
-- 955"1
-- 994"0
-- 1004.0 - - 1026"2
-- 1099"0 - - 1150"4 - - 1183"2 -- 1230"6 -- 1276.8 -- 1309"2
35~
34~
3421 .24~
25g 350 330 230 32~
22, 0 261 36~ t 34o~
261 261 2110 31~
3o~
200 19°1 29, 0 28*
27,0 2610 1810 1710 1610 1510 14, o 13~.
12 °
T a b l e 1
o- and m-fluorobenzaldehyde emission (Contd.)
(I) (2) (3) (4)
06.1 2.5 - - 1397.4
23138-6 4 - 1464.9
15.7 2.5 - - 1487.8
2 3 0 1 7 . 0 1 - 1586.5
2 2 9 8 3 . 0 4 - - 1620.5
13.6 5 - - 1689.9
06.5 7 - - 1697.0
22884-5 9 - 1719.0
60"2 5 - - 1743"3
51"8 4 - - 1750"7
4 6 ' 6 2 - - 1754"9
38"4 2 - - 1765'1
34"6 2 - - 1768"9
22705"2 1"5 - - 1898"3
22673"4 1 - - 1930"1
22597"5 2"5 - - 2 0 0 6 " 0
22343"4 1"5 - - 2260"1
2 2 0 8 9 " 0 1 - - 2514"5
40-6 2 - - 2562"9
21656"1 3 - - 2 9 4 7 ' 4
0 9 . 0 4 - 2994"5
2 1 5 7 7 . 0 1 - 3 0 2 6 . 5
2 1 2 6 6 . 7 2 - 3 3 3 6 . 8
21181"2 6 - - 3422"3
19503"0 4 - - 5100"5
10'0
9'0 8~
7'0
60 36]
6? 36i 6'0
61 25[
6~. 351
6, 0 3 4 |
6° 25]
6~(cis-O)
6'0 25'0 60 35, 0
260
6'0 22'0
6 ° 1 2 ~
6'0 19'0
6'0 14~
6~. 13~
61 12g 6; 71o
6 . 0 61
1 7 5
176 M K Haque and S N Thakur
interval of 843 cm -1 is associated with a very strong vibronic band 19 °. This is the intense component of the doublet observed by Green and Harrison (1976) in the infrared spectrum that persists at low tempera.tures also. The vibrational intervals of 627 and 1026 cm -1 appear with the bands of medium and weak intensities and have been assigned as 210 and 180 respectively. These vibrational frequencies also represent those components of the doublet bands in the infrared spectrum that have been observed by Green and Harrison (1976) in the solid phase.
The bands 26 °, 270, 280, 29 °, 30 °, 31 °, 32 °, 331 °, 340, 350 and 36 o all involve out-of plane vibrations. The rotational contours of these bands are diffuse and appear to have two peaks with separation of 3 to 4 cm -1.
4. lb The .~-- -X emission bands: The vibronic bands observed in this system are given in table 2. The 0~ band is expected to have a type C rotational contour; as can be seen from figures la and lb, the observed rotational contour o f the 0 ° band o f this system is considerably different from, and not as sharp, as that o f the 0 ° band o f the triplet-singlet system. The torsional vibration (v3s) appears in the vibronio band at 26223 cm -x and it shows a diffuse rotational contour about 12 cm -x in width.
The most prominent sequence band at 26361 cm -~ has been interpreted as 36ll which fits in very well with the assignment of an absorption band at 26470 cm -~ as 36 °.
The next intense sequence interval of --19 cm -1 has been assigned as 25~ with 250 showing a very diffuse rotational contour in a band at 26148 cm -1 and 25o 1 is seen in the absorption spectrum at 26478 om -1. The sequence interval of --10 cm -x is the third in the order of decreasing intensities and it has been assigned as 3511. In the fluorescence emission spectrum 35 o has not been observed, but there is a strong band in the absorption spectrum at 26534 cm -x which may be assigned as 351o. The value of v"3s from the phosphorescence spectrum is 212 cm -1.
The only other prominent band in the fluorescence system i~ located between 25626 and 25618 cm -x consisting of two broad peaks. The rotational contour of this band does not have any resemblance with that of the 00 ° band. The separation of the higher wavenumber peak from the 0o ° band agrees with an out-of-plane ring vibration o f 708 cm -x involving mainly the motion of the hydrogen atoms. This vibration also appears in the excited state in the absorption spectrum with a frequency of 692 cm -~.
Table 2. Wavenumbers and assignments of the principal band groups in the A--X fluorescence spectrum of o-fluorobenzaldehyde.
Wavenumber Relative Separation
from 0g band Assignment
(cm -I) intensity (cm_l)
(1) (2) (3) (4)
26384"4 5 + 51'1 36~
60"6 9 ÷ 27"3 361
33"3 10 0 0o °
26273.9 7 -- 59'4 0o ° (cis-O)
48-0 3 -- 85"3 36~
23.2 3 -- 110"1 36, 0
(Contd.)
o- and m-fluorobenzaldehyde emission
Table 2 (Contd.)
177
(1) (2) (3) (4)
26147"9 3 -- 185"4 25°
10"2 2'5 -- 223"1 36~ °
26053"7 2"5 -- 279"6 340
03"5 2 - 329"8 360
25987"8 1'5 - 345'5 240
25872"9 3 -- 460"4 330
32"3 1 - 501"0 230
05"0 2 -- 528"3 32]
25777"8 2"5 -- 555'5 221 °
04'4 1"5 -- 628"9 210
25626.1 4 -- 707"2 31°
25572"9 1 -- 760"4 30°
33'3 2 -- 800'0 200
25489'3 1'5 -- 844"0 190
85.1 1"5 -- 848'2 2910
25373"7 1'5 -- 959.6 280
40-3 2 -- 993"0 270
16"8 3 -- 1016"5 260
06.5 3 -- 1026.8 18 °
27.6 2 -- 1105"7 17~
2 5 1 8 0 ' 2 1"5 -- 1153'I 16°
52'0 2 -- 1181"3 150
07"I I -- 1226'2 140
67"7 2 -- 1265'6 130
39"3 2 -- 1294"0 12~ °
178 M K Haque and S N Thakur
A vibronic band at 26274.6 em -x in the absorption spectrum and one at 26273.9 cm -1 in the fluorescence spectrum could not be assigned as sequence bands and may be assigned tentatively as the 0°0 band of the .~ 1A" -- X XA' system of the cis-O retainer. The relative intensity of this probable ~ band of the eis-O rotamer compared with that of the 0 ° band of the trans-O returner is more in the emission spectrum and less in the absorption spectrum. The intensity factor and the fact that it is red-shifted with respect to the 0 ° band of the trans-O rotamer suggest that the relative stability of the cis-O rotamer increases in going from the ground electronic state to the first excited singlet (n~r*) state. A band at 24545.9 cm -1 in the triplet- singlet spectrum may also be tentatively assigned as the 0°0 band of the metastable cis-O retainer. The separation of this band from the 0 ° band is -- 57.6 era -x, which may be compared with the separation -- 59.4 cm -x of the cis-O 00 ° band from the trans-O 0°0 band in the fluorescence spectrum. Miller et al (1967) have estimated a separation of about I00 cm -x between the ground state potential minima of the trans.-O and the eis-O retainers. Our temperature dependence studies of the absorp- tion spectrum, however, do not show an appreciable increase in the relative intensity of t h e ' proposed' cis-O 00° band.
4.2 m-fluorobenzaldehyde
4.2a The A - - X emission bands: The region of the 0 ° band of the triplet-singlet emis- sion consists of two types of prominent bands. The rotational contours of one type show two peaks separated by 2-3 cm -x whereas those of the other type show only one peak.
At first sight it appears as if the single-peaked and the double-peaked rotational contours in the 0 g band region belong to two different retainers. In such a ease one expects to observe a different pattern of single- and double-peaked bands in the region of 60 band. However, all the sequence bands associated with the 0~ band have also been found to be associated with the 60 band (figures 3b and 3c). If the potential minima for the two rotamers had the same separation in the two electronic states, the 0 ° bands of the two retainers would be coincident, but even then one would expect to get two bands in the 60 band region due to different frequencies of the C = O stretching vibration in the two retainers. The absence of such bands indi- cates that only one retainer is important in the electronic spectrum, and accordingly the prominent bands observed in the spectrum have been assigned to the trans-O retainer (Haque and Thakur 1979). We also tried to explain the vibrational structure by assuming that the rotational contour of the 0 ° band has two closely-spaced peaks (2 cm -1) separated by a single peak on the lower frequency side. This assumption, however, leads to a number of inconsistencies in the vibrational analysis. The assumption of rotational contours of different types for the 0o ° band and the sequence bands (figure 3b) leads to the most consistent assignments of a majority of bands in the T-S emission spectrum.
Keeping in view the fact that m-fluorobenzaldehyde is a 1, 3-di-light substituted benzene for planar vibrations, and a 1-1ight-3-heavy substituted benzene for out-of- plane vibrations (Varsanyi 1969) the assi~ments of the vibronic bands of the a-X system have been made as given in table 3, using Mulliken's notation (Mulliken 1955).
o- and m-fluorobenaaldehyde emission 179 Table 3. Wavenumbers and assignments o f the principal band groups in the a - X phosphorescence system o f m-fluorobenzaldehyde.
Wavenumber Relative Separation
from 0 g band Assignment
(cm -~) intertsity (cm_l)
(1) (2) (3) (4)
25179.0 0"5 + 195'0 34~; 35o x
50"8 2 + 166"8 2501
39"6 1 + 155'6 25~
34"8 2 + 150"8 35~
28"6 4 + 144"6 34[
07'6 1"5 + 123"6 36~
00.0 1"5 + 116"0 3601
25012.1 2 + 28"1 36]
03"6 3 + 19"6 36~
24993"7 6 + 9"7 361
84"0 10 0 0g
76"0 3 -- 8"0 251 36~
1
66"2 8 -- 17'8 25, ~
59"8 5 -- 24.2 35x t 361
51"3 8 -- 32'7 35, t
44.0 5 -- 40'0 25]
34"9 5 -- 49"1 2511 35x t
29"0 5 -- 55.0 25~
26"9 5 -- 57"1 345
24919.0 1 -- 65"0 35~
24890"6 1 -- 93"4 36~
75.5 4 -- 108.5 364; 34~
24798"1 3 -- 185"9 25 o
69-3 2 -- 214"6 36~; 25|
59'6 3 -- 224.4 350
32"2 4 -- 251"8 34°; 351
24603.6 2 -- 380"4 25 o
(Con~)
180
T a b l e 3
M K Haque and S N Thakur (Contd.)
(1) (2) (3) (4)
24579-6 1-5
42-2 1
27"7 1
24481"8 1"5
62-0 1
05"2 6
24337"3 1"5
08"6 2
24219"1 2"5
01-9 2"5
24129"0 1
24098-8 2
79"0 2
22"3 1
23979-3 3
11"0 1
23853"6 7
20"3 2
23764"8 1"5
24"8 2
23686"0 4
23599"6 5
24.3 3"5
01"6 2
23398"0 4
7 5 ' 8 4
23254"3 8
22978'2 1"5
21549"0 4
19876"0 3
-- 404"4 - - 441.8
-- 456"3 - - 502"2 - - 522"0
-- 578"8 - - 646.7 -- 675"4 - - 764"9 - - 7 8 2 q
-- 855.0
-- 885"2 - - 905"0 - - 961"7 - - 1004.7 - - 1070"0 - - 1130"4 - - 1163.7
-- 1218"4
-- 1259"2
-- 1298'0
-- 1384-4
-- 1459"7
-- 1482"4
-- 1586"0
-- 1608.2 - - 1729'7
-- 2005"8
- - 3435"0
-- 5108"0
24,o 33,°; 35~
23, o 34°
22o 2611 21, o 31°
201 o 3OO 29°
19°
28o 27,o
18°; 261°
171 o 16, o 157 141°
13~
12~
11°
101°
8,*
7, o
6,*
26o 6°
6°
o- and m-fluorobenzaldehyde emission 181 Most of the sequence bands have been assigned on the basis of Deslandre's schemes for v~, v~, v~ and v~6. The sequence interval of -~- 9.7 cm -1 has been assigned to the torsional vibration (v~) of the CHO group. All the four members of this sequence that appear on the plates show two prominent rotational peaks each. The sequence interval of -- 17.8 om -1 has been assigned to the in-plane bending vibration of the CHO group (v26). The out-of-plane CHO wagging vibration is found to give rise to the sequence interval o f - 32.7 om -x.
The most intense band after the 0 ° band is at 0-1729.7 om -1 which corresponds to the C = O stretching vibration. In the cis-O rotamer of m-fluorobenzaldehyde, due to a weak intramoleeular hydrogen bonding, the C = O stretching frequency is expected to decrease from its value in benT_aldehyde (1728 crn-1), whereas in the case of trans-O rotamer no such decrease is expected. Therefore, the observed value of 1729"7 em -~ has been taken to correspond to the trans-O rotamer.
The vibronic bands 16~ ° and 18~ ° appear very prominently, and have the same struc- ture as the 0 ° band. Of these, the first band corresponds to the in-plane C-H bending vibration of the ring with a frequency of 1130 cm -1 whereas the second band cor- responds to the in-plane bending vibration of the ring involving a frequency of 1005 cm -1. Other vibronic bands that appear with medium intensities are 11~, 121°
34~, 7~, 8 °, 9~, and 10~ ° with intensities decreasing in that order. A band at 0-1259.2 cm -x, assigned as 13~, appears with an appreciable intensity and it may be correlated with the higher frequency component of a pair of bands observed by Miller et al (1967) at 1256 and 1248 cm -1. This vibration may involve C - - C stretching as well as C - - F stretching vibrations and it also favours the trans-O rotamer as the emitting species.
4.2b The A - - X emission bands: The vibronio bands observed in this system and their assignments are given in table 4. A band at 26621.3 crn -1 which has about half the intensity of the 0°0 band has been identified as due to the torsional vibration of the Table 4. Wavenumbers and assignments of the principal band groups in the .~-~"
fluorescence spectrum of m-fluorobenzaldehyde.
Wavenumber Relative Separation
(cm -1) intensity from 0g b a n d Assignment
(1) (2) (3) (4)
26797.0 0.5 + 67.3 36~
79.0 0.5 q- 49-3 36|
56.6 2 + 26.9 361
29.7 10 0 0g
12.2 9 -- 17.5 351
05.0 3 - 24.7 251
26696.1 6 -- 33'6 35|
84.8 3 - 4 4 ' 9 25~
21"3 5 -- 108"4 36, 0
(Contd.)
1 8 2 M K Haque and S N Thakur
T a b l e 4 (Contd.)
(1) (2) (3) (4)
26542"4 4 - - 187.3 25~
13"9 2 - - 215.8 35~
26470.3 2 - - 2 5 9 . 4 340
26323"1 3 ' 5 - - 4 0 4 . 6 24, °
26297.1 2 - - 4 3 2 . 6 33~
78"7 5 - - 451.3 230
09"7 0"5 - - 520"0 22~
26169"6 2 - - 5 6 0 " I 320
26083"3 1-5 - - 646"4 210
53"5 2 - - 676"2 311 °
25967"6 3 - - 762.1 2 0 o
25946"8 4 - - 7 8 2 . 9 30°
2 5 8 7 2 . 2 3"5 - - 857.5 29~
4 5 ' 4 5 - - 884.3 190
2 0 . 7 1 - - 909"0 28~
25768"7 0"5 - - 961-0 27~
31.5 5 - - 9 9 8 . 2 181 °
25657"3 2 - - 1072.4 170
0 0 . 7 I - - 1129.0 16°1
2 5 5 6 9 ' 6 3 - - 1160.1 15~
04"7 3 - - 1225.0 141°
25470"2 1 - - 1259.5 13~
31"1 2 - - 1298.6 120
25"4 4 - - 1404-3 l l 0
2 5 2 8 4 ' 4 6 - - 1456"3 10~
44"7 4 - - 1485.0 90
25144"8 3 - - 1584"9 81°
19"7 4 - - 1610.0 7 ]
o- and m-fluorobenzaldehyde emission 183 CHO group in the ground state and assigned as 36~ °. The corresponding vibration in the excited state (3610.) has been observed at 26869.1 cm -~ in the absorption spec- trum (Haque 1979). This vibration gives rise to the main sequence with a sequence interval of + 26.9 cm -z. The in-plane (vm) and the out-of-plane (vs5) vibrations of the CHO group give rise to an intense band at 26542.4 cm -1 and a relatively weak band at 26513.9 cm -1 respectively. The sequences involving these vibrations have also been observed. An intense sequence band (35]) with a sequence interval o f - - 17.5 cm -~ appears at 26712.2cm -1 and a fairly intense band (25]) with a sequence interval o f - 24-7 cm -1 has been identified at 26705.0 cm -1. All the above sequences show the first two or three members in the emission as well as in the vapour absorption spectra (Haque 1979). Other vibronic bands that appear with appreciable intensities are 23~ °, 29 °, 18~ °, 10~, 9~, 8~ °, 15 °, 14~, 30~ °, 31° with intensities decreasing in that order.
5. Results and discussion
The vibrational frequencies of the two molecules in their electronic ground states obtained from the T 1 -~ S O and S I -~ S O emissions have been summarized in table 5.
The vibrational intervals not observed in the triplet-singlet or singiet-singlet emission have been taken from the infrared and Raman spectra (Green and Harrison 1976).*
The descriptions of vibrational modes in the assignment of the vibronic bands given in the last column of table 5 are those given in Varsanyi (1969). Some of the frequen- cies of the two molecules although arranged in the same row bearing the same vibra- tion numbering as in Mulliken (1955) correspond to different vibration numberings in Wilson (1955). Thus in m-fluorobenzaldehyde the frequency 1259 cm -1 corres- ponds to C-C stretch (via), 1164 to i.p. C-H bend (Vgb), 1130 to i.p. C-H bend (VlSb), 1070 to i.p. C-H bend (v18~), 1005 to ring bend (v~), 404 to i.p. C-H bend (v15), 905 to o.19. C-H bend (Vl~a) and 855 cln -1 to o.p. C-H bend umbrella (vl~b). The vibrational assignments given in the last column of table 5 corresponding to these frequencies are for o-fluorobenzaldehyde.
In o-fluorobenzaldehyde all the eleven out-of-plane vibrations appear in the fluore- scence spectrum, vsl being the most prominent. In the triplet-singlet emission also, all the out-of-plane vibrations are present, although weakly. In m-fluorobenzalde- hyde ten of these vibrations appear in fluorescence and nine in phosphorescene. The appearance of non-totally symmetric vibrations in single quantum is electronically forbidden and the vibronic bands involving them result from Herzberg-Teller type intensity stealing. Zwarich and Goodman (1970) have examined the Herzberg-Teller mechanism in the case of benzaldehyde, and have predicated that vsl is the most active mode in both phosphorescence and fluorescence spectra. It is observed that v3~ is involved in a very weak band in the phosphorescence spectrum although it appears very strongly in the fluorescence emission of o-isomer whereas in m-isomer it appears strongly both in fluorescence and in phosphorescence. This observation may be compared with that in the electronic emission of p-fluorobenzaldehyde where vs~ does not appear at all in the phosphorescence although it gives rise to the strongest
*Captions to tables3 and 4 in this paper (Green and Harrison 1976) have been interchanged due to printing errors. Table 3 contains data for ortho compounds and table 4 those for meta- compounds.
184 M K Haque and S N Thakur
Table 5. Ground state vibrational frequencies in o- and m-fluorobenzaldehyde.
Wavenumber (cm -~)
Vibration* o-fluoroben- m-fluoroben- Assignment**
zaldehyde zaldehyde
1 3089 a 3082 b
2 3078 a 3074
3 3054 a
4 3025 a 3030 b
5 2867 a 2843 a
6 1719 1730
7 1620 1608
8 1587 1586
9 1488 1482
10 1465 1460
11 1397 1384
12 1309 1298
13 1277 1.259
14 1231 1218
15 1183 1164
16 1150 1130
17 1099 1070
18 1026 1005
19 843 885
20 800 765
21 627 647
22 539 522
23 506 456
24 342 404
25 184 186
26 1004 1003
27 994 962
28 955 905
29 847 855
30 764 782
31 708 675
32 526 560
33 456 442
34 278 252
35 212 224
36 110 108
C-H stretch C-H stretch C-H stretch C-H stretch
aldehyde C-H stretch C = O stretch C-C stretch (vsb) C-C stretch (vsa) C-C stretch (vl,b) C-C stretch (v19a) aldehyde C-H bend i.p. C-H bend (v3) C-C stretch (vl~)
~-CHO stretch (vTa) (v~3)
i.p. C-H bend (v15) i.p. C-H bend (yah) i.p. C-H bend (Vl~b) i.p. ring bend (v~2) ring breathing (v~) (veo)
(v,b)
i.p. C-H bertd (visa) i.p. ring-CHO bend (V~a) aldehyde H wag o.p. C-H bend (vs)
o.p. C-H bend umbrella (vxTD o.p. C-H bend (rata)
o.p. C-H bend (vlt) o.p. ring bend (v4) o.p. ring bend (v16a) o.p. ring bend (Vaeb) o.p. C-F bend (vloa)
¢,-CHO wag
¢-CHO torsion
*(Mulliken 1955)
**(Varsanyi 1969; Wilson et al 1955)
aLiquid phase infrared values (Green et a11976)
~Raman values (Green and Harrison 1976)
vibronie b a n d o f the fluorescence system ( H a q u e a n d T h a k u r 1978). O n l y t w o o f the out-of-plane vibrations have been observed in the fluorescence spectrum o f p-fluorobenzaldehyde ( H a q u e a n d T h a k u r 1978). T h u s the Herzberg-Teller m e c h a n i s m appears t o be m o r e effective in the spectra o f o- a n d m-fluorobenzalde- h y d e f o r the presence o f electronically f o r b i d d e n b a n d s t h a n in t h e s p e c t r u m o f p-fluorobenzaldehyde.
o- and m-fluorobenzaldehyde emission 185 In the phosphorescence spectrum of o-fluorobenzaldehyde the vibronic bands involving intervals of 1277, 1231 and 843 cm -1 appear very prominently, and these have been identified as C-C stretching, ¢-CHO stretching and in-plane ring bending frequencies respectively. The corresponding bands with intervals of 1259, 1218 and 885 cm -a respectively also appear prominently in the phosphorescence spectrum of m-fluorobenzaldehyde. In the phosphorescence spectrum of p-fluorobenzaldohyde, however, none of these vibrations is associated with the intense bands.
The large number of vibronic bands lying to the higher frequency side of the 0o ° bands in the triplet-singlet emissions of both the molecules shows that the vibrational populations in the triplet states in these cases are quite different from those in bonzal- dehyde and in p-fluorobenzaldehyde.
The tentative assignment of the 0 ° bands of the cis-O retainer of o-fluorobenzal- dehyde in the fluorescence and the phosphorescence spectra at 26274 and 24546 cm -1 respectively gives a triplet-singlet energy separation of 1728 cm -1 in the cis-O rot0aner as compared to 1730 cm -1 in the more stable trans-O rotamer.
Koyanagi and Goodman (1979) have assigned some of the prominent bands in the a - - X system of isotopic benzaldehydes as involving the out-of-plane aldehyde hydrogen wagging vibration (v26). The 26~ band lies at 0-587 cm -1 in the phosphore- scence spectrum of benzaldehyde -h e and at 0-548 em -~ and at 0-578 cm -x in those of benzaldehyde -ld 1 and benzaldehyde -Rd s respectively. It is found that v2e de- creases by about 409/0 in going from the ground electronic state to the 7'1 state. Similar prominent bands have been observed in the spectra of o- and m-fluorobenzaldehyde and have been assigned in analogy with the spectra of isotopic benzaldehyde. The bands at 22597.5 cm -t and at 22978.2 cm -1 in the phosphorescence spectra of o- and m-fluorobenzaldehyde respectively have been assigned as 260 leading to a value
1 t t •
of 1003 cm- for v=e m both the molecules. The 26~ bands are observed at 0-568.6 em -1
~nd 0-578 em -1 in the spectra of o- and m-fluorobenzaldehydo respectively. The uncertain assignment of a band at 0-587.6 cm -t in the p-fluorobenzaldehyde phos- phorescence spectrum (Haque and Thakur 1978) should now be changed to 2611; the 260 band has been identified at 23477.3 cm -1 leading to a value of 1004 cm -~ for v~6.
In the vapour phase phosphorescence emissmn of chlorobenzaldehydes (Haque et al 1980) also 26~ t can be identified with prominent bands at 0-539 cm -1 in o-chloro- benzaldehyde at 0-575 cm -1 in m-chlorobenzaldehyde and at 0-583 cm -1 in p-chloro- benzaldehyde. The percentage decrease in v26 in each of these molecules is similar to that in benzaldehyde. There seems to be a very slight substituent effect in v~6 which decreases in going from ortho to meta to para substitution in both the fluoro and the chlorobenzaldehydes.
Acknowledgements
The authors would like to thank Dr J M Hellas, Chemistry Department, University of Reading, England, for providing the necessary facilities to SNT to record the spectra. MKH is also thankful to the University Grants Commission, New Delhi for fmancial assistance during the course of the investigation and to Bhagalpur University for granting him the necessary leave.
186 M K Haque and S N Thakur References
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