COMMUNICATIONS TO THE EDITOR
1435
light, not only acts as an energy carrier but also redistributes the fluorine between the reaction products. Acknowledgment. This work was supported by Grant 9530/13 from the Defense Research Board of Canada.
40
UNIVERSITY OF CALIFORNIA SANTABARBARA, CALIFORNIA OF CANADA ROYAL MILITARY COLLEGE KINGSTON, ONTARIO, CANADA K
GLENNH. MILLER
J. R. DACEY
RECEIVED MARCH4. 1965
a
0 I-
W
a
The Spectroscopy and Photosensitization
3 v)
of Various Photochromic Spiropyrans
W
E
Sir: The purpose of this communication is to report on the emission spectra, assignment of excited states, ‘Q photosensitization, certain aspects of the absorption \ Y o spectra of several spiropyrans, and mechanisms for the 2: photochromic process. More complete details of 0 y o \ O 0 results and interpretation will follow in the future. Table I presents a summary of the most pertinent in.O $.*e. S O formation relative to the points described above. o % h . ~ ~ The 6’-nitro derivative has an emission with sharp I I I I l e*-. vibrational structure showing a 1450 vibrational pro” 35cI 50 loo TIME, MINUTES. 250 gression in 2MeTHF. Such a spacing corresponds to Figure 1. Pressure of reaction cell aa a function of time. an NO stretching frequency. The spectrum is also Indicated ratios are the initial pressures Xe/c-C4F8. The sharp in EPA and the emission origin is slightly blue 0.6/20.7 curve is displaced upward by 20 torr units. shifted compared to 2MeTHF. The separation of the origin of emission from the longest wave length band (4200 em. maximum is approximately 8800 cm. ture could be in excess of 200°, so that with our system from the onset). All of these facts strongly indicate fluorination with XeFz seems probable. that the emission corresponds to a phosphorescence For reaction 1, the energy available is approximately originating from an n,a* triplet state. Furthermore, 254 kcal. (194 kcal. from Xe a t 1469 A. and about 60 it has been possible to enhance the emission by sensitizkcal. from two Xe-F bonds). Approximately 220 of ing with benzophenone a t 77’K. as well as to enhance this is required to break two C-F bonds; the remainder photocoloration a t - 100’. This is consistent with is insufficient to fragment the c-C4Fabut could open the the state assignment. Enhancement has very recently c - C ~ F ~ * -C4Fe(8) been shown for two different compounds.’ Thus, the mechanism for the photochromic process would appear ring. Various isomers of this diradical could form, to be excitation (to lowest a,n* or n , s * singlet states), depending on how the cyclic radical was formed and internal conversion to the lowest n,a* singlet, interopened. The condensation of two of these radicals to system crossing to the lowest, n , s * triplet, followed by form a C8 fluorocarbon yields the main product found. molecular rearrangement to the colored species. Ob2C4Fa CeHn (9) viously, the earlier steps would be bypassed by sensitization. This product may be more or less fluorinated by the The emission for 6’-nitro,8’-methoxy (Table I) is XeF2. Its formation would cause the pressure drop assigned as a phosphorescence originating in an n , r * observed, the rate being constant for a constant rate of triplet state. The mechanism for the photochromic formation of Xes* We conclude that the main initial step is reaction 1. It is interesting that Xe, when activated with 1469-A. (1) T. Bercovici and E. Fischer, J. Am. Chem. Soc., 86, 5687 (1964).
._
‘.‘.
.
-+
+
Volume 69, Number 4
April 1965
COMMUNICATIONS TO THE EDITOR
1436
Table I : Spectroscopic and Photosensitization Properties of Various Spiropyrans -Absorption, Compound
Solventa
colorless formOnset Longest ,,A, in A. A.
EPA 2MeTHF HC
-3700 -3600 -3650
3200 (ab) 3200 (sh) 3200 (sh)
4140 4080 4080
5330 (sh) 5900 6500
Pink-red Pink-red Pink
Benzophenone BenEophenone Benzophenone
No
-3950
3300
4670
6150 (max.)
Pink-red
... ...
2MeTHF
-3950
3350
4740
6190 (max.)
Pink-red
Bensophenone Benzaldehyde Benzophenone
EPA 2MeTHF
-4000
5080 5180
6270 6160 (max.)
Blue Blue
6050
Pink-red
6100
Pink-red
1,3,3-TrimetIiylindolinobenzospiropyran (TIB) 6 -Nitro T I B EPA
6 -Nitro, 8 -Ioethoxy
TIB
...
... 3540
----Emission Colorless form, A.
originsColored form, R.
Color of colored form
-PhotosensitizationPhotoEmis- Photocolorsensitirer aionb ationC
... ...
...
No
Yea
... ...
Yes w No Yes vu
Yes w Yes vw Yea w
... ...
CH3 1,3,3-Trimethylindolino-8-naphthospiropyran
2MeTHF
3595
EPA
3585
3660and 7100 (max.) 3700 and 7200 (max.)
Benrophenone Quinoxaline
...
Yea vw Yes vw
2 parts of ethyl ether, 5 parta of isopentane, 2 parts of ethyl alcohol; 2MeTHF: 2-methyltetrahydrofuran; HC: 2 parts of methylcyclohexane, Refers to comparative formation of colored form by Refers to photosensitization of emission of colorless form a t 77'K. vs = very strong (-5O:l); w = weak (1.5-3:l); vw very weak (
