750-850 nm; cf. Table 1). ..... near infrared region at 780-860 nm (cf. Table l) .... Chem. 1987, 91, 5184-5193. - [Iob] K. Y. Law. F. C. Bailev. Dves Pipm. 1988. 9.
D. Keil, H. Hartmann, C. Reichardt
935
Synthesis and Spectroscopic Characterization of New NIR Absorbing (2-Thieny1)- and (4-Dialkylaminoaryl)-Substituted Croconic Acid Dyes Dietmar Keil”, Horst Hartmann*b, and Christian ReichardtC Synthec GmbH, Technologiepark Wolfen-Thalheim”, 0-4400 Wolfen, Germany Department of Chemistry, Technical University of Merseburgb, New address: Fachhochschule Merseburgb, Geusaer StraDe, 0-4200 Merseburg, Germany Department of Chemistry, University of Marburg“, Hans-Meenvein-StraDe, W-3550 Marburg, Germany Received January 15, 1993
Key Words: Croconic acid / Dyes / Thiophenes / Calculations, PPP / Near infrared (NIR) absorbing dyes By condensation of croconic acid (7) with 2-(dialky1amino)thiophenes 6a-d and with 3-(dialkylamino)phenols9a-c the deeply colored, sparingly soluble croconic acid dyes (”croconines”) 8a-d a n d 10a-c, respectively, have been prepared.
These croconines have been characterized by UV/Vis, IR, and ‘H-NMR spectrometry. They exhibit strong electronic absorption bands in the near infrared (NIR) region (h,,, = 750-850 nm; cf. Table 1).
In connection with the development of cheap and powerful laser diodes emitting in the near infrared (NIR) region (h > 750 nm), organic dyes with electronic absorption bands in this part of the electromagnetic spectrum are of particular practical interest“]. Depending on their chemical constitution, NIR dyes transform the energy of the absorbed light by means of various photophysical or photochemical processes into other forms of energy and, incidentically,change their constitutional structure or the structure of the surrounding medium by these processes. In this way, new data recording and data storing media can be constructed[*]. A special type of organic dyes capable to absorb light in the NIR region are the squaric acid derivatives (“squaraines”) of the general formula 1. These dyes 1 exhibit, in addition to their strong absorptivity in the NIR region, a high photostability and a high aggregation tendency, the latter originating from the low solubility of these dyes in most organic solvents or polymers used as embedding media for data recording Squaric acid dyes such as 1, now known for more than thirty years, are easy to prepare by condensation of the commercially avaible squaric acid with N,N-disubstituted aniline~[~]. Because of the unique properties of squaraines like 1, many efforts to modify their chemical structure have been reported. Apart from the replacement of the N,N-dialkylaniline moieties attached to the fourmembered ring by various other types of electron-donating carboand heterocycles such as azulene~[~], pyrroles161, or heterocyclic methylene bases[’] as in dye 2, a replacement of the central squaric acid ring by the croconic acid moiety has been performed to give croconic acid dyes (“croconines”) such as 3 and 4. Such croconines can be synthesized, in analogy to 1 and 2, by condensation of croconic acid with N,N-dialkylanilinesr8]or with heterocyclic methylene basedg]. Both types of croconines, i.e. 3 and 4, exhibit a strong long-wavelength electronic absorption in the NIR region, which is bathochromically shifted by ca. 100 nm with respect to the corresponding absorption maxima of the squaraines 1 and 2[7,10]. Recently, we have found that, instead of N,N-dialkylanilines, 2(dialky1amino)-substituted thiophenes can be used in the condensation reaction with squaric acid to give the squaraines 5[”]. With
respect to the large bathochromic band shifts observed in going from squaraines to analogously substituted croconines, the synthesis of 5-dialkylamino-2-thienyl-substitutedcroconic acid dyes[l21 as well as further examples of their 4-(dialkylarnino)phenyl-~ubstituted analogs would be of practical interest. The chemistry of deltaic, squaric, and croconic acid and their derivatives has been reviewed very recently[l3I.
Liebigs Ann. Chem. 1993,935-939
In this paper we report on the preparation o f the croconines 8a-d and 10a-c, which can be formally classified, due Scheme 1
0
qd - p \R
0
MNR O0
R2N
0
0VCH VerlagsgesellschaftmbH, D-69451 Weinheirn, 1993 0170-2041/93/0909-0935 $10.00+.25/0
D. Keil, H. Hartmann, C. Reichardt
936
Table 1. Longest-wavelength UVNis/NIR absorption maxima (in nm) of the croconine dyes 8a-d and 10a-d, measured in dichloromethane at room temperature
Scheme 2
8a 8b
7
6a-d
8c 8d
nO
786 793 798 192.5
;:2 ;2:
10a 10b 1oc
817 822 855
5.34 5.36 5.29
Because of the low solubility in all organic solvents, highly diluted dye solutions had to be measured and, therefore, the molar decadic extinction coefficients are not of the usual precision. La]
6,8 R2N
I
b
a
Me,N-
d
C
@-
0-
n
OWN-
to their structural resemblance to vinylogous amides, as merocyanines with a strong dipolaric character. The condensation of the 2-(dialky1amino)thiophenes 6a-d with croconic acid (7) in a boiling 1:l mixture of 1butanol and toluene leads to the 2-thienyl-substituted croconines 8a-d in yields of 55-70%. Because of their low solubility in organic solvents, the dyes formed crystallize mostly spontaneously from the reaction mixture already during the reaction procedure. The starting compounds 2(dialky1amino)thiophenes 6a-d are easily available from 2mercaptothiophene and suitable secondary amine~['~I. The analogous condensation of 3-(dialky1amino)phenols 9a-c with croconic acid (7) under the same reaction conditions gives the aryl-substituted croconines 10a-c in yields of 45-55%. The new croconines 8a-d and 10a-c are deeply colored, high-melting (m.p. > 360°C), microcrystalline solids with a very low solubility in most organic solvents. The croconines 8a-d, particularly 8a and 8b, can be sublimed under reduced pressure without decomposition. It should be mentioned that the condensation reaction affording the dyes 8a-d does only occur with 2-(dialkylamino)thiophenes without substituents in their 4-position. With 4-substituted 2-(dialky1amino)thiophenes such as 4Scheme 3
9a-c
7
10a-c
Q910
a
b
C
aryl-substituted derivative~['~], which normally react with squaric acid to give correspondingly substituted derivatives of the squaraines 5, no reaction with croconic acid takes place. Obviously, the neighborhood of the 4-aryl group to the reacting 5-position prevents the electrophilic attack of the 1,3-carbonyl groups of croconic acid at this position. Similarly, the condensation of croconic acid with unsubstituted N,N-dialkylanilines, as erroneously reported in the patent literatureI8],is also not possible. Only after introduction of a 3-hydroxy group into the N,N-dialkylanilines, the desired condensation reaction of 9a-c with 7 takes place as shown in Scheme 3. This observation has also been made recently by other authors[l61. Since the molecular size of croconic acid is only slightly larger than that of squaric acid, the presence of altogether five functional groups surrounding the five-membered ring obviously causes some additional strain which prevents the formation of a stabilized fully conjugated, planar chromophoric system. This strain can be partly compensated by the presence of hydroxy groups in the meta position of the two 1,3-aryl rings in 10, so that it is obviously possible to form intramolecular hydrogen bonds of these hydroxy groups with the carbonyl groups at the central croconic acid moiety and to planarize the chromophore in this way. The new croconines 8a-d and 10a-d have been characterized by UVNisINIR (Table l), mass, IR (Table 2), and 'H-NMR spectroscopy (Table 3). Their mass spectra are characterized by the occurrence of (M+ + 2) peaks[l71,by analogy with the mass spectra of the corresponding squaraines 1[18], indicating a similar fragmentation behavior of both types of dyes 8 and 10. The mass spectra of 8a-d and 10a-c will be reported and discussed in detail in a subsequent paper["]. The IR spectra of all thiophene-containing croconines 8a-d exhibit characteristic absorption bands at 1600- 1660 cm-', which can be attributed to core vibrations of the heteroaryl-substituted central croconic moiety, whereas the origin of the other bands at 3080, 1570- 1600, 1520- 1530, 1400-1420, and 880 cm-' is not understood as yet. The IR spectra of the dyes 10a-c exhibit, in addition to some intensive absorption bands at 2800-2900, 1600, and 1500 cm-' and in close resemblance to the IR spectra of 8a-d, characteristic absorption bands at 1650- 1670 and 3400-3450 cm-', stemming from the core vibrations of the Liebigs Ann. Chem. 1993, 935-939
Substituted Croconic Acid Dyes
931
Table 2. Analytical and infrared data for the croconine dyes 8a-d and 1Oa-c Molecular formula
Yield
M. p .
Elemental analysis
Main IR absorptions
(Molar mass)
(%I
["CI
C
(in K B r )
C17H16N203S2 (360.4)
55
>360 Calcd. 56.66 4.47 7.77 17.79 Found 56.85 4.59 7.87 17.85
8b
C21H20N203S2 (412.5)
60
>360 Calcd. 61.15 4.89 6.79 15.54 3050w, 2900-2850w, 1640m, 1620m, Found 60.91 4.93 6.52 15.17 1570s, 1520s, 1400s, 880s
8c
C23H24N203S2 (440.6)
70
>360
Calcd. 62.70 5.49 6.36 14.55 Found 62.81 5.53 6.16 14.42
3080w, 2930-2850w, 1660m, 1640m, 1580111, 1520111, 1400s, 880m
C21H20N205S2 (444.5)
65
>360
Calcd. 56.74 4.53 6.30 14.42
3080111, 2900-2800m, 1660111,1630m,
Found
1600111,1530111, 1420s, 8 8 0 s
'21H20N2'5 (380.4)
55
>360
Calcd. 66.31 5.30 7.36 F o u n d 65.68 5.40 7.04
3400m, 2900-2800w, 1660m, 1600s, 1500m
45
>360
Calcd. 68.79 6.47 6.42 68.62 6.77 6.56
3450m, 2850-2950w, 1670m, 1600s, 1500s
>360 Calcd. 71.88 5.82 5.78
3450m, 2900-2800w, 1650w, 1600m,
No.
lob C25H28N205
Found
(436.5) 50
lo' '2gH2EN2O5
Found
(484.6)
H
N
S
[cm-'1
3080w, 2900-2850w, 1640m, 1620m, 1570s, 1520s
56.52 4.51 6.03 14.15
70.52 5.87 5.99
1500m
Table 3. 'H-NMR data for the croconine dyes 8c and lOc, measured in CDC13, and of two corresponding compounds without and with the squaric acid moiety for comparison of the chemical shifts
B
d
X
H (in
Ha
Hb
HC
d
Hd
c
He
A)
6.75(m)
6.06(m)
3.05(m)
1.55( m )
Squaric acid moiety (in A ) Croconic acid moiety (in A )
7.95(d)
6.33(d)
3.50(t)
1.72( m )
8.75(m)
6.55(d)
3.79(m)
1.75( m )
H (in 8 ) Squaric acid moiety (in B ) Croconic acid moiety (in B )
4.40(s) 11.4(s) 12.0(s) 14.7(s) 15.2(s)
6.64(d) 7.51(s) 7.64(s)
2.68(t) 3.34(t)
2.64(t) 2.71(m)
1.95(m) 1.95(m)
8.85(s)
3.34(m)
2.71(t)
1.90(m)
J[Hzl
Reference
6.51(m)
3.6(iab)
[la] t 1lb 1
4.7( Ja 6.03(m) -
,,)
4.5(Jab)
This work
8.1(Jbx)
This work This work
-
This work
9.02(s)
central croconic moiety and the OH stretching vibrations of the hydroxyaryl substituents, respectively. It is noteworthy that in the IR spectra of both types of croconines, i.e. 8a-d and 10a-c, no absorptions in the region 1700- 1900 cm-', typical of C = O stretching vibrations, can be detected. The absence of such IR bands obviously reflects the significant contribution of dipolar mesomeric structures to the electronic ground state of these dyes, as exemplarily shown for the croconines 8 in Scheme 4. Liebigs Ann. Chem. 1993, 935-939
H(X)
The strong dipolar character of the croconines 8 and 10 follows also from quantum-chemical calculations performed by means of the usual PPP approach. According to these calculations, the carbon atoms of the central fivemembered ring, in particular the carbon atom which is linked to the electronegative oxygen atom in meso (8) position, are strongly positively charged, whereas the three oxygen atoms linked at the five-membered ring are significantly negatively charged. Thus, the electronic ground state of
D. Keil, H. Hartmann, C. Reichardt
938 Scheme 4
‘0
0 ‘
I
11
lf
8a
n
A
t o,’
3
8”
-
R2N
4’
4 8
0,
NA 1
f
l
5 7
2\s 6
these croconines does not exhibit, as visualized in Figure 1 for the parent compound 8a taken into account the calculated charge density distribution (see Table 4), such a significant charge alternation along the whole conjugated n system, as one expects if these dyes are considered as heterocyclic analogs of the Michlers Hydrol Blue type dye 11. This dye 11 is characterized, due to its structural resemblance to polymethine by a strong charge alternation along its conjugated n system. In the first excited state, however, the electronic structure of the croconines 8 is much stronger related to the Michlers Hydrol Blue dye 11 and, hence, more polymethinic. Representative for all dyes, the ‘H-NMR chemical shifts of 8c and 10c are given in Table 3. They are in agreement with the proposed structures and do not show any peculiarities. The ‘H-NMR spectrum of 10c contains two singlets of different intensity at 6 = 8.85 and 9.02 for the two equivalent phenyl hydrogens and two singlets at F = 14.7 and 15.2 for the two equivalent hydroxyl hydrogens suggesting the presence of two rotamers in the CDC13 solution of this compound. These rotamers are in equilibrium with each other by rotation around the bound between the central croconic moiety and the hydroxy-arylamino substituent. As expected for important potential applications, all new croconines 8 and 10 exhibit in solution very intense, longwavelength, rather narrow n-n* absorption bands in the near infrared region at 780-860 nm (cf. Table l), with a slight inflection at their shorter-wavelength side and similar to the UVNisINIR spectra of known croconine dyes[9b].In contrast to the UVNisINIR spectra in solution, solid croconine dyes exhibit a considerable broadening of the longwavelength absorption band, shifting the long-wavelength edge of this absorption band up to ca. 900 nm, as shown for dye 8a in Figure 2. A comparison of the Amax values of the new croconine dyes 8 and 10 with the UVNisINIR spectra of analogously substituted squaraine dyes 1 and 5 shows that in the 2-aminothiophene series a bathochromic band shift of ca. 140 nm is observed in going from 5 to 8, and in the dialkylaminoaryl series
3’
6’
11
Figure 1. Calculated charge density distribution Q, (Qr = Z, - qr) in the ground and first excited state of the 1,3-bis(2-dimethylamino5-thienyl)croconine 8a and of the 1,3-bis[4-(dimethylamino)phenyl]trimethinium ion 11
a band shift of ca. 180 nm in going from 1 to 10; thus, indicating a much stronger auxochromic strength of the croconines acid moiety as compared to the squaric acid moiety. We thank Mrs. C. Miiller, Technologie-Park Wolfen-Thalheim, Mrs. D. Miinster, Filmfabrik Agfa Wolfen AG, and Mr. G Schafer, University of Marburg, for their help in recording and interpreting the absorption spectra. We thank the Fonds der Chemischen Industrie, Frankfurt (Main), for financial support of this work. Table 4. K-Electron distribution (q,) in the ground and first excited state of the croconine dye 8a as well as the Michlers Hydrol Blue type dye 11; the atom numbering r is given in Figure 1
I
11
1
1.7040
1.5895
-0.0145
1.6509
1.5980
-0.0529
2
1.0144
1.0305
+0.0151
0.9805
1.0071
+0.0266
3
1.1458
1.1227
-0.0231
1.1265
0.0569
-0.0596
4
0.9484
0.9900
+ O ,0 4 1 6
0.9509
1.0294
+0.0785
5
1.0635
1.0098
-0.0537
1.1189
0.9960
-0.1229 +0.0089
6
1.7322
1.7402
+ o . 0080
1.7375
1.7464
7
0.3702
1.0830
+o. 1 1 2 8
0.8989
1.0979
+o. 1990
8
0.7425
0.5945
-0.0479
1.0718
0.9166
-0.1552
3
1.6977
1.5949
-0.1029
I0
(3.7555
0.7725
+O. 0155
11
>.A413
1.4171
-0.0245
Liebigs Ann. Chem. 1993, 935-939
Substituted Croconic Acid Dyes
939
(dialkylamino)phenol(9a-c, 2.00 mmol) in 25 ml of I-butanol and 25 ml of toluene was boiled under reflux for a few minutes, during which time a precipitate already formed. After cooling to room tem-
0.8 -
Absorption Frequency C / 103.cm-'-
T. Imasaka, K. Tanaka, N. Ishibashi, Anal. Chem. 1990, 62, 374-378. - [Ib] J. Fabian, R. Zahradnik, Angew. Chem. 1989, lol, 693-710; Angew, Chem. Ed, ~ ~ 1989, ~ 28,1 677-694, , 121 M. E-elius, G. Pawlowski, H. W, Vollmann, Angew. Chem, 1989, 101, 1475-1502; Angew. Chem. Int. Ed. Engl. 1989, 28, 145-1471, r31 K. Y Law, Chem. Rev. 1993, 93, 449-486. Table 5. Parameters used for performing the PPP calculations for K41 [4aI G. Maahs, P. Hegenberg, Angew. Chem. 1966, 78,927-931; 8a and 11 Angew. Chem. Int. Ed Engl. 1966, 5, 888-893. - [4b1 A. H. Schmidt, Synthesis 1980, 961-994. - [4c] H.-E. Sprenger, W. Ziegenbein, Angew. Chem. 1966, 78, 937-938; Angew. Chem. Int. Ed. Engl. 1966, 5, 894. L5] W. Ziegenbein, H.-E. Sprenger, Angew. Chem. 1966, 78, 937; Angew. Chem. Int. Ed. Engl. 1966, 5, 893. 0.58 140 1.o -C= 1 11.42 L6] [6a] A. Treibs, K. Jacob, Angew. Chem. 1965, 77, 680-681; An8.24 140 1.o -N(CH,b 2 21.22 gew. Chem. Int. Ed. Engl. 1965, 4, 694. - [6b] A. Treibs, K. 170 0.7 2 20.00 9.16 -SJacob, Liebigs Ann. Chem. 1966, 699, 153-167. -0 [cl 1 19.28 4.70 130 1.2 L71 H.-E. Sprenger, W. Ziegenbein, Angew. Chem. 1967, 79, 2.70 130 1.2 =O 1 17.28 581-582; Angew. Chem. Int. Ed. Engl. 1967, 6, 553-554. [*I Canon K. K., Jpn. Pat. 5914150 1984; Chem. Abstr. 1985, la] Core charge. - rb] pCc= 2.318 eV. - I[' Linked at the 2 position 102, 15216~. 191 [9a1 K. Miura, T. Ozawa, J. Iwanami (Mitsubishi Chemical Inof the croconine moiety. dustries Coi), Jpn. Pat. 6134092, 1986; Chem. Abstr. 1987, 106, 41718r. - 9b] S. Yasui, M. Matsuoka, T. Kitao, Dyes Pigm. 1988, 10, 13-22; Chem. Abstr. 1989, 110, 175116J. - [9c] A. Experimental Guy, H. Deporter (Agfa-Gevaert A.-G.), D.O.S. 1930224, 1970; Chem. Abstr. 1970, 72, 122944t. Melting points: Kofler heating-table microscope. - Elemental [lo] [Ioa] K. Y Law, . I Phys. Chem. 1987, 91, 5184-5193. - [Iob] K. analyses: CHNS-932-Analyzer (Fa. Leco Corp., St. Joseph/ Y. Law. F. C. Bailev. Dves Pipm. 1988. 9. 85- 107; Chem. Abstr. USA). - IR: Spectrometer Specord IR 70 (Fa. Carl Zeiss, Jena), 1988, i08, 152134.-' ' H. Hartmann. D. Keil. T. Moschnv. Dtsch. Pat. (DDR) with KBr pellets. - UVNisMIR: Spectrometer Specord M 40 (Fa. ['I] D. 294962, 1990; Chem. Abstr: 1992, II6,~P108306g.Carl Zeiss, Jena) and U-3410 (Fa. Hitachi, Tokyo), with 1-cm Keil, H. Hartmann, T. Moschny, Dyes Pigm. 1991, 17, 19-27; quartz cells and dichloromethane (Uvasol; Fa. Merck, Darmstadt) Chem. Abstr. 1991, 115, 234662n. as solvent. The dyes were weighted in (ca. 0.1 mg in 50 ml of [I2] H. Hartmann, D. Keil, T. Moschny, Dtsch. Pat. (DDR) 294961, 1990; Chem. Abstr. 1992, 116, 108305f. CH2CI2)with the electronic microbalance 4503 Micro (Fa. SarG. Seitz, P. Imming, Chem. Rev. 1992, 92, 1227-1260. torius, Gottingen). The absorption spectrum of solid 8a was meas- [I3] [I4] S. Scheithauer, H. Hartmann, R. Mayer, Z. Chem. 1966,8, ured by means of a transparent polyester sheet (polyethyleneglycol 181-183. H. Hartmann, .I Prakt. Chem. 1967, 36, terephthalate) on which the dye was fixed by sublimation in vacuo IPrakt. Chem. 50-72. - [lk] H. Hartmann. S. Scheithauer. . 1969, 311, 827-843. (
