A sandwich device was prepared by electrodeposition of an insoluble layer of oligomerized tris(4-(2-thienyl)phenyl)amine onto conducting indium-tin oxide coated glass, spin coating the stacked platinum compound, tetrakis(p-decylphenylisocyano)platinum tetranitroplatinate, from toluene onto the oligomer layer, and then coating the platinum complex with aluminum by vapor deposition. This device showed rectification of current and gave electroluminescence. The electroluminescence spectrum (.lambda..sub.max =545 nm) corresponded to the photoluminescence spectrum of the platinum complex. Exposure of the device to acetone vapor caused the electroemission to shift to 575 nm. Exposure to toluene vapor caused a return to the original spectrum. These results demonstrate a new type of sensor that reports the arrival of organic vapors with an electroluminescent signal. The sensor comprises (a) a first electrode; (b) a hole transport layer formed on the first electrode; (c) a sensing/emitting layer formed on the hole transport layer, the sensing/emitting layer comprising a material that changes color upon exposure to the analyte vapors; (d) an electron conductor layer formed on the sensing layer; and (e) a second electrode formed on the electron conductor layer. The hole transport layer emits light at a shorter wavelength than the sensing/emitting layer and at least the first electrode comprises an optically transparent material.

TECHNICAL FIELD 
The present invention relates generally to molecular electronics, and, more 
particularly, to vapor-sensitive, molecular light emitting diodes based on 
thin films of certain platinum complexes. 
BACKGROUND ART 
The inventors have recently published reports that have enucleated and 
explained the unusual "vapochromic" changes in absorption and emission 
spectra that result when certain stacked platinum complexes are exposed to 
organic vapors; see, e.g., C. L. Exstrom et al, Chemical Materials, Vol. 
7, pp. 15-17 (1995) and C. A. Daws et al, Chemical Materials, Vol. 9, pp. 
363-368 (1997). 
A typical experiment involves a solution, crystal or solid film of 
material, such as tetrakis(p-decylphenylisocyano)platinum 
tetracyanoplatinate (I) (see FIG. 1, which depicts the chemical formula of 
the compound, where the dashed vertical line indicates the c-axis) that 
forms stacks of alternating cations and anions with strong inter-platinum 
interactions. These salts exhibit an intense absorption band in the 
visible region. Exposing the stacks to small molecule vapors, such as 
acetone or chloroform, leads to sorption of the vapor molecules in the 
free volume between the stacks, and produces shifts in the absorption and 
emission spectra. These "vapochromic" or "vapoluminescent" changes are 
usually reversible so that the original spectrum is regained quickly after 
the vapor is removed. Such an effect has potential application for sensor 
technology. 
Molecular LEDs are under intense investigation; see, e.g., R. H. Friend in 
Conjugated Polymers and Related Materials, W. R. Salaneck et al, Eds., 
Chapter 22, Oxford University Press (1993); A. J. Heeger, in Conjugated 
Polymers and Related Materials, W. R. Salaneck et al, Eds., Chapter 4, 
Oxford University Press (1993); R. H. Friend et al, in "Physical 
Properties of Polymers Handbook", J. E. Mark, Ed., AIP Press, New York 
(1996); Y. Yang, MRS Bulletin, pp. 31-38 (June 1997); T. Tsutsui, MRS 
Bulletin, p. 39-45 (June 1997); and W. R. Salaneck et al, MRS Bulletin, p. 
46-51 (June 1997). However, to the best of the inventors' knowledge, there 
is only one unspecific report from the patent literature which describes 
this type of LED sensor; see, U.S. Pat. No. 5,629,533, issued to D. E. 
Ackley et al May 13, 1997. 
Vapochromic platinum complexes and salts have been the subject of a patent; 
see, U.S. Pat. No. 5,766,952, issued to Kent R. Mann et al on Jun. 16, 
1998. Such complexes change color when exposed to certain organic vapors. 
DISCLOSURE OF INVENTION 
In accordance with the present invention, a molecular light emitting diode 
is provided. The molecular LED employs an organic complex that acts as 
both a sensor to certain organic molecules, or analyte vapors, and as an 
active light emitter. The molecular LED of the present invention 
comprises: 
(a) a first electrode; 
(b) a hole transport layer formed on the first electrode; 
(c) a sensing/emitting layer formed on the hole transport layer, the 
sensing layer comprising a material that changes color upon exposure to 
the analyte vapors; 
(d) an electron conductor layer formed on the sensing layer; and 
(e) a second electrode formed on the electron conductor layer, wherein the 
hole transport layer emits light at a shorter wavelength than the 
sensing/emitting layer and wherein at least the first electrode comprises 
an optically transparent material. The device is preferably formed on a 
transparent dielectric substrate, on which the first electrode is formed. 
Also in accordance with the present invention, a method is provided for 
detecting analyte vapors. The method comprises: 
(a) providing the above-described vapochromic LED; 
(b) introducing the analyte vapors to the sensing layer; and 
(c) biasing the first electrode positive with respect to the second 
electrode. 
Further in accordance with the present invention, methods are provided for 
forming the vapochromic LED. 
Other objects, features, and advantages of the present invention will 
become apparent upon consideration of the following detailed description 
and accompanying drawings, in which like reference designations represent 
like features throughout the FIGURES. The drawings referred to in this 
description should be understood as not being drawn to scale except if 
specifically noted.

BEST MODES FOR CARRYING OUT THE INVENTION 
In accordance with the present invention, a vapochromic light emitting 
diode is provided, which is sensitive to certain organic vapors and emits 
optical radiation upon exposure to the organic vapor. Referring to FIG. 2a 
the general structure of the LED 10 is formed on a substrate 12 and 
comprises: 
(a) a first electrode 14 formed on the substrate; 
(b) a hole transport layer 16 formed on the first electrode; 
(c) a sensing/emitting layer 18 formed on the hole transport layer 
(d) an electron conductor layer 20 formed on the sensing/emitting layer; 
and 
(e) a second electrode 22 formed on the electron layer. 
The material comprising the substrate 12 comprises any optically 
transparent dielectric material having sufficient thickness and strength 
to support the layers formed thereover. An example of a suitable substrate 
material includes quartz. 
The first electrode 14 comprises indium tin oxide (ITO) or polyaniline or 
other such material that is both electrically conductive and transparent 
in at least the optical region. For example, ITO is transparent from about 
250 to 2600 nm. The deposition of the first electrode 14 on the substrate 
12 is performed by any of the conventional techniques for these materials. 
The hole transport layer 16 serves to move positive charge to the 
emitting/sensing layer 18. Hole transport materials are discussed in 
greater detail below. Such materials may be formed by vacuum deposition, 
chemical vapor deposition, spin-casting, and the like. 
The sensing layer 18 changes color upon exposure to a chemical vapor 24 and 
emits light 26. For many sensing layer materials, the effect is reversible 
upon removal of the chemical vapor. That is, the color of the sensing 
layer 18 reverts to its original color. However, there are some sensing 
materials that do not exhibit this color reversal. Many of these sensing 
materials may be returned to their original color upon introduction of a 
second chemical vapor. 
The electron conductor layer 20 serves to move electrons from the second 
electrode 22 to the sensing/emitting layer 18. Electron conductor 
materials that would be useful in the practice of the present invention 
are well-known to those skilled in this art; an example of such a material 
is aluminum tris(8-hydroxyquinoline). Such materials may be formed by 
vacuum deposition, chemical vapor deposition, spin-casting, and the like. 
The second electrode 22 comprises an electrically conductive material, 
preferably a metal having a comparatively low work function, thus easily 
giving up electrons. Examples of such metals include aluminum, magnesium, 
calcium, indium, and silver or mixtures thereof. The deposition of the 
second electrode 22 on the electron conductor layer 20 is performed by any 
of the conventional techniques for these materials. 
Two or more of the active layers (hole transport layer 16, sensing/emitting 
layer 18, and electron conductor layer 20) may be combined in a single 
layer. FIG. 2b depicts combining all three layers into a single layer 18', 
while FIG. 2c depicts combining the hole transport layer 16 and the 
sensing/emitting layer 18 into a single layer 18". The compositions of the 
active layers 16, 18, and 20 are discussed below. 
In order to increase the efficiency of the LED 10 for sensing the chemical 
vapor, it is preferred that some means for introducing the chemical vapor 
to the sensing layer 18 be provided. Such means may comprise, for example, 
pinholes in the second electrode 22. 
Because it is a better film-former than Compound I, the inventors chose to 
study tetrakis(p-decylphenylisocyano)platinum tetranitroplatinate (II) as 
the stacked platinum complex. This material, which serves as both the 
electron conductor layer 20 and the sensing layer 18, is also vapochromic 
as described above, except that unlike most vapochromic compounds, the 
color change does not reverse when the vapor is removed; see, e.g., C. L. 
Exstrom Ph.D. Dissertation, University of Minnesota, 1995. The color can, 
however, be changed by introducing another vapor, and switched back and 
forth in a reversible manner with the sequential use of two different 
vapors; see, e.g., Exstrom, supra. Examples of vapors useful for switching 
the color back include alcohols, specifically, methanol, ethanol, and 
2-propanol, diethyl ether, hexane, and acetonitrile. 
In principle, the device 10, 10', 10" of the present invention is sensitive 
to any molecules which can penetrate the sensing/emitting layer 18, 18', 
18". However, in practice, larger molecules are not as detectable as 
smaller molecules. In particular, the device is measurably sensitive to 
simple organic molecules, such as methanol, ethanol, iso-propanol, diethyl 
ether, acetonitrile, hexane, acetone, benzene, dichloromethane, and 
chloroform, as well as to simple inorganic molecules, such as water vapor 
and ammonia, to name a few. The sensitivity of the device is limited only 
by the same factors that limit the vapochromic effect in general. 
Concentrations on the order of high parts per billion (ppb) have been 
detected, and it is expected that optimized devices will be able to detect 
concentrations down to the low ppb region. 
Compound II was prepared from a mixture of p-decylphenylisonitrile, 
cis[n-(CH.sub.3 CN).sub.2 PtCl.sub.2 and [n-C.sub.4 H.sub.9).sub.4 
N][Pt(NO.sub.2).sub.4 in acetonitrile. The purified product gave 
appropriate combustion analysis and spectroscopic data. 
In the present study, the quantum yield for photoluminescence (PL) from a 
thin film of Compound II was 3.8% (excited at 438 nm, measured at 540 nm). 
Devices were prepared by spin-casting Compound II from toluene onto ITO 
(indium-tin oxide) coated glass, followed by vapor deposition of aluminum 
to provide a single layer device, comprising ITO/Compound II/Al, which 
turned out to be very resistive, unstable, and did not show rectification. 
Spin-casting of Compound II is conveniently performed using toluene as the 
spin-coating solvent. 
No electroluminescence (EL) was observed for Compound II in the single 
layer device before electrical breakdown. The presence of 
electroluminescence before electrical breakdown would mean that the 
electrical properties (i.e., hole or electron conduction) of the material 
were not appropriate to form a LED by itself. 
Recently, the inventors have shown that it is possible to form single layer 
LEDs, comprising ITO/Compound IV/Al, by anodically oligomerizing 
tris(p-thienylphenyl)amine (Compound III), to form Compound IV; se, Y. 
Kunugi, Synthetic Metals, Vol. 89, pp. 227-229 (1997): Compound IV is not 
a vapochromic material, but does emit light at a constant wavelength. The 
structure of this single layer LED is that depicted in FIG. 2b; it will be 
noted that this is an emitting device only, in which Compound IV performs 
the functions of hole transport and electron conductor, as well as 
emitting. The device of FIG. 2b is capable of sensing vapors only if the 
layer 18" comprises a vapochromic material, as well as performing the hole 
transport and electron conductor functions. 
##STR1## 
where n is of a value sufficient to form a cross-linked, insoluble 
polymer. 
Because the layer of Compound IV is insoluble in solvents such as toluene, 
it seemed possible to spin-cast a second layer of Compound II on top of 
Compound IV without layer interdiffusion. In this way, Compound IV could 
be used as a hole transport layer to make a two-layer device. This 
approach succeeded when the inventors prepared ITO/Compound IV/Al.sub.q3 
/Al, using the well-known emitter, aluminum tris(8-hydroxyquinoline). 
Here, the inventors prepared the two-layer device ITO/Compound IV/Compound 
II/Al by electrooligomerizing Compound III (700 nm layer of Compound IV) 
into ITO, spin-coating Compound II from toluene (200 nm layer of Compound 
II), and vapor depositing aluminum (200 nm). As described above, the 
electrooligomerization was performed by oxidizing Compound III in 
acetonitrile, lithium perchlorate providing the oxidized form of Compound 
IV, and then reducing it to form the neutral Compound IV. The amount of 
Compound IV on the surface was estimated coulometrically and 
spectroscopically. FIG. 2c depicts this two active layer device 10", which 
is essentially the same as that of FIG. 2a, except that the thin film 18" 
of Compound II serves as both the sensing layer and the electron conductor 
layer. Compound IV is the hole conducting layer 20. 
The device ITO/Compound IV/Compound II/Al (2 mm diameter) gave 
rectification of the applied current (FIG. 3) favoring electron flow from 
Al through the molecular layers to ITO. The EL spectrum corresponded 
closely to the PL spectrum of Compound II cast from toluene onto ITO 
(.lambda..sub.max 540 nm, FIG. 4). The current/EL intensity curve was 
nearly linear up to 25 V and we estimate that the photon/electron 
efficiency is about 0.01%. When the device was exposed to argon saturated 
with acetone vapor, the EL spectrum changed dramatically to 
.lambda..sub.max 575 (FIG. 5). This spectrum is quite different from the 
PL spectrum of Compound IV. Thus, it appears that the EL comes primarily 
form Compound II, not Compound IV, which emits at lower wavelength. If the 
device is left at zero current for several days, open to the room 
atmosphere, no change in the spectrum is seen. Exposure of the device to 
toluene vapor in argon causes the spectrum to revert to the original. 
Molecular LEDs are often not very stable when they are in use, especially 
in air. Use as a sensor, however, only requires that the device be 
occasionally pulsed. Using a 1 mA cm.sup.-2, 3 sec pulse every 5 min gives 
a stable emission intensity for an hour. When acetone vapor in argon was 
introduced, the spectrum did not change for about 5 min, but then quickly 
shifted to the longer wavelength (FIG. 6). Since there is no evidence that 
the color changes from the outside diameter of the device moving inward, 
and because the rate of change is independent of the diameter of the 
device, we hypothesize that the acetone penetrates through the thin 
aluminum film 14, which, due to its thinness, probably has pinholes. This 
is consistent with the sudden change of the color after a few minutes (as 
the acetone diffusion front reaches the molecular layers and rapidly 
diffuses into them, changing the structure and EL spectrum). 
These preliminary results suggest that it will be possible to have a device 
of this type report on the arrival of certain vapors with a pulse of 
light. Since different organic vapors elicit different optical responses, 
a variety of chemicals can be detected. Using reversible examples of 
stacked complexes may provide the possible of quantifying the response, 
and using alternative device geometries should improve the response time. 
Other hole transport layers 16 include materials such as poly(p-phenylene 
vinylene) (PPV) polymers of symmetric and asymmetric triaryl amines, such 
as 4,4'-bis(phenyl-m-tolylamino)biphenyl (TPD), oligothiophenes with 
diaryl amino substituents, such as 
2,5-bis{4-[bis(4-methylphenyl)amino]phenyl}thiophene (BMA-1T), 
5,5'-bis{4-[bis(4-methylphenyl)amino]phenyl}-2,2'-bithiophene (BMA-2T), 
5,5"-bis{4-[bis(4-methylphenyl)amino]phenyl}-2,2':5',2"-ter thiophene 
(BMA-3T), and 
5,5'"-bis{4-[bis(4-methylphenyl)amino]phenyl}-2,2':5',2":5",2'"-quarterthi 
ophene (BMA-4T). Still other hole transport materials are well-known in 
this art, and may be used in the practice of the present invention. 
The sensing/emitting layer 18 must have the characteristics of emitting 
layers of conventional LEDs plus the vapochromic properties needed for 
color change. There are other active sensing/emitting layers 18 that are 
also known that allow the analyte molecules (organic vapor) to penetrate 
the lattice and induce a solvatochromic shift. Examples of such active 
emitting layers 18 are disclosed, e.g., in U.S. Pat. No. 5,766,952, and 
include (I) neutral platinum complexes comprising platinum complexed by 
four ligands wherein two ligands are negatively charged groups selected 
from the group consisting of CN.sup.-, NO.sub.2.sup.-, NCO.sup.-, 
NCS.sup.-, Cl.sup.-, Br.sup.-, I.sup.-, and oxalate and the remaining two 
ligands are selected from the group of at least one and at most two 
arylisonitrile groups and a Fisher carbene (i.e., (C(Y).dbd.NH--C.sub.6 
H.sub.4 -alkyl group), wherein Y can be O-alkyl, NH-alkyl, or 
N(alkyl).sub.2, including (a) Pt(CN--C.sub.6 H.sub.4 -alkyl group).sub.2 
X.sub.2, (b) Pt(CN--C.sub.6 H.sub.4 -alkyl group)(C(Y).dbd.NH--C.sub.6 
H.sub.4 -alkyl group)X.sub.2, and (II) double-complex salts represented by 
the general formulae (a) [Pt(CN--C.sub.6 H.sub.4 -alkyl group).sub.4 
][PtX.sub.4 ], (b) [Pt(Phen)(CN--C.sub.6 H.sub.4 -alkyl group).sub.2 
][PtX.sub.4 ] and (c) Pt(bpy)(CN--C.sub.6 H.sub.4 -alkyl group).sub.2 
][PtX.sub.x ], where alkyl group comprises an alkyl group of at least 4 
carbon atoms, X is selected from the group consisting of CN.sup.-, 
NO.sub.2.sup.-, NCO.sup.-, NCS.sup.-, and 1/2 oxalate, Phen is 
1,10-phenanthroline or an alkyl-substituted phenanthroline and bpy is 
2,2'-bipyridine or an alkyl-substituted bipyridine. Further examples of 
suitable vapochromic materials include tetra-alkyl metallo porphyrins with 
para substituents, such as hydrogen and C.sub.1 to C.sub.10 alkyl. Yet 
additional examples include salts of the anion [Ru(bipyridine)(CN).sub.4 
].sub.2, suitable substituted oligothiophenes with large substituents, and 
organic dye molecules that are solvatochromic and have solvent-permeable 
crystal lattices. 
The second electrode 22 may be a unitary layer, as shown in FIGS. 2a-2c, or 
may be digitated, as shown in FIG. 7, to expose the underlying layer (18' 
or 20, as the case may be). Such a configuration provides an alternative 
to relying on a thin conducting electrode with pinholes for introducing 
the vapor to be detected to the sensing/emitting layer 18 (or 18' or 18"). 
Light emitted from the sensing/emitting layer 18 (or 18' or 18") may be 
coupled through the substrate 12 into an optical fiber 28 for transmission 
to a remote site. FIG. 8 depicts such a configuration. 
Detection of color emitted by the device 10 may be done by the human eye or 
by any other color-discriminating sensor. Such color-discriminating 
sensors are well-known in the art. 
More than one device 10 (four such devices 10a, 10b, 10c, 10d are shown) 
may be formed on the substrate 12, each device formed with a different 
vapochromic compound 18 (e.g., 18a, 18b, 18c, 18d), to permit 
identification of a mixture of vapors. FIG. 9 depicts this configuration. 
INDUSTRIAL APPLICABILITY 
The molecular LED disclosed herein is expected to find use in molecular 
devices, particularly as sensors. 
Thus, there has been disclosed a molecular light emitting diode. It will be 
appreciated by those skilled in this art that various changes and 
modifications of an obvious nature may be made, and all such changes and 
modifications are considered to fall within the scope of the invention, as 
defined by the appended claims.