Is there a known chemical that makes it possible to control the opacity of a large surface?
There are 


thermochromic materials that change color in response to heat
photochromic materials that change color in response to light
and electrochromic materials than change color in response to an applied electric field.


All of these materials are sub-categories of Smart Glass. I suspect that electric field sensitive Smart Glass materials are what you are looking for.  There are several different types of such Smart Glass materials.


Those with suspended particles that are normally randomly aligned, but when an electric field is applied they align an alter the light transmission properties of the material.  These materials have already found commercial application (airplane) and are moving down to lower cost applications (autos)
Electrochromic devices where a molecule undergoes a chemical change between colored and non-colored states when a field is applied.
Liquid crystal materials dissolved in a polymer matrix are normally randomly aligned, but, when a field is applied they align thus altering the light transmission properties of the material.


There are also some non-electrochromic smart glass technologies that are also discussed in the Smart Glass Wikipedia article linked above.  These are generally lower tech and therefore lower cost.  They work based on


Having several aligned windows, but with the alignments in different directions and then manually moving the windows into place to adjust the opacity.
Having two reflective windows with a space in between.  When the space is filled with a material having the same refractive index as the reflective windows, the assembly becomes transparent.
Using multiple polarized panels and moving them into or out of place to adjust opacity.


Which method you choose will depend upon how quick you need the materials to change color, the level of manual intervention you're willing to tolerate and how much your willing to spend.  If you google "Smart Glass" you'll find that this field is already well advanced.