Brightening rhymes with:
frightening, enlightening, frightening, heightening, lightening, tightening, battening, captaining, disheartening, enlightening, fattening, flattening, frightening, heartening, heightening, lightening, shortening, smartening, straightening, sweetening, threatening, tightening
Brightening sounds like:
breakdowns, breasting, brightens, brightness, bursting