Chemicals + lightning = The Flash.
Sheldon is well known to love the comic character The Flash.
This shirt is an equation for how you create that character.
As you can see from the link above, the shirt actually appeared in a much earlier episode of the show, season six episode 21, which aired on April 25th, 2013, over a year before the CW version of The Flash started airing (October 2014).
From the Flash Wikipedia page:
Barry Allen is an assistant scientist from the Criminal and Forensic Science Division of Central City Police Department. Barry had a reputation for being very slow, deliberate, and frequently late, which frustrated his fiancée, Iris West. One night, as he was preparing to leave work, a freak lightning bolt struck a nearby shelf in his lab and doused him with a cocktail of unnamed chemicals.
The same thing happened to Wally West:
Wallace Rudolph "Wally" West is the nephew of both Iris West and Barry Allen, by marriage, and was introduced in The Flash (vol. 1) #110 (1959). When West was about ten years old, he was visiting his uncle's police laboratory, and the freak accident that gave Allen his powers repeated itself, bathing West in electrically charged chemicals.
This is faithfully reproduced in the 1990 TV show The Flash:
John Wesley Shipp as Barry Allen / Flash: A forensic scientist in the Central City Police Department (CCPD) who gains the power of super speed after his lab is struck by lightning causing him to be doused with chemicals.
This is also at least partially how Barry Allen in the 2014 TV show The Flash gains his powers:
Fourteen years after his mother's death, an advanced particle accelerator malfunctions during its public unveiling, bathing the city center with a previously unknown form of radiation during a severe thunderstorm. Allen is struck by lightning from the storm and doused with chemicals in his lab. Awakening after a nine-month coma, he discovers he has the ability to move at superhuman speeds.
Yes, there's the addition of the radiation but the basic formula still applies.