William Hart, Lead Singer of Music Group The Delfonics, Dead at 77

William "Poogie" Hart, the Grammy-award-winning lead singer of the R&B band The Delfonics, has died, TMZ has learned.

His son, Hadi, tells TMZ ... William was having trouble breathing so he was recently taken to Temple University Hospital in Philadelphia, where he passed away Thursday from complications during surgery.

William and his brother, Wilbert, launched the Philly-based group in the late '60s with Randy Cain, Ritchie Daniels and Thom Bell. The group would have a major impact on the development of the Philadelphia sound, known as T.S.O.P.

William and Thom exclusively wrote all of the songs, including such hits as "La-La (Means I Love You)," "Didn't I (Blow Your Mind This Time)," "Break Your Promise," "I'm Sorry," and "Ready or Not Here I Come (Can't Hide from Love)."

In 1971, The Delfonics won a Grammy Award for "Didn't I (Blow Your Mind This Time)" for Best R&B Performance By A Duo Or Group, Vocal Or Instrumental.

'Didn't I' and 'La-La' were also featured in Quentin Tarantino's film "Jackie Brown." Other movies that have popularized their music were Spike Lee's "Crooklyn" and "The Family Man" with Nicolas Cage.

Their songs "Ready or Not Here I Come (Can't Hide from Love)" and "Funny Feeling" were played in the video game "Grand Theft Auto V" on a fictional radio station The Lowdown 91.1.

The original band split in 1975 after achieving twelve top-20 hits on Billboard's R&B and Soul Single Chart. Former members created splinter groups that continued to perform through the '90s and 2000s.

William was 77.